Thrombosis in Clinical Practice

Thrombosis in Clinical Practice Thrombosis in Clinical Practice Edited by Andrew D Blann, Gregory YH Lip Haemostasis...

Author: Andrew Blann | Gregory Y.H. Lip | AGG Turpie

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Thrombosis in Clinical Practice

Thrombosis in Clinical Practice Edited by

Andrew D Blann, Gregory YH Lip Haemostasis Thrombosis and Vascular Biology Unit University Department of Medidne City Hospital, Birmingham, UK Alexander GG Turpie Department of Medidne McMaster University Hamilton, Canada

LONDON AND NEW YORK

© 2005 Taylor & Francis, an imprint of the Taylor & Francis Group First published in the United Kingdom in 2005 by Taylor & Francis, an imprint of the Taylor & Francis Group, 2 Park Square, Milton Park, Abingdon, Oxon OX14 4RN TeL:+44 (0)20 7017 6000 Fax.: +44 (0) 20 7017 6699 E-mail: [email protected] Website: http://www.tandf.co.uk/medicine This edition published in the Taylor & Francis e-Library, 2005. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to http://www.ebookstore.tandf.co.uk/.” All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of the publisher or in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of any licence permitting limited copying issued by the Copyright Licensing Agency, 90 Tottenham Court Road, London W1P 0LP. Although every effort has been made to ensure that all owners of copyright material have been acknowledged in this publication, we would be glad to acknowledge in subsequent reprints or editions any omissions brought to our attention. Although every effort has been made to ensure that drug doses and other information are presented accurately in this publication, the ultimate responsibility rests with the prescribing physician. Neither the publishers nor the authors can be held responsible for errors or for any consequences arising from the use of information contained herein. For detailed prescribing information or instructions on the use of any product or procedure discussed herein, please consult the prescribing information or instructional material issued by the manufacturer. A CIP record for this book is available from the British Library. Library of Congress Cataloging-in-Publication Data Data available on application ISBN 0-203-64035-7 Master e-book ISBN

ISBN 0-203-67765-X (Adobe e-Reader Format) ISBN 1 84214 163 5 (Print Edition) Distributed in North and South America by Taylor & Francis 2000 NW Corporate Blvd Boca Raton, FL 33431, USA Within Continental USA Tel: 800 272 7737; Fax: 800 374 3401 Outside Continental USA Tel: 561 994 0555; Fax: 561 361 6018 E-mail: [email protected] Distributed in the rest of the world by Thomson Publishing Services Cheriton House North Way Andover, Hampshire SP10 5BE, UK Tel: +44(0)1264 332424 E-mail: [email protected] Composition by Newgen Imaging Systems (P) Ltd, Chennai, India

Contents Contributors

vi

Preface

x

1 The pathophysiology of thrombosis Lina Badimon and Juan Jose Badimon 2 Epidemiology of coagulopathy Andrew J Catto 3 Commonly used anticoagulant and antiplatelet drugs: aspirin, heparin, and warfarin Andrew D Blann 4 Bleeding risk and hemorrhage: why it happens and what to do about it Gualtiero Palareti, Benilde Cosmi, and Cristina Legnani 5 Thrombophilia and venous thrombosis Pieter W Kamphuisen, Harry R Büller, and Frits R Rosendaal 6 Antithrombotic therapy for atrial fibrillation Ioannis Karalis and Gregory YH Lip 7 Valves Walter Ageno and Alexander GG Turpie 8 Percutaneous coronary intervention Freek WA Verheugt 9 Coronary artery disease: coronary revascularization Petr Widimsky, Zbynĕk Straka, and Martin Pĕnička 10 Peripheral arterial disease M Burress Welborn III, Franklin S Yau, and G Patrick Clagett 11 Antithrombotic therapy for ischemic stroke Tarvinder S Dhanjal and Matthew Walters 12 Management of venous thromboembolism during pregnancy Ian A Greer and Andrew J Thomson 13 Thrombophilia Ian Jennings 14 Systemic thrombosis in children M Patricia Massicotte, Paul Monagle, and Anthony K Chan 15 New drugs and directions Bernd Jilma Index

1 18 36

66 85 105 130 140 152 190 230 252 270 294 323

351

Contributors Walter Ageno Associate Professor of Medicine Department of Clinical Medicine University of Insubria Varese, Italy Juan Jose Badimon Cardiovascular Biology Research Laboratory Mount Sinai School of Medicine New York, USA Lina Badimon Director The Cardiovascular Research Center, CSIC-ICCC Barcelona, Spain Andrew D Blann Senior Lecturer and Consultant Clinical Scientist Haemostasis, Thrombosis and Vascular Biology Unit University Department of Medicine Birmingham, UK Harry R Büller Academic Medical Center Department of Vascular Medicine Meibergdreef Amsterdam The Netherlands Andrew J Catto Senior Lecturer, Honorary Consultant Physician and MRC Clinician Scientist Academic Unit of Molecular Vascular Medicine Leeds, UK Anthony K Chan Associate Professor Department of Pediatrics McMaster University Hamilton, Ontario, Canada

G Patrick Clagett Department of Surgery University of Texas Dallas, Texas, USA Benilde Cosmi Department of Angiology and Blood Coagulation University Hospital S. Orsola-Malpighi Bologna, Italy Tarvinder S Dhanjal British Heart Foundation Research Fellow and Specialist Registrar in Cardiology Division of Cardiovascular Sciences Institute of Biomedical Research University of Birmingham Birmingham, UK Ian A Greer Depute Dean, Faculty of Medicine Regius Professor, Obstetrics and Gynaecology Division of Developmental Medicine Reproductive and Maternal Medicine University of Glasgow Glasgow, UK Ian Jennings UK National External Quality Assessment Scheme for Blood Coagulation Sheffield, UK Bernd Jilma Division of Haematology and Immunology Department of Clinical Pharmacology Vienna University Hospital Wien, Austria Pieter W Kamphuisen Department of Internal Medicine University Medical Center Nijmegen Nijmegen, The Netherlands Ioannis Karalis Research Fellow Haemostasis Thrombosis and Vascular Biology Unit University Department of Medicine Birmingham, UK

Cristina Legnani Department of Angiology and Blood Coagulation University Hospital S. Orsola-Malpighi Bologna, Italy Gregory YH Lip Consultant Cardiologist and Professor of Cardiovascular Medicine Director, Haemostasis Thrombosis and Vascular Biology Unit University Department of Medicine Birmingham, UK M Patricia Massicotte The Peter Olley Chair in Pediatric Thrombosis Pediatric Thrombosis Program, Clinical Director Professor, Department of Pediatrics University of Alberta Edmonton, Alberta Canada Paul Monagle Division of Laboratory Services Royal Children’s Hospital Department of Pediatrics University of Melbourne Melbourne, Australia Gualtiero Palareti Department of Angiology and Blood Coagulation University Hospital S. Orsola-Malpighi Bologna, Italy Martin Pĕnička Cardiovascular Center Aalst Aalst, Belgium Frits R Rosendaal Department of Clinical Epidemiology, and Hemostasis and Thrombosis Research Center Leiden The Netherlands Zbynĕk Straka Cardiocentre Charles University Srobarova Prague, Czech Republic

Andrew J Thomson Consultant in Obstetrics & Gynaecology and Honorary Senior Lecturer Department of Obstetrics and Gynaecology Royal Alexandra Hospital Paisley, Scotland Alexander GG Turpie Department of Medicine McMaster University Hamilton, Canada Freek WA Verheugt Professor of Cardiology Heartcenter Department of Cardiology University Medical Center St Radboud Nijmegen, The Netherlands Matthew Walters Clinical Scientist Gardiner Institute Western Infirmary Glasgow University of Glasgow Glasgow, UK M Burress Welborn III Assistant Professor of Surgery UT Southwestern Medical Center Division of Vascular Surgery Dallas, Texas, USA Petr Widimsky Cardiocenter Charles University Srobarova Prague, Czech Republic Franklin S Yau Department of Surgery University of Texas Dallas, Texas, USA

Preface As thrombosis will cause (via cardiovascular disease, cancer, and connective tissue diseases) the majority of morbidity and mortality in the Western and developed world, then reduction of this major risk factor is clearly a problem that must be addressed. This will be even more pertinent as the new and developing world is rapidly catching up and acquiring the disease profile of the old. This book tries to address/highlight the questions relating to the pathophysiology and clinical practice related to thrombosis. Our initial questions, therefore, begin with “how can we detect thrombosis?”, and “how can we prevent them?” In order to answer the first question, we need first to get to grips with the mechanistic nuts and bolts of thrombosis, and Chapters 1 and 2 are designed to help out in this respect. Chapter 3 provides a historical perspective of antithrombotic therapies. However, overambitious use of antithrombotic agents can lead to hemorrhage—a topic discussed in Chapter 4. Most episodes of thrombosis, and almost all of the life-threatening events, occur in the arteries—thromboses in the venous circulation are discussed in Chapter 5. Chapters that follow provide a systematic and comprehensive view of the various conditions that lead to (arterial) thrombosis in adult life and different ways to treat them. For example, atrial fibrillation is covered in Chapter 6, artificial valves in Chapter 7, coronary interventions in Chapters 8 and 9 and peripheral artery disease in Chapter 10. Stroke is addressed in Chapter 11, and Chapter 12 discusses problems associated with pregnancy. Despite addressing all the possible risk factors for thrombosis (such as smoking and diabetes), some individuals are still prone to thrombosis. This relatively new syndrome—thrombophilia, often the result of a genetic polymorphism—is discussed in Chapter 13. Chapter 14 looks at problems associated with thrombosis in children and acknowledging the shortcomings of existing therapies. We conclude with Chapter 15 on new drugs and directions. This latter chapter comes at a time when the next few years will see the introduction of entirely new classes of antithrombotic agents. Our expectant readers are physicians, general practitioners, nurse practitioners, and other healthcare workers who care for patients presenting with thrombosis-related problems, and thus, the scope is necessarily wide. Evermindful of the fast-moving pace of this area, we have included as many up-to-date guidelines as are feasible. We thank our excellent colleagues for their help, encouragement, and contribution. Andrew D Blann Gregory YH Lip Birmingham, United Kingdom Alexander GG Turpie Hamilton, Canada January 2005

1 The pathophysiology of thrombosis Lina Badimon and Juan Jose Badimon Introduction Arterial and venous thrombosis and thromboembolism cause life-threatening episodes, disabilities, reduction in quality of life, and death. Whereas arterial thrombi are predominantly formed by platelets, venous thrombi are intravascular deposits composed predominantly of fibrin and red cells, with a variable platelet and leukocyte component [1]. Growing thrombi may locally occlude the lumen, or embolize and be washed away by the blood flow to occlude distal vessels. However, thrombi may be physiologically and spontaneously lysed by mechanisms that block thrombus propagation. Thrombus size, location, and composition are regulated by hemodynamic forces (mechanical effects), thrombogenicity of exposed substrate (local molecular effects), relative concentration of fluid phase and cellular blood components (local cellular effects), and the efficiency of the physiologic mechanisms of control of the system, mainly fibrinolysis [2, 3]. Mechanisms of thrombus formation Physiologically, blood constituents do not attach nor adhere to intact endothelial structures. However, the exposure of subendothelial structures, or foreign cardiovascular devices, to flowing blood initiates complex mechanisms that trigger platelet activation and deposition, fibrin formation and deposition, leukocyte accretion, and erythrocyte entrapping in variable flow-dependent characteristics. The dynamics of platelet deposition and thrombus formation are modulated by the type of injury and the local geometry at the site of damage [2–4] (Table 1.1). Platelets The recognition of subendothelial ligands by platelets involves (a) adhesion, activation, and adherence to recognition sites on the thromboactive substrate (extracellular matrix proteins; e.g. von Willebrand factor, collagen, fibronectin, vitronectin, laminin), (b) spreading of the platelet on the surface, and (c) aggregation of platelets with each other to form a platelet plug or white thrombus. The efficiency of the platelet recruitment will depend on the underlying substrate and local geometry. A final step of recruitment of other blood cells also occurs; erythrocytes, neutrophils, and occasionally monocytes are found on evolving mixed thrombus (Figure 1.1). Platelet function depends on adhesive interactions and most of

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Table 1.1 Factors modulating thrombus formation. Nature of the exposed substrate • Degree of injury (mild vs severe arterial injury) • Composition of atherosclerotic plaque • Residual mural thrombus Local fluid dynamics • Shear stress • Tensile stress Systemic thrombogenic factors Hypercholesterolemia Catecholamines (smoking, cocaine, stress, etc.) Smoking Diabetes Homocysteine Lipoprotein (a) Infections (Chlamidia pneumoniae, Helicobacter pylori, Cytomegalovirus) Hypercoagulable state (fibrinogen, von Willebrand factor, Tissue factor, factor VII) Defective fibrinolytic state, etc.

the glycoproteins on the platelet membrane surface are receptors for adhesive proteins. Many of these receptors have been identified, cloned, sequenced, and classified within large gene families that mediate a variety of cellular interactions [5] (Table 1.2). The understanding of the biochemical events involved in platelet activation has progressed significantly. Most platelet aggregation agonists seem to act through the hydrolysis of platelet membrane phosphatidylinositol by phospholipase C, which results in the mobilization of free calcium from the platelet-dense tubular system. Exposed matrix from the vessel wall and thrombin generated by the activation of the coagulation cascade as well as circulating epinephrine are powerful platelet agonists. Adenosine diphosphate (ADP) is a platelet agonist that may be released from hemolyzed red cells in the area of vessel injury. Each agonist stimulates the discharge of calcium and promotes the subsequent release of its granule contents. Platelet-related ADP and serotonin stimulate adjacent platelets, further enhancing the process of platelet activation. Arachidonate, which is released from the platelet membrane by the stimulatory effect of collagen, thrombin, ADP, and serotonin, is another platelet agonist. Arachidonate is converted to thromboxane A2 by the sequential effects of cyclooxygenase and thromboxane synthetase. Thromboxane A2 not only promotes further platelet aggregation but is also a potent vasoconstrictor [6].

The pathophysiology of thrombosis

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The most abundant family of platelet surface receptors is the integrin family, which includes GPIIb/IIIa, GPIa/IIa, GPIc/IIa, the fibronectin receptor, and the vitronectin receptor, in decreasing order of magnitude. Another gene family present in the platelet membrane glycocalyx is the leucine-rich glycoprotein family represented by the GPIb/IX complex, receptor for von Willebrand factor (vWF) on unstimulated platelets that mediates adhesion to subendothelium and GPV. Other gene families include the selectins (such as GMP-140) and the immunoglobulin domain protein (HLA class I antigen and platelet/endothelial cell adhesion molecule 1, PECAM-1). Unrelated to any other gene family is the GPIV (IIIa) (5). The GPIb/IX complex consists of two disulfide-linked subunits (GPIbα and GPIbβ) tightly (not covalently) complexed with GPIX in a 1:1 heterodimer. GPIbβ and GPIX are transmembrane glycoproteins and form the larger globular domain. The major role of GPIb/IX is to bind immobilized vWF on the exposed vascular subendothelium and initiate adhesion of platelets. GPIb does not bind

Figure 1.1 Diagram of platelet-vessel wall and platelet-platelet interaction with the participation of the coagulation and spontaneous fibrinolysis systems. (Continuous lines: pathways of activation; Dashed lines: pathways of inhibition.)


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soluble vWF in plasma. Apparently, it undergoes a conformation change upon binding to the extracellular matrix and then exposes a recognition sequence for GPIb/IX. The cytoplasmic domain of GPIb/IX has a major function in linking the plasma membrane to the intracellular actin filaments of the cytoskeleton and functions to stabilize the membrane and to maintain the platelet shape [7]. Randomly distributed on the surface of resting platelets are about 50,000 molecules of GPIIb/IIIa. The complex is composed of one molecule of GPIIb (disulfide-linked large and light chains) and one of GPIIIa (single polypeptide chain). It is a Ca2+-dependent heterodimer, noncovalently associated on the platelet membrane [8]. Calcium is required for maintenance of the complex and for binding of adhesive proteins. On activated platelets, the GPIIb/IIIa is a receptor for fibrinogen, fibronectin, vWF, vitronectin, and thrombospondin [9, 10].

Table 1.2 Platelet membrance glycoprotein receptors. Glycoprotein receptor

Function

Ligand

GPIIb/IIIa

• Aggregation, adhesion at high shear rate

Fg, vWF, Fn, Ts, Vn

Receptor Vn

• Adhesion

Vn, vWF, Fn, Fg, Ts

GPIa/IIa

• Adhesion

C

GPIc/IIa

• Adhesion

Fn

GPIcN/IIa

• Adhesion

Ln

GPIb/IX

• Adhesion

vWF, T

GPV

• Unknown

Substrate T

GPIV (GPIIIb)

• Adhesion

Ts, C

GMP-140 (PADGEM)

• Interaction with leucocytes

Unknown

PECAM-1 (GPIIa)

• Unknown

Unknown

Fg: fibrinogen; vWF: von Willebrand factor; Fn: fibronectin; Ts; thrombospondin; Vn: vitronectin; C; collagen; Ln: laminin; T: thrombin; PECAM-1: platelet/endothelial cell adhesion molecule 1.

Thrombin plays an important role in the pathogenesis of arterial thrombosis. It is one of the most potent known agonists for platelet activation and recruitment. The thrombin receptor has 425 amino acids with seven transmembrane domains and a large NH2terminal extracellular extension that is cleaved by thrombin to produce a “tethered” ligand that activates the receptor to initiate signal transduction [11, 12]. Thrombin is a critical enzyme in early thrombus formation, cleaving fibrinopeptides A and B from fibrinogen to yield insoluble fibrin, which effectively anchors the evolving thrombus. Both free and fibrin-bound fibrin thrombin are able to convert fibrinogen to fibrin allowing propagation of thrombus at the site of injury. Therefore, platelet activation triggers intracellular signaling and expression of platelet membrane receptors for adhesion and initiation of cell contractile processes that induce shape change and secretion of the granular contents. The expression of the integrin IIb/IIa


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(αIIbβ3) receptors for adhesive glycoprotein ligands (mainly fibrinogen and vWF) in the circulation initiates platelet-to-platelet interaction. The process becomes perpetuated by the arrival of platelets brought by the circulation. Most of the glycoproteins in the platelet membrane surface are receptors for adhesive proteins or mediate cellular interactions. It has been shown that vWF binds to platelet membrane glycoproteins in both adhesion (platelet-substrate interaction) and aggregation (platelet-platelet interaction), leading to thrombus formation at high shear rates [7, 13, 14]. In Figure 1.2, a simplified diagram shows different platelet membrane receptors involved in platelet activation. Ligand binding to the different membrane receptors triggers platelet activation with different relative potencies. A lot of interest has been recently generated on the platelet ADP-receptors (P2Y12, P2Y1R, P2X1R) because of available pharmacological inhibitors [15].

Figure 1.2 Simplified diagram of platelet receptors and ligands. Activation of the coagulation system The activation of the coagulation cascade leads to the generation of thrombin, which is a powerful platelet agonist that contributes to platelet recruitment in addition to catalyzing the formation and polymerization of fibrin. Fibrin is essential to the stabilization of the platelet thrombus and its withstanding removal forces by flow, shear, and high intravascular pressure (Figure 1.3). The proteins which compose the clotting enzymes do not collide and interact on a random basis in the plasma but interact in complexes in a highly efficient manner on platelet and endothelial surfaces. The major regulatory events in the coagulation (activation, inhibition, generation of anticoagulant proteins) occur on membrane surfaces. It is interesting to note that venous thrombosis, which is predominantly constituted by fibrin clots occurs in areas of stasis and low-shear-rate conditions typical of the venous system. Therefore, the low local-shear-rate conditions and flow recirculations developing


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in the poststenotic areas may explain fibrin accumulation. Tissue factor (TF) appears to be a major procoagulant factor in the vascular space immediately underlying the endothelial lining of arteries, a site that might be readily accessible upon local injury or upon rupture of an atherosclerotic plaque. The blood coagulation system involves a sequence of reactions integrating zymogens (proteins susceptible to be activated to enzymes via limited proteolysis) and cofactors (nonproteolytic enzyme activators) in three groups: (a) the contact activation (generation of factor XIa via the Hageman factor), (b) the

Figure 1.3 Analysis of early thrombosis on human atherosclerotic plaques perfused in the Badimon perfusion chamber for 3 minutes. Immunofluorescence analysis of a mural growing thrombus on an atherosclerotic plaque. Red (rhodamine) marks fibrin-fibrinogen deposition and green (fluorescein) marks platelet deposition. conversion of factor X to factor Xa in a complex reaction requiring the participation of factors IX and VIII, and (c) the conversion of prothrombin to thrombin and fibrin formation [16] (Figure 1.4). The triggering surfaces for in vivo initiation of contact activation have been suggested to be sulfatides and glycosaminoglycans of the vessel wall. The physiologic role of this system is unclear, however, because the absence of Hageman factor, prekallikrein, or highmolecular-weight kininogen does not induce a clinically apparent pathology. Factor XI deficiency is associated with abnormal bleeding. Activated factor XI induces the activation of factor IX in the presence of Ca2+. Factor IXa forms a catalytic complex with factor VIII on a membrane surface and efficiently activates factor X in the presence of Ca2+. Factor IX is a vitamin K-dependent enzyme, as are factor VII, factor X, prothrombin, and protein C.


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Factor VIII forms a noncovalent complex with vWF in plasma and its function in coagulation is the acceleration of the effects of IXa on the activation of X to Xa. Absence of factor VIII or IX produces the hemophilic syndromes. The TF pathway, previously known as extrinsic coagulation pathway, through the TFfactor VII complex in the presence of Ca2+ induces the formation of Xa. A second TFdependent reaction catalyzes the transformation of IX into IXa. TF is an integral membrane protein that serves to initiate the activation of factors IX and X and to localize the reaction to cells on which TF is expressed. Other cofactors include factor VIIIa, which binds to platelets and forms the binding site for IXa, thereby forming the machinery for the activation of X, and factor Va, which binds to platelets and provides a binding site for Xa. The human genes for these cofactors have been cloned and sequenced. In physiologic conditions, no cells in contact with blood contain active TF, although cells such as monocytes and polymorphonuclear leukocytes can be induced to synthesize and express TF [16]. Activated Xa converts prothrombin into thrombin. The complex which catalyzes the

Figure 1.4 Simplified diagram of the coagulation cascade and thrombosis. formation of thrombin consists of factors Xa and Va in a 1:1 complex. The activation results in the cleavage of fragment F1.2 and formation of thrombin from fragment 2. The interaction of the four components of the “prothrombinase complex” (Xa, Va, phospholipid, and Ca2+) yield a more efficient reaction [17]. Activated platelets provide procoagulant surface for the assembly and expression of both intrinsic Xase and prothrombinase enzymatic complexes [18]. These complexes respectively catalyze the activation of factor X to factor Xa and prothrombin to thrombin. The expression of activity is associated with the binding of both the proteases, factor IXa and factor Xa, and the cofactors, VIIIa and Va, to procoagulant surfaces. The binding of IXa and Xa is promoted by VIIIa and Va, respectively, such that Va and likely VIIIa provide the equivalent of receptors for the proteolytic enzymes. The surface of the


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platelet expresses the procoagulant phospholipids that bind coagulation factors and contribute to the procoagulant activity of the cell. Blood clotting is blocked at the level of the prothrombinase complex by the physiologic anticoagulant-activated protein C and oral anticoagulants. Oral anticoagulants prevent posttranslational synthesis of γ-carboxyglutamic acid groups on the vitamin K-dependent clotting factors, preventing binding of prothrombin and Xa to the membrane surface. Activated protein C cleaves factor Va to render it functionally inactive. Loss of Va decreases the role of thrombin formation to negligible levels [19]. Thrombin acts on multiple substrates, including fibrinogen, factor XIII, factor V, factor VIII, and protein C, in addition to its effects on platelets. It plays a central role in hemostasis and thrombosis. The catalytic transformation of fibrinogen into fibrin is essential in the formation of the hemostatic plug and in the formation of arterial thrombi. It binds to the fibrinogen central domain and cleaves fibrinopeptides A and B, resulting in fibrin monomer and polymer formation [18–20]. The fibrin mesh holds the platelets together and contributes to the attachment of the thrombus to the vessel wall. The presence of a residual mural thrombus predisposes to recurrent thrombotic episodes. Two main contributing factors for the development of rethrombosis have been identified. First, a residual mural thrombus may encroach into the vessel lumen resulting in increased shear rate in an artery or areas of stasis in venous thrombi, which facilitates the activation and deposition of platelets on the lesion. Second, the presence of a fragmented thrombus appears to be one of the most powerful thrombogenic surfaces. Thus, following lysis, thrombin becomes exposed to the circulating blood, leading to activation of the platelets and coagulation, further enhancing thrombosis. In these conditions, the antithrombin activity of heparin is limited for three main reasons. First, a residual thrombus contains active thrombin bound to fibrin, which is thus poorly accessible to the large heparin-antithrombin III complexes; second, a plateletrich arterial thrombus releases large amounts of platelet factor 4, which may inhibit heparin; third, fibrin II monomer, formed by the action of thrombin on fibrinogen, may also inhibit heparin. Conversely, molecules of hirudin and other specific antithrombins are at least ten times smaller than the heparin-antithrombin III complex, have no natural inhibitors, and therefore have greater accessibility to thrombin bound to fibrin. Spontaneous anticoagulation and fibrinolysis The control of the coagulation reactions occurs by diverse mechanisms, such as hemodilution and flow effects, proteolytic feedback by thrombin, inhibition by plasma proteins (such as antithrombin III (ATIII)) and endothelial celllocalized activation of an inhibitory enzyme (protein C), and fibrinolysis. Although ATIII readily inactivates thrombin in solution, its catalytic site is inaccessible while bound to fibrin, and it may still cleave fibrinopeptides even in the presence of heparin. Thrombin has a specific receptor in endothelial cell surfaces, thrombomodulin, that triggers a physiologic anticoagulative system [21, 22]. The thrombin-thrombomodulin complex serves as a receptor for the vitamin K-dependent protein C that is activated and released from the endothelial cell surface. Activated protein C blocks the effects of factors V and VIII and limits thrombin effects. Endogenous fibrinolysis represents a repair mechanism such as


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endothelial cell regrowth and vessel recanalization. Fibrinolysis involves catalytic activation of zymogens, positive and negative feedback control, and inhibitor blockade. Thrombus formation in different locations Thrombosis of left ventricular and left atrial chambers Intracavitary mural thrombi develop frequently in patients with acute myocardial infarction, chronic left ventricular infarction, chronic left ventricular aneurysm, dilated cardiomyopathy, and atrial fibrillation. The clinical significance of thrombosis in the cardiac chambers derives from its potential for systemic embolism, which also depends on dynamic forces of the circulation [23, 24]. In the first few days after acute myocardial infarction, leukocyte infiltration separates endothelial cells from their basal lamina. The resulting exposure of subendothelial tissue to blood serves as the nidus for thrombus development. Specific endocardial abnormalities have also been identified histologically in surgical and postmortem specimens from patients with left ventricular aneurysms and at necropsy in patients with idiopathic dilated cardiomyopathy. Both experimental and clinical [25, 26] studies have emphasized the importance of wall motion abnormalities in the development of left ventricular thrombi, and it seems clear that stasis of blood in regions of akinesis or dyskinesis is the essential factor. Similarly, stasis is important in the development of atrial thrombi [27] when effective mechanical atrial activity is impaired, as occurs in atrial fibrillation, atrial enlargement, mitral stenosis, and cardiac failure. Stasis is associated to conditions of low shear rate in which activation of coagulation factors rather than of platelet leads to fibrin formation and constitutes the predominant pathogenic mechanism in the development of intracavitary thrombi. Although a hypercoagulable state is controversial, a hypercoagulable tendency in this condition was suggested, and it is conceivable that a systemic procoagulant tendency arises during the acute stage of myocardial infarction and predisposes to thromboembolic events. More relevant is experimental evidence that the surface of a fresh thrombus is itself highly thrombogenic, producing at least a local, if not a systemic, hypercoagulable state [28]. The problems of thromboembolism originating from the cardiac chambers prompt consideration of the balance between the effects of regional injury, stasis, and procoagulant factors that favor thrombus formation, and dynamic forces of the circulation, which are responsible for the migration of thrombotic material into the systemic circulation. Even though stasis favors thrombus formation within the sac of a left ventricular aneurysm, isolation from dynamic circulatory forces protects against embolic migration. In diffuse dilated cardiomyopathy, on the other hand, mural thrombus is not isolated from the circulation and the embolic risk is higher.


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Prosthetic heart valves and thromboembolism Mechanical prosthesis. The pathologic events leading to thromboembolism already develop during surgery. The damaged peri-valvular tissue and prosthetic materials activate platelets as soon as blood starts flowing across the valve, and immediate platelet deposition results [29]. Prosthetic materials activate the intrinsic clotting system, with subsequent fibrin formation [30]. In addition, the abnormal hemodynamic characteristics of mechanical prosthetic devices promote mainly fibrin generation and, less importantly, platelet activation. Finally, the process of fibrin thrombus formation can be facilitated in areas with stasis and decreased blood flow such as in the left atrium during atrial fibrillation and in the left ventricle during low cardiac output state secondary to left ventricular dysfunction [31]. Biologic prosthesis. Bioprosthetic valves are considerably less thrombogenic mainly because of the biologic properties of materials used in their construction and also because of characteristic axial flow profile and leaflet pliability. Venous thrombosis and pulmonary embolism Venous thrombi usually form in regions of slow or disturbed flow and begin as small deposits that frequently arise in large venous sinuses in the calf, in valve cusp pockets either in the deep veins of the calf or thigh, or in venous segments that have been exposed to direct trauma [32–34], The major predisposing factors to venous thrombosis are activation of blood coagulation and venous stasis, whereas in arterial thrombosis it is vascular wall damage, usually of atherosclerotic origin [35–38]. Nevertheless, wall damage may predispose to venous thrombosis in special circumstances. Arterial thrombosis The formation of a thrombus within an artery with obstruction of blood flow and oxygen supply to the target organ produces the acute ischemic syndromes. These thrombotic episodes largely occur in response to atherosclerotic lesions that have slowly progressed to a high-risk inflammatory/prothrombotic stage. Although distinct from one another, the atherosclerotic and thrombotic processes appear to be closely interrelated as the causal presentation of ischemic syndromes in a complex multifactorial process named atherothrombosis [39]. Atherosclerotic disease in the coronary artery system may manifest in the form of stable or unstable angina, acute myocardial infarction, or sudden cardiac death. Atherosclerotic disease in the cerebral arterial system (including the intracranial and extracranial arteries) can manifest in the form of transient ischemic attack (TIA) and cerebral ischemic infarction. In contrast to that of acute coronary syndromes, the pathophysiologic process leading to acute cerebral ischemia is less well defined but appears to involve multiple etiologic mechanisms. Atherosclerotic disease of the abdominal or leg arteries can result in acute and chronic ischemia of the extremities and usually involves thrombosis, embolism, or both, originating from atherosclerotic plaques.


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Thrombosis in the acute coronary syndromes Fissuring or rupture of an atherosclerotic plaque in the coronary arteries plays a fundamental role in the development of the acute coronary syndromes, as has been clearly shown in patients who died suddenly or shortly after an episode of unstable angina or myocardial infarction. Disrupted atherosclerotic plaques are commonly associated with the formation of mural or occlusive thrombi usually anchored to fissures in the ruptured or ulcerated plaque. Angiographic studies have documented the presence of intraluminal thrombi both in unstable angina and acute myocardial infarction. The incidence of thrombi in unstable angina varied significantly among different studies, in part related to the interval between anginal symptoms and arteriographic study. The shorter this interval, the higher the likelihood of finding occlusive thrombi. Severity of vessel wall damage is probably the most important of all the previously mentioned “thrombogenic risk factors” in the acute coronary syndromes. It is likely that when injury to the vessel wall is mild, the thrombogenic stimulus is relatively limited and the resulting thrombotic occlusion transient, as occurs in unstable angina. On the other hand, deep vessel injury secondary to plaque rupture and ulceration results in exposure of collagen, lipids, and other elements of the vessel media, leading to relatively persistent thrombotic occlusion and myocardial infarction. Although a substantial proportion of episodes of unstable angina and acute myocardial infarction are caused by plaque fissuring or rupture with superimposed thrombosis, other mechanisms that alter the balance between myocardial oxygen supply and demand need to be considered [2, 3]. In patients with stable coronary disease, angina commonly results from increases in myocardial oxygen demand beyond the ability of stenosed coronary arteries to increase its delivery. In contrast, unstable angina, non-Q wave myocardial infarction, and Q wave myocardial infarction represent a continuum of the disease process and are usually characterized by an abrupt reduction in coronary flow. In unstable angina, disruption of an atherosclerotic plaque may lead to acute changes in plaque morphology and reduction in coronary flow. Transient episodes of thrombotic vessel occlusion at the site of plaque injury may occur, leading to angina at rest. This thrombus is usually labile, resulting in only temporary vessel occlusion. In addition to plaque disruption, other mechanisms contribute to reduced coronary flow. Platelets attach to the damaged endothelium or to exposed media and release vasoactive substances including thromboxane A2 and serotonin, leading to aggregation of neighboring platelets and vasoconstriction. Alterations in perfusion and myocardial oxygen supply probably account for two-thirds of episodes of unstable angina; the rest may be caused by transient increases in myocardial oxygen demand. In non-Q wave infarction, the angiographic morphology of the responsible lesion is similar to that seen in unstable angina, suggesting that plaque disruption is common to both syndromes. About one-fourth of patients with non-Q wave infarction have a completely occluded infarct-related vessel at early angiography, with the distal territory usually supplied by collaterals. The presence of ST segment elevation in the electrocardiogram, early peak in plasma creatine kinase, and high angiographic patency rate of the involved vessel suggest that complete coronary occlusion followed by early


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reperfusion (within the first 2 h) or resolution of vasospasm, are pathogenetically important in non-Q wave infarction. In Q wave infarction, plaque rupture is commonly associated with deep arterial injury or ulceration, resulting in the formation of a fixed and persistent thrombus, leading to abrupt cessation of myocardial perfusion and necrosis. The coronary lesion responsible for the infarction is frequently only mild to moderately stenotic, which suggests that plaque rupture with superimposed thrombosis is the primary determinant of acute occlusion rather than of the severity of the underlying lesion [40]. While severe preexisting lesions often lead to complete vessel occlusion, myocardial infarction does not commonly supervene, perhaps owing to adequate collateral flow. In perhaps onefourth of patients, coronary thrombosis results from superficial intimal injury or blood stasis in areas of high-grade stenosis. In sudden cardiac death, two mechanisms predominate. Sudden death related to ischemia probably involves a rapidly progressive coronary lesion in which plaque rupture and resultant thrombosis lead to myocardial hypoperfusion and fatal ventricular arrhythmias. Absence of collateral flow to the myocardium distal to the occlusion or platelet microthrombi may contribute to the development of sudden ischemic death. Additionally, fatal ventricular arrhythmias are common in patients, after extensive myocardial infarction or other forms of cardiomyopathy, in whom a substrate for the generation and maintenance of ventricular tachycardia or fibrillation exists (Figure 1.5). Thrombosis in cerebrovascular disease Clinical manifestations of atherosclerotic disease in the cerebral circulation (intracranial and extracranial) results in a spectrum of acute

Figure 1.5 Thrombosis on an eroded (non-ruptured) human atherosclerotic coronary artery. Original preparation from the Eulalia Study on Sudden Death (H & E stain).


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cerebral ischemic syndromes ranging from TIAs to full-blown cerebral infarction [41, 42]. Similar to the pathogenesis of acute coronary syndromes, thrombosis over a disrupted plaque plays a key pathogenic role in the majority of patients suffering from these vascular events. However, in contrast to acute coronary syndromes, other pathogenic mechanisms such as intracranial hemorrhage, subarachnoid hemorrhage, and cardiogenic embolism also play a role in a substantial proportion of these patients. TIAs are brief episodes of focal loss of brain function, thought to be due to ischemia, that can usually be localized to that portion of brain supplied by one vascular system, and to which no other cause can be found. Though arbitrary, by convention, episodes lasting less than 24 hours are classified as TIAs although the longer the episode, the greater the likelihood of finding a cerebral infarct by computer tomography. TIAs commonly last 2– 15 minutes and are rapid in onset. Each TIA leaves no persistent deficit, and there are often multiple attacks. Most patients have TIAs that include motor symptoms. It is common for amaurosis fugax (monocular blindness) to occur without other symptoms during the episode. In general, TIAs evolve from two causes: focal low blood flow and embolism. The mechanism of focal low flow in TIAs is not well defined. Probably, a critical stenotic or occluded artery reduces flow to a focal area of normal brain. Certainly, poor collateral circulation to the ischemic area plays a prominent role, but factors such as viscosity, vessel wall compliance, and other unknown factors are needed to explain why the reduction is transient. The presence of fibrin or fibrin-platelet mural thrombus is found overlying an atherosclerotic plaque in two-thirds of cases with hemispheric TIAs. In contrast, evidence of ulceration on the plaques, which are widely regarded as sources of emboli, is only found in part of the cases with TIA, and at a similar rate in the asymptomatic cases. That the incidence of ulceration was about the same in TIA and asymptomatic groups appeared to preclude an important role for ulceration as the etiology of repetitive TIAs; however, investigations are underway on this topic with the use of more sophisticated technologies. Intraplaque hemorrhage is present in over one-third of cases of TIAs. Future investigation into the pathogenesis of carotid intraplaque hemorrhage might indicate that this is the result of small plaque fissuring and dissection, as recently demonstrated in acute coronary syndromes. Overall, the evidence from different studies indicate that probably in a substantial proportion of patients, TIAs result from progressive luminal narrowing leading to precarious hemodynamic insufficiency. One can also infer that mural thrombi at the subocclusive stage can either contribute to the obstruction or creation of an emboli. Large plaque ulceration or intraplaque hemorrhage did not correlate well with clinical ischemic events, but probably play a role in plaque growth. It has been assumed that fragments of mural thrombi of extracranial atheromatous plaque break and cause TIAs. However, the proportion of patients with TIAs of embolic origin is not clearly defined. As discussed previously, pathologic studies in patients with TIA indicated that about 30% of endarterectomy specimens obtained demonstrated no overlying thrombi. Furthermore, probably in a substantial proportion of patients with TIAs, cardiogenic emboli (emboli from cardiac chambers) appears to be the pathogenic mechanism. Clinical data indicated that at least 25% of all cerebral ischemic events and more than a third of cerebral ischemic events in the elderly are associated with atrial fibrillation. In addition, about 1 in 3 patients with atrial fibrillation will experience a


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cerebral ischemic event during lifetime. These confounding data have complicated our understanding of the pathogenic mechanisms involved in transient ischemic events. Further prospective clinical studies are needed in order to assess the relative importance of various pathogenic processes in different risk groups and to formulate accurate therapeutic strategies. The clinical hallmark of a stroke (acute cerebral infarction) is the abrupt development of a focal neurologic deficit due to ischemia or bleeding in a particular territory [43]. Intracerebral or subarachnoid hemorrhage accounts for 15% of strokes. The vast majority of strokes (up to 85%) are ischemic, arising from atherosclerotic disease. The prevalence of presumed cardioembolic stroke varies between 15% and 30% suggesting either patient population differences or variability in application of diagnostic criteria. In addition to the lack of validated, reliable, clinical diagnostic criteria for differentiating between cardioembolic and no embolic ischemic stroke, a significant proportion of the patients have risks for both mechanisms of stroke. Thus, about 30% of patients with acute ischemic stroke will have a potential cardiac source of embolism, but about one-third of these patients will also have concomitant cerebrovascular atherosclerosis that could also be responsible for brain ischemia. Thrombosis in peripheral arterial disease The most characteristic symptom of peripheral arterial disease is intermittent claudication, which occurs when the oxygen demand of exercising skeletal muscle exceeds the flow reserve of the diseased limb arteries. Characteristically, intermittent claudication occurs reproducibly with exercise at a given workload and is relieved by rest. Thus, intermittent claudication represents the skeletal muscle counterpart to stable angina pectoris. Peripheral arterial disease tends to be slowly progressive and may remain asymptomatic for long periods, particularly when collateral circulation is well developed [44]. An abrupt onset of ischemic rest pain or sudden worsening of intermittent claudication may be due to thromboembolism, or thrombosis in situ complicating an atherosclerotic lesion. Embolism in a previously healthy artery is more likely when symptoms occur suddenly in a patient without prior claudication, and with risk factors for thromboembolism (such as atrial fibrillation, left ventricular failure, prosthetic heart valves, or aortic aneurysm). When this occurs, advanced ischemia is common, owing to inadequate time for development of collateral circulation. A syndrome of microembolization may also occur, commonly with atherothrombotic materials released from a proximal arterial lesion (such as an aortic aneurysm). The clinical presentation associated with microembolization is that of digital pain, cyanosis, or gangrene, with preserved pulses and skin temperature in the extremity. This may also occur as a complication of arterial catheterization. Sudden worsening of intermittent claudication, or progression of intermittent claudication to rest pain or gangrene, is often seen with thrombotic or embolic occlusion of a preexisting stenotic lesion. When in situ thrombosis complicates an existing lesion, the presumed mechanism in many cases is plaque rupture or fissuring as previously described, but is also probably due to stasis of blood flow with a severely stenotic lesion in some cases. The clinical outcome in such cases is most dependent on the adequacy of the collateral circulation.


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Because peripheral arterial disease is frequently asymptomatic for long periods, the frequency of intermittent claudication is likely to underestimate the incidence of significant peripheral arterial disease. Acknowledgments The work reported in this chapter has been partially supported by grants from Plan Nacional de Salud SAF2000/0174; FIS C03–01. References [1] Badimon L, Badimon JJ, Fuster V. Pathogenesis of thrombosis. In: Fuster V, Verstraete M (eds), Thrombosis in Cardiovascular Disorders. Philadelphia, PA: WB Saunders Company; Chapter 2, 1992:17–39. [2] Badimon L, Chesebro JH, Badimon JJ. Thrombus formation on ruptured atherosclerotic plaques and rethrombosis on evolving thrombi. Circulation 1992; 86:III 74–III 85. [3] Fuster V, Badimon L, Badimon JJ, Chesebro JH. The pathogenesis of coronary artery disease and the acute coronary syndromes. (Part I). N Engl J Med 1992; 326:242–50. (Part II). N Engl J Med 1992; 326:310–18. [4] Badimon L, Badimon JJ, Fuster V. Pathogenesis of thrombosis. In: Verstraete M, Fuster V, Topol EJ (eds), Cardiovascular Thrombosis. Philadelphia, PA: Lippincot-Raven Publisher; 1998:23–44. [5] Kieffer N, Phillips DR. Platelet membrane glycoproteins: functions in cellular interactions. Annu Rev Biol 1990; 6:329–57. [6] Brass LF. The biochemistry of platelet activation. In: Hoffman R, Benz EJ Jr, Shattil SJ, Furie B, Cohen HJ (eds), Hematology: Basic Principles and Practice. New York: Churchill Livingstone; 1991:1176–97. [7] Meyer D, Girma JP. von Willebrand factor: structure and function. Thromb Haemost 1993; 70:99–104. [8] Fitzgerald LA, Phillips DR. Calcium regulation of the platelet membrane glycoprotein IIb-IIIa complex. J Biol Chem 1985; 260:11366–76. [9] Coller B. Platelet GPIIb/IIIa antagonists: the first anti-integrin receptor therapeutics. J Clin Invest 1997; 99:1467–71. [10] Ginsberg MH, Xiaoping D, O’Toole TE, Loftus JC, Plow EF. Platelet integrins. Thromb Haemost 1993; 70:87–93. [11] Vu TH, Hung DT, Wheaton VI, Coughlin SR. Molecular cloning of a functional thrombin receptor reveals a novel proteolytic mechanism of receptor activation. Cell 1991; 64:1057–68. [12] Coughlin SR. Thrombin receptor structure and function. Thromb Haemost 1993; 70:184–7. [13] Badimon L, Badimon JJ, Turitto VT, Vallabhajosula S, Fuster V. Platelet thrombus formation on collagen type I. Influence of blood rheology, von Willebrand factor and blood coagulation. Circulation 1988; 78:1431–42. [14] Badimon L, Badimon JJ, Turitto VT, Fuster V. Role of von Willebrand factor in mediating platelet-vessel wall interaction at low shear rate: the importance of perfusion conditions. Blood 1989; 73:961–7. [15] Gachet C. ADP receptors of platelets and their inhibition. Thromb Haemost 2001; 86:222–32. [16] Nemerson Y. Mechanism of coagulation. In: Williams WJ, Beutler E, Erslev AJ, Lichtman MA (eds), Hematology. New York: McGraw-Hill; 1990:1295–304.


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[17] Mann KG. Factor VII-activating protease: coagulation, fibrinolysis, and atherothrombosis? Circulation 2003; 107:654–5. [18] Mann KG. Thrombin formation. Chest 2003; 124 (3 Suppl.):4S–10S. [19] Mann KG, Nesheim ME, Church WR, Haley P, Krishnaswamy S. Surface dependent reactions of the vitamin K dependent enzyme complexes. Blood 1990; 76:1–16. [20] Nemerson Y, Williams WJ. Biochemistry of plasma coagulation factors. In: Williams WJ, Beutler E, Erslev AJ, Lichtman MA (eds), Hematology. New York: McGraw-Hill; 1990:1267– 84. [21] Francis CW, Marder VJ. Physiologic regulation and pathologic disorders of fibrinolysis. In: Colman RW, Hirsh J, Marder VJ, Salzman EW (eds), Hemostasis and Thrombosis: Basic Principles and Clinical Practice. Philadelphia, PA: Lippincott; 1987: 358–79. [22] Collen D, Lijnen HR. Molecular and cellular basis of fibrinolysis. In: Hoffman R, Benz EJ Jr, Shattil SJ, Furie B, Cohen HJ (eds), Hematology: Basic Principles and Practice. New York: ChurchillLivingstone; 1991:1232–42. [23] Tegeler CH, Downes TR. Thrombosis and the heart. Semin Neurol 1991; 11:339–52. [24] Adams PC, Cohen M, Chesebro JH, Fuster V. Thrombosis and embolism from cardiac chambers and infected valves. J Am Coll Cardiol 1986;8: 76B–87B. [25] Mikell FL, Asinger RW, Elsperger KJ. Regional stasis of blood in the dysfunctional left ventricle: echocardiographic detection and differentiation from early thrombosis. Circulation 1982; 66:755–63. [26] Merino A, Hauptman P, Badimon L, Badimon JJ, Cohen M, Fuster V, Goldman M. Echocardiographic “smoke” is produced by an interaction of erythrocytes and plasma proteins modulated by shear forces. J Am Coll Cardiol 1992;20: 1661–8. [27] Shrestha NK, Moreno FL, Narcisco FV, Torres L, Calleja H. Two-dimensional echocardiographic diagnosis of left atrial thrombus in rheumatic heart disease: a clinicopathologic study. Circulation 1983; 67:341–7. [28] Fuster V, Halperin JL, Left ventricular thrombi and cerebral embolism (editorial). N Engl J Med 1989; 320:392–4. [29] Fuster V, Badimon L, Badimon JJ, Chesebro JH. Prevention of thromboembolism induced by prosthetic heart valves. Semin Thromb Hemost 1988; 14:50–8. [30] Alpert JS. The thrombosed prosthetic valve. Current recommendations based on evidence from the literature. J Am Coll Cardiol 2003; 41:659–60. [31] Lengyel M, Fuster V, Keltai M, et al. Guidelines for management of left-side prosthetic valve thrombosis: a role for thrombolytic therapy. J Am Coll Cardiol 1997; 30:1521–6. [32] Mannucci PM. Venous thrombosis: the history of knowledge. Pathophysiol Haemost Thromb 2002; 32:209–12. [33] The potential role of direct thrombin inhibitors in the prevention and treatment of venous thromboembolism. Chest 2003; 124(3 Suppl.):40S–48S. [34] Tovey C, Wyatt S. Links diagnosis, investigation, and management of deep vein thrombosis. BMJ 2003; 326:1180–4. [35] Levi M, Dorffle-Melly J, Johnson GJ, Drouet L, Badimon L; Subcommittee on Animal, Cellular, and Molecular Models of Thrombosis and Haemostasis of the Scientific and Standardization Committee of the International Society on Thrombosis and Haemostasis. Usefulness and limitations of animal models of venous thrombosis. Thromb Haemost 2001; 86:1331–3. [36] Kearon C. Natural history of venous thromboembolism. Circulation 2003; 107:I22–I30. [37] Anderson FA, Spencer FA. Risk factors for venous thromboembolism. Circulation 2003; 107:I9–I16. [38] White RH. The epidemiology of venous thromboembolism. Circulation 2003; 107:I4–I8. [39] Badimon L, Badimon JJ, Vilahur G, Segales E, Llorente V. Pathogenesis of the acute coronary syndromes and therapeutic implications. Pathophysiol Haemost Thromb 2002; 32:225–31. [40] Falk E, Shah PK, Fuster V. Coronary plaque disruption. Circulation 1995; 92:657–71.


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[41] Adams RJ, Chimowitz MI, Alpert JS, Awad IA, Cerqueria MD, Fayad P, Taubert KA; Stroke Council and the Council on Clinical Cardiology of the American Heart Association; American Stroke Association. Coronary risk evaluation in patients with transient ischemic attack and ischemic stroke: a scientific statement for healthcare professionals from the Stroke Council and the council on Clinical Cardiology of the American Heart Association/American Stroke Association. Circulation 2003; 108:1278–90. [42] Graham CA. Transient cerebral ischemia demands urgent evaluation. Stroke 2003; 34:2451–2. [43] Hack W, Kaste M, Bogousslavsky J, Brainin M, Chamorro A, Lees K, Leys D, Kwiecinski H, Toni P, Langhorne P, Diener C, Hennerici M, Ferro J, Sivenius J, Gunnar N, Bath P, Olsen TS, Gugging M; European Stroke Initiative Executive Committee and the EUSI Writing Committee European Stroke Initiative Recommendations for Stroke Management—update 2003. Cerebrovasc Dis 2003; 16:311–37. [44] McDermott MM, Mandapat AL, Moates A, Albay M, Chiou E, Celic L, Greenland P. Knowledge and attitudes regarding cardiovascular disease risk and prevention in patients with coronary or peripheral arterial disease. Arch Intern Med 2003; 163:2157–62.

2 Epidemiology of coagulopathy Andrew J Catto Introduction Thrombosis is the commonest cause of mortality in the Western world. Thrombotic disease can be broadly classified as that arising in the venous system (at low blood flow and pressure) and in the arterial system (at high flow and pressure). A thrombus is a deposit arising from the constituents of the blood on the lining of the heart or blood vessels. Thrombi occur anywhere in the circulation, and an occlusive thrombus occupies the lumen of the vessel resulting in cessation of flow and tissue damage distal to the occlusion. Virchow, as early as 1856, proposed that three major factors determine the site and extent of thrombus. Specifically, the mechanical effects in which blood flow is predominant, the constituents of the blood, and, finally, the vessel wall. The interactions of these three factors determine the type of thrombus formed. There are basic differences between arterial and venous thrombosis such as the composition of the thrombi (platelet rich in arterial and fibrin rich in venous), and the presence of vascular wall damage (atheroma) in arterial thrombosis. However, these distinctions are not absolute and they share common underlying mechanisms. Disturbance of hemostasis is central to the pathogenesis of all thrombosis, even though it differs in nature depending on anatomical location. As hemostasis and thrombosis are similar (if not entirely identical) processes, the concept of thrombosis as “hemostasis in the wrong place” [1] provides some basis for considering how platelet function and coagulation systems might contribute to the pathogenesis of thrombosis. Arterial thrombosis There is little doubt that the development of arterial thrombosis is the product of multiple genetic and environmental risk factors in both atherosclerosis and thrombosis. Acute thrombosis occurring in a ruptured, usually lipid rich, atherosclerotic plaque is the event that precipitates the development of acute myocardial infarction (MI), cerebral thrombosis, and acute peripheral arterial occlusion. Plaque rupture is followed by platelet aggregation to exposed subendothelial von Willebrand factor (vWF) through glycoprotein complexes. A cascade of intracellular signaling events with subsequent platelet activation follows platelet adhesion to the subendothelial binding of vWF and fibrinogen to a conformationally active form of glycoprotein IIb/IIIa. Platelet aggregation results in further propagation of the expanding platelet thrombus.

Epidemiology of coagulopathy

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The traditional view of arterial thrombus held that disorders such as myocardial infarction and ischemic stroke were largely attributed to the modifiable risk factors such as hypertension, cigarette smoking, hyperlipidemia, and obesity. However, it has been known for some time that at least 30–40% of the variance in risk of MI is attributed to factors other than these conventional risk factors [2, 3]. Furthermore, a strong body of evidence exists to support the efficacy of antithrombotic therapies in both the primary and secondary prevention of atherothrombotic events. There is also evidence from younger subjects with MI suggesting that their atherosclerotic burden is less when compared with elderly MI subjects. Consequently, the dominant pathogenic mechanisms for MI in younger patients may be more likely to involve perturbations of the hemostatic balance. What is the evidence for the hemostatic system in arterial thrombosis? Pathological studies indicate that acute thrombosis in the coronary and cerebral vasculature involves activation of both platelets and coagulation systems. Over the past two decades, there have been a considerable number of studies that support a role for the hemostatic system in the development of MI. These include both case-control and prospective studies [4–8]. However, while case-control and crosssectional studies assess the relationship between a given circulating marker and the development of arterial thrombosis, it is not possible to exclude the possibility that any observed elevation (or indeed reduction) of a given circulating factor is not the result of an acute phase reaction associated with the thrombotic event itself [9]. Although this problem is overcome in prospective studies, any association may be confounded by underlying subclinical atherosclerosis, which in turn influences the measured levels of hemostatic factor. In the case of vascular disease, any association is further complicated by interactions between the other hemostatic factors as well as the “traditional” cardiovascular risk factors such as smoking. Arguably, the firmest evidence derives from intervention studies, in which pharmacological therapies lower the level of the circulating factor. In the case of hemostatic factors, there are very few effective agents that reduce levels of hemostatic factors. For example, a number of drugs are known to lower fibrinogen levels [10, 11], but toxicity has tended to limit their effectiveness. Of the numerous individual constituents of the coagulation cascade, a small number have provided most use in our understanding of the epidemiology of thromboembolic disease (Table 2.1). Fibrinogen There is a strongly, generally consistent relationship of fibrinogen levels with the risk of MI and that of ischemic stroke. This has been demonstrated in both case-control and large well-characterized prospective studies [12–14], including the Framingham Study [15]. In the Framingham Study, there was a clear relationship between fibrinogen and incidence of ischemic heart disease (IHD) in both men and women, and stroke in men but not women (possibly an artifact arising from small numbers). They also reported an association between high fibrinogen and hypertension. In the Leigh study of IHD,


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incidence in males aged 40–69, there was also suggestive evidence of an interaction between systolic blood pressure and plasma fibrinogen in those in the

Table 2.1 Hemostatic and thrombotic Coagulation factors, platelet factors, and activation of coagulation Fibrinogen Factor VII Factor VIII Factor XIII von Willebrand factor Prothrombin fragment 1+2 Thrombin-antithrombin III Fibrin degradation products D-dimer Factor V Fibrinolytic factors

Tissue plasminogen activator Plasminogen activator inhibitor-1 Clot lysis time Inflammatory factors C-reactive protein Serum amyloid A Interleukins Cellular adhesion molecules Other factors Homocysteine

highest tertile of levels [16]. Furthermore, there is evidence that cholesterol and fibrinogen are also additive in coronary risk, as shown in PROCAM [17] and ECAT [18] where coronary rates were highest in individuals with elevation in both factors than elevations of any one factor alone. A number of mechanisms have been proposed to account for the effect of fibrinogen on vascular risk including an increase in fibrin formation, raised plasma viscosity, promoting platelet aggregation, and vascular and smooth muscle cell proliferation [19]. When studying fibrinogen, consideration should be taken as to the possible confounders as fibrinogen levels increase with age, menopause, hypertension, and obesity as well as positively correlating with total and LDL cholesterol [20], and inversely correlates with


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exercise and hormone replacement therapy [21]. However, the most important environmentat determinant of fibrinogen levels is cigarette smoking [22], probably mediated through interleukin-6. Factor VII Factor VII (FVII) is an inactive vitamin K-dependent zymogen synthesized in the liver and secreted as a single-chain glycoprotein of 48 kD. On contact with tissue factor, it is converted to the two-chain active form by selective proteolysis by factor Xa, and the factor VIIa/ tissue factor complex then activates factor X and factor IX. Levels of FVII are influenced by multiple environmental factors including advancing age, gender, dietary lipids, body mass index, and triglyceride [23–25]. Much of the epidemiologic interest in FVII stems from the findings of the Northwick Park Heart Study (NPHS), a whole population study (as distinct from case-control study) in which cases, and those so far unaffected by clinically manifest IHD, were drawn from the same industrial population. The primary objective of NPHS was to assess hemostatic function in the pathogenesis of IHD [4]. The two variables showing clear prospective associations with subsequent IHD were factor VII coagulant activity (F:VIIC) and fibrinogen. NPHS has been one of the few studies to demonstrate prospective findings on FVII, although case-control studies have also found associations with FVII and IHD or ischemic stroke risk [26–29], including work from our Unit [30]. Some of the observed differences may be attributed to the assay and the NPHS F:VIIc assay is more sensitive to two-chain VIIa. The clotting assay, factor VIIC, reflects the total FVII antigen, but it can also be influenced by the amount of FVII that is activated (factor VIIa) in the plasma. The latter can be measured directly with an assay that utilizes truncated tissue factor as stimulant [31]. It has been suggested that the factor VIIC assay used in the NPHS is particularly sensitive to factor VIIa. There is, however, no good evidence that factor VIIa is an informative marker with respect to the prediction of cardiovascular disease [32]. Subsequent prospective studies have failed to confirm an association between FVII and risk of CHD [33–35]. FVII is not related to risk of venous thrombosis [36]. Factor VIII and von Willebrand factor Congenital deficiency or dysfunction of vWF, which is a multimeric glycoprotein circulating in blood as a noncovalent complex with procoagulant FVIII [37], results in moderate to severe bleeding and congenital FVIII deficiency is associated with a decreased mortality from IHD [38]. However, it has been recognized that vWF is involved in the pathogenesis of IHD. In prospective studies of middle aged and older adults [39], elevated FVIII activity, vWF activity, and vWF antigen have been implicated in arterial thrombosis [34, 35, 40] as well as in large case-control studies of ischemic stroke [41], in which raised vWF was shown to be an independent determinant of poststroke mortality. In a number of prospective studies, with atherosclerotic disease, vWF levels have been independently associated with risk of thrombotic complications [6, 18, 42]. The precise role of FVIII in this process is not clear. It seems likely that elevated FVIII/vWF


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circulates as a complex and most studies show these factors to be highly correlated. FVIII levels are influenced by ABO blood group, as subjects with non-O exhibit, a higher FVIII concentration, and increased risk of IHD [43]. Factor XIII Until recently, there was relatively little information relating changes in factor XIII (FXIII) level or activity to human arterial disease. Plasma FXIII is a tetrameric molecule composed of two A-subunits of 83.2 kD and two B-subunits of 79.7 kD that are held together noncovalently in a heterologous tetramer of 325.8 kD [44]. Two animal studies have provided evidence for a role for circulating FXIII in the pathogenesis of both MI and thromboembolic disease [45, 46]. In a canine electrically induced MI model treated with an FXIII inhibitor, fibrinolytic therapy led to increased clot lysis compared to animals not pretreated with the inhibitor. In a ferret model of pulmonary embolus, FXIII cross-linking is associated with increased resistance to exogenous tPA therapy. Both studies conclude that activated FXIII has an important role in fibrinolysis-resistant clot formation and may affect outcomes in these two pathologically distinct syndromes that have fibrin formation as a common feature. Two of the earlier published case-control studies report associations between FXIII Val34Leu and a history of MI. In a study of subjects with angiographically proven coronary artery disease and a history of MI, FXIII Leu34 was significantly less common in those with a history of MI [47]. In those subjects with Leu34 who had a previous MI, there were higher levels of the fibrinolytic inhibitor PAI-1, the PAI-1 4G/4G genotype was more common, and there was evidence of interactions with other clotting factors and with features of the insulin-resistance syndrome [48, 49]. A mixed postmortem and casecontrol study of survivors of MI in Finnish subjects again suggested a protective effect of Leu34, but it was unable to reproduce the PAI-1 4G/4G genotype associations [50]. Interestingly, this study found the prevalence of the protective allele to be lowest in the Eastern Kainuu region, which has the highest prevalence of MI, similar to that reported in the high-risk South Asian community in England [51]. Fibrin is the main protein constituent of the blood clot, which is stabilized by factor XIIIa through an amide or isopeptide bond that ligates adjacent fibrin monomers. The relationship between FXIII and the major constituent of a thrombus, fibrin, has been reviewed elsewhere [52]. Proteins C, S, and antithrombin III There is good evidence that congenital deficiencies of proteins C, S, and antithrombin III (ATIII) are associated with an increased risk of venous thrombosis. The evidence linking these deficiency states with arterial thrombosis is less well defined [53]. In an analysis of 311 largely younger patients with clinically evident ischemic stroke, deficiencies of proteins S, C, and ATIII were present in 11.9%, 2.2%, and 2.2% of cases, respectively [54]. The small numbers of subjects and heterogeneous nature of the clinical phenotypes, as well as control matching makes interpretation of these studies difficult. For example, in a recent study of young subjects with ischemic stroke, deficiencies of the naturally occurring anticoagulants is not uncommon, although congenital deficiencies are


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uncommon [55–57]. There is some evidence to suggest that protein C and S deficiency may be present in 5–15% of cases with ischemic stroke [58–60]. However, in clinical practice, congenital deficiencies of proteins C, S, and ATIII are not clinically important, as the majority of patients with ischemic insults are elderly, and prothrombotic deficiencies are unlikely to be playing a major role in disease pathogenesis. Activated protein C resistance and factor V Leiden Activated protein C (APC) is a naturally occurring anticoagulant. Resistance to the anticoagulant effect of APC (APCR) is most common due to a single nucleotide polymorphism (G1619A) in the gene-encoding factor V—factor V Leiden (FVL). The procoagulant effect results from a reduced rate of factor Va inactivation. The available evidence indicates that FVL is the most common genetic risk factor associated with venous thrombosis. In relation to arterial thrombosis, the evidence that FVL plays any role is more limited. In relation to MI there is evidence for [61, 62] and against [63–65], including our own findings in ischemic stroke [66]. There is some data to suggest that approximately 20% of cases of pediatric ischemic stroke are associated with FVL. A number of studies have linked APCR with arterial thrombosis in the absence of FVL. A study of 826 adults with evidence of carotid and femoral atherosclerosis exhibited decreased response to APC, which seemed independent of FVL, supporting a role for other environmental and genetic factors [67]. Fibrinolysis in arterial thrombosis The fibrinolytic system comprises a circulating pro-enzyme plasminogen, which is converted by plasmin to the plasminogen activators tissue-plasminogen activator (tPA) and urokinase. Plasminogen activator inhibitor-1 (PAI-1) is the principal inhibitor of fibrinolysis and is the main counterbalance to natural fibrinolysis. PAI-1 is synthesized by platelets, endothelium, and vascular smooth muscle cells (SMC). There is data from several prospective studies to suggest that endogenous fibrinolytic activity at baseline was found to predict future cardiovascular morbidity and mortality. This has been shown for healthy men at risk from stroke [68] and MI [69] and also young patients suffering MI. One of the key problems associated with the interpretation of fibrinolysis studies is based in part upon whether the detected alteration in fibrinolytic function is considered to be a primary process or the result of pre (or sub) clinical atherosclerosis and a resultant dysfunctional endothelium. Other complicating factors include measurement difficulties for tPA and PAI-1 as there is significant circadian variation for the fibrinolytic parameters. There is also spontaneous transformation of PAI-1 to an inactive or latent form as well as release of PAI-1 from platelets necessitating careful and specific blood collection techniques [70]. Furthermore, as plasma tPA and PAI-1 are strongly correlated, it is difficult, if not impossible to separate the effects of these factors in an epidemiological study and depending on the variables controlled for in a regression model, the relative importance of tPA and PAI-1 tends to vary [71]. In addition, the tPA:Ag assay measures both free tPA and tPA complexed to PAI-1, which increases with high PAI-1 levels due to delayed clearance [72]. Furthermore, increased PAI-1 levels


24

correlate with obesity, hyperinsulinemia, and raised triglycerides, which further complicate interpretation of circulating levels. The available evidence suggests that for risk prediction, the more robust marker is tPA:Ag as opposed to PAI-1:Ag or PAI-1:Ac. In the prospective Physicians Health Study (healthy men) [69] and ECAT study, both show statistically large increases in risk of coronary thrombosis for subjects with high baseline tPA:Ag and in ECAT, the relative risk associated with a 1 SD change was larger for tPA:Ag than either PAI-1:Ag or PAI1:Ac. It seems likely that the balance between net activation and net inhibition of fibrinolysis is the most important factor in fibrinolytic potential as shown in measures of global clot lysis time in NPHS [73]. Venous thrombosis Venous thromboembolic disorders are a range of conditions characterized by in situ thrombus formation and the variable presence of embolic manifestations. The clinical presentation depends to a large extent on the site and extent of thrombus formation and on the underlying cause. The spectrum encompasses thrombosis arising in the deep veins of legs (DVT) with embolism to the pulmonary vasculature (pulmonary embolism—PE). However, venous thrombosis is recognized to occur in other sites including the upper limbs, cerebral venous system, and the retinal veins. PE and DVT account for more than 250,000 hospitalizations in the US and constitute the third commonest cause of death after acute ischemic coronary syndromes and stroke. Approximately two-thirds of cases are first time episodes and the remainder arises in patients with recurrent venous thromboembolism (VTE) [74]. In the absence of genetic deficiencies, thrombosis occurs in the older population, largely in the context of environmental factors such as surgery, obesity, and underlying malignant disease. There is increasing evidence of the importance of hemostatic mechanisms in the pathogenesis of venous thrombosis. The function and regulation of coagulation and fibrinolytic systems and factors have been extensively studied. Precisely how they functionally integrate, in vivo, is still unknown. The role of inhibitory mechanisms in preventing venous thrombosis was established after the identification of the link between inherited deficiencies of antithrombin, protein C, protein S, and venous thrombosis. The general importance of the anticoagulant pathway involving protein C and protein S was established by the widespread prevalence among white populations and the clinical consequences of the phenotype of APCR and its main causative gene mutation, factor V G1691A (factor V Arg506Gln, factor V Leiden). There has been slower appreciation of the procoagulant effect of increased levels of coagulation factors in venous disease, but recent work has highlighted the likely importance of prothrombin and FVIII levels in this regard. (See Table 2.2 for the hemostatic and thrombotic markers for venous thrombosis.) Proteins C, S, and ATIII in venous thrombosis Despite their importance, proteins C, S, and ATIII deficiency together account for less than


25

Table 2.2 Hemostatic and thrombotic markers for venous thrombosis. Factor V Prothrombin Endothelial cell protein C/activated protein C receptor Protein C Protein S Antithrombin III Thrombomodulin Factor VIII and vWF Homocystinemia Factor VII Fibrinolytic factors?—uncertain

10% of all cases of recurrent venous thrombosis. In unselected patients with (VTE), ATIII deficiency occurs in 1.1% of subjects compared with 2.4% of selected patients [75]. Patients with ATIII deficiency appear to be at greater risk for VTE than those with protein C or S deficiency, and in one report 85% of patients with ATIII deficiency suffered a thromboembolic event by the age of 50 [76]. The prevalence of heterozygous protein C and S deficiency among the general population is low and around 5–10% of selected patients with VTE and around 50% of heterozygotes up to 50 years suffer a thrombosis [77]. APC resistance in venous thrombosis Between 20% and 60% of patients with recurrent VTE display APCR [78] and for the majority of subjects this is the result of FVL. Unlike arterial thrombosis, FVL and APCR are important risk factors for VTE. Approximately 3–5% of the general population are FVL heterozygotes (FVL homozygotes are rare) [79]. While APCR is not as strong a risk factor for VTE as the thrombophilias referred to earlier, it is associated with a 3- to 7-fold increase in risk of VTE [80], and FVL also enhances the prothrombotic tendency in the presence of other VTE risk factors, for example, approximately 60% of women experiencing a VTE during oral contraceptive use are APCR [81]. There is also a thrombotic interaction between FVL and the other heritable thrombophilias [82]. D-dimer and venous thrombosis D-dimer is a cross-linked fibrin degradation product that circulates in plasma after lysis of fibrin by plasmin. In DVT and PE patients, endogenous fibrinolysis occurs resulting in increased circulating cross-linked fibrin degradation products. D-dimer has been used as a diagnostic test for VTE, although the results need to be carefully interpreted. The utility


26

of the D-dimers in the diagnosis of venous thrombosis has been extensively reviewed elsewhere [83, 84]. Other coagulation factors and venous thrombosis Elevated FVIII, FIX, and FXI are all linked with an increased thrombotic risk. In the Leiden Thrombophilia Study (LETS), 25% of subjects with first ever VTE had FVIII levels greater than 150% of normal compared with only 11% of healthy controls [36] and plasma levels of greater than 150 IU/dl were associated with a five times risk for an initial event as also shown in other studies [85]. Subjects who had previously had a VTE and showed higher FVIII levels were at a particularly elevated risk of VTE recurrence [86]. Interestingly, studies have also shown that plasma levels of FIX and FXI also increase risk of VTE. In LETS, subjects with FIX and FXI levels above the 90th percentile had a 2.5- and 2.2-fold risk, respectively, of VTE, compared with those beneath the 90th percentile [87, 88]. In 100 healthy individuals and 129 patients with VTE, FVIII levels correlated only with vWF but not with risk of thrombosis, although levels of tPA and PAI-1 were increased, which is interesting as these are usually associated with arterial thrombosis [89], and the lack of association of venous thrombosis with fibrinolysis was confirmed elsewhere [90]. By contrast, in the US Longitudinal Investigation of Thromboembolism Etiology (LITE) study of 159 patients with VTE, FVIII and vWF were linearly associated with increased risk of VTE, but there was no association of VTE with fibrinogen or C-reactive protein levels, or white cell count [91]. In another study, fibrinogen levels were more strongly related with risk of venous thrombosis and in one study the risk of venous thrombosis with high fibrinogen levels was increased 4-fold [92]. FXIII that has been implicated in arterial thrombosis has also been associated with reduced levels of FXIII occurring in patients with venous thrombosis [93]. Homocysteine and venous thrombosis The evidence directly linking raised homocysteine (Hcy) with risk of VTE is unknown, although there is some evidence that mild to moderate elevations in Hcy are independent risk factors for VTE. In LETS, 10% had Hcy levels above the 95th percentile with an odds ratio (OR) for VTE of 2.5 [94]. In a meta-analysis of ten case-control studies, similar findings were observed [95]. It is probable that raised Hcy is a risk factor for recurrent VTE as well. Other hemostatic factors There is a panoply of other hemostatic factors that are beyond the scope of this chapter. These include lipoprotein A [96], thrombomodulin [97] as well the platelet receptor glycoproteins IIb/IIIa [98, 99], platelet Ia/Iia [100] and Ib/IX [101]. Other inflammatory markers associated with thrombosis include C-reactive protein, serum amyloid A, cellular adhesion molecules, and the interleukins [102].


27

Conclusions Arterial disease occurs in a high-pressure, high-flow system with atheromatous disease as

Table 2.3 Summary of the outcome of investigations of hemostatic polymorphisms. Polymorphism

Phenotype

Association of genotype with disease

Venous thromboembolic disease Factor V G1 691A, Arg506Gln

APCR

Clear risk factor alone or in combination with other factors, genetic or acquired

Prothrombin G20210A

Altered levels, mechanism uncertain Moderate risk factor alone or in combination with other risk factors

Factor V HR2 haplotype

Slight APCR; glycosylation isoforms altered?

Weak risk factor alone or in combination with factor V G1691A?

Factor XIII Val34Leu

Increased activation rate

Preliminary results suggest 34Leu is protective against thrombosis

EPCR 23-bp repeat

Predicted altered expression on cell surface

Preliminary results suggest it a weak/moderate risk factor

Arterial occlusive disease Fibrinogen β chain— Associated with altered level 455G/A

Inconsistent

Fibrinogen β chain Bcl I,

Associated with altered level

Inconsistent

Fibrinogen a Thr313 Ala

Altered clot struture?

Inconsistent

Factor VII Arg353Gln

Altered level

Inconsistent

Factor VII HVR4

Probably altered level

Unknown

Factor VII—402G/A

Altered level

Unknown

Factor VII—401G/T

Altered level

Unknown

Factor VII—323 0/10 Altered level

Unknown

Factor XII Val34Leu Increased activation rate

Preliminary results suggest 34Leu is protective against MI

Factor V G1691A, Arg506Gln

APCR

Possibly in selected patients and in combination with acquired risk factors

Prothrombin

Altered level

Possibly in selected patients, and in


G20210A EPCR 23-bp repeat

28

combination with acquired risk factors Predicted altered expression on cell surface

Preliminary results suggest it may be a weak risk factor

tPA Alu insertion/del Unlikely

Inconsistent

PAI-1–675 4G/5G

Inconsistent

Inconsistent

PAI-1 (CA)n

Suggestive of altered level

Unknown

PAI-1 Hind III

Suggestive of altered level

Unknown

Gp IIIa Pro33Leu

No effect on fibrinogen binding, but Inconsistent increased sensitivity to activation?

Gp IαVNTR

Unknown

Preliminary results show association

Gp Ib; Iα C3550T, Thr145Met

Unknown

Inconsistent

Gp Ia/IIa, α2 C807T

Associated with altered surface expression of receptor, altered collagen binding

Inconsistent

Figure 2.1 This figure shows the interaction between genes, environment, and the intermediate phenotype. In the illustration, the intermediate phenotype is circulating fibrinogen and the final phenotype is thrombotic stroke. a dominant feature. Hemostatic factors have been extensively studied as possible risk determinants for disease. The most consistent associations have been found for fibrinogen. Although it is formally possible that fibrinogen is merely a marker of another underlying process, such as the acute-phase reaction [103], there is reasonably strong evidence that fibrinogen is causally involved in thrombotic disease. For each of the other


29

hemostatic factors, though a causal role is plausible, the evidence for this is less strong. It is interesting to note that hemostatic risk factors for atherothrombotic disorders are largely those that have been associated to some extent with classical risk markers such as features of insulin resistance [104], some of which is likely to be heritable, as shown in recent twin studies [105], These observations indicate the importance of environmental influences and emphasize the complexity of the processes involved in vascular disease. When individual hemostatic factors do contribute to risk, it is highly likely to be a complex interaction with other metabolic factors. This understanding further increases the importance of lifelong risk interactions and may suggest an explanation for some of the inconsistencies in case-control studies of arterial disease. Some of the inconsistencies in hemostatic studies are likely to arise from deficiencies of study size and power, plurality of clinical endpoints (MI, unstable angina, coronary artery disease, progression of arterial disease, stroke), and these have been often selected in a postdata collection search for significance. Other difficulties include recruitment bias in the selection of patients, controls, or both. Future challenges are likely to arise from the knowledge base arising from the human genome project. The study of interindividual genetic variation and the potential for study of functional variants of candidate genes is likely to alter our understanding of the importance of these thrombotic risk factors. The genetic basis of hemostatic risk factors has been thoroughly reviewed elsewhere [106]. A summary of the currently known polymorphisms and the association with vascular disease is shown in Table 2.3. Although the goal must be risk prediction, it is most likely that genomics will provide insights into thrombotic mechanisms. It will be necessary for clinicians to integrate hemostatic factors into our evolving knowledge of the genetics, biochemistry, and epidemiology of vascular disease. References [1] Macfarlane RG. Blood coagulation and thrombosis: introduction. Br Med Bull 1955; 11:1–4. [2] Sacco RL, Ellenberg JH, Mohr JP, Tatemichi TK, Hier DB, Price TR. Infarcts of undetermined cause: the NINCDS stroke data bank. Ann Neurol 1989; 25:382–90. [3] Grover SA, Coupal L, Hu XP. Identifying adults at increased risk of coronary disease. How well do the current cholesterol guidelines work? JAMA 1995; 274:801–6. [4] Meade TW, Mellows S, Brozovic M, Miller GJ, Chakrabarti RR, North WR, et al. Haemostatic function and ischaemic heart disease: principal results of the Northwick Park Heart Study. Lancet 1986; 2:533–7. [5] Wilhelmsen L, Svärdsudd K, Korsan-Bengtsen K, Larsson B, Welin L, Tibblin G. Fibrinogen as a risk factor for stroke and myocardial infarction. N Engl J Med 1984; 311:501–5. [6] Jansson JH, Nilsson TK, Johnson O. von Willebrand factor in plasma: a novel risk factor for recurrent myocardial infarction and death. Br Heart J 1991; 66:351–5. [7] Hamsten A, Walldius G, De Faire U, Dahlen G, Szamosi A, Landou C, et al. Plasminogen activator inhibitor in plasma: risk factor for recurrent myocardial infarction. Lancet 1987; 2:3–9. [8] Ridker PM, Hennekens CH, Stampfer MJ, Manson JE, Vaughan DE. Prospective study of endogenous tissue plasminogen activator and risk of stroke. Lancet 1994; 343:940–3. [9] Carter AM, Catto AJ, Bamford JM, Grant PJ. Gender-specific associations of the fibrinogen B β 448 polymorphism, fibrinogen levels, and acute cerebrovascular disease. Arterioscler Thromb Vasc Biol 1997; 17:589–94.


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[50] Wartiovaara U, Perola M, Mikkola H, Totterman K, Savolainen V, Penttila A, et al. Association of FXIII Val34Leu with decreased risk of myocardial infarction in Finnish males. Atherosclerosis 1999; 142:295–300. [51] McCormack LJ, Kain K, Catto AJ, Kohler HP, Stickland MH, Grant PJ. Prevalence of FXIII V34L in populations with different cardiovascular risk. Thromb Haemost 1998; 80:523–4. [52] Ariens RA, Lai TS, Weisel JW, Greenberg CS, Grant PJ. Role of factor XIII in fibrin clot formation and effects of genetic polymorphisms. Blood 2002; 100:743–54. [53] De SV, Leone G, Mastrangelo S, Tripodi A, Rodeghiero F, Castaman G, et al. Clinical manifestations and management of inherited thrombophilia: retrospective analysis and followup after diagnosis of 238 patients with congenital deficiency of antithrombin III, protein C, protein S. Thromb Haemost 1994; 72:352–8. [54] Barinagarrementeria F, Cantu-Brito C, De La PA, Izaguirre R. Prothrombotic states in young people with idiopathic stroke. A prospective study. Stroke 1994; 25:287–90. [55] Douay X, Lucas C, Caron C, Goudemand J, Leys D. Antithrombin, protein C and protein S levels in 127 consecutive young adults with ischemic stroke. Acta Neurol Scand 1998; 98:124– 7. [56] Munts AG, van Genderen PJ, Dippel DW, van Kooten F, Koudstaal PJ. Coagulation disorders in young adults with acute cerebral ischaemia. J Neurol 1998; 245:21–5. [57] Margaglione M, D’Andrea G, Giuliani N, Brancaccio V, De Lucia D, Grandone E, et al. Inherited prothrombotic conditions and premature ischemic stroke: sex difference in the association with factor V Leiden. Arterioscler Thromb Vasc Biol 1999; 19:1751–6. [58] Bonduel M, Sciuccati G, Hepner M, Torres AF, Pieroni G, Frontroth JP. Prethrombotic disorders in children with arterial ischemic stroke and sinovenous thrombosis. Arch Neurol 1999; 56:967–71. [59] Nowak-Gottl U, Strater R, Heinecke A, Junker R, Koch HG, Schuierer G, et al. Lipoprotein (a) and genetic polymorphisms of clotting factor V, prothrombin, and methylenetetrahydrofolate reductase are risk factors of spontaneous ischemic stroke in childhood. Blood 1999; 94:3678– 82. [60] Kenet G, Sadetzki S, Murad H, Martinowitz U, Rosenberg N, Gitel S, et al. Factor V Leiden and antiphospholipid antibodies are significant risk factors for ischemic stroke in children. Stroke 2000; 31:1283–8. [61] Rosendaal FR, Siscovick DS, Schwartz SM, Beverly RK, Psaty BM, Longstreth WT Jr, et al. Factor V Leiden (resistance to activated protein C) increases the risk of myocardial infarction in young women. Blood 1997; 89:2817–21. [62] Doggen CJ, Cats VM, Bertina RM, Rosendaal FR. Interaction of coagulation defects and cardiovascular risk factors: increased risk of myocardial infarction associated with factor V Leiden or prothrombin 20210A. Circulation 1998; 97:1037–41. [63] Emmerich J, Poirier O, Evans A, et al. Myocardial infarction, Arg 506 to Gln factor v mutation, and activated protein C resistance. Lancet 1995; 345:321. [64] Kontula K, Ylikorkala A, Miettinen H, Vuorio A, Kauppinen-Mäkelin R, Hämäläinen L, et al. Arg506Gln factor V mutation (Factor V Leiden) in patients with ischaemic cerebrovascular disease and survivors of myocardial infarction. Thromb Haemost 1995; 73:558–60. [65] Cushman M, Rosendaal FR, Psaty BM, Cook EF, Valliere J, Kuller LH, et al. Factor V Leiden is not a risk factor for arterial vascular disease in the elderly: results from the Cardiovascular Health Study. Thromb Haemost 1998; 79:912–15. [66] Catto AJ, Carter AM, Ireland H, Bayston TA, Philippou H, Barrett J, et al. Factor V Leiden gene mutation and thrombin generation in relation to the development of acute stroke. Arterioscler Thromb Vasc Biol 1995; 15:783–5. [67] Kiechl S, Muigg A, Santer P, Mitterer M, Egger G, Oberhollenzer M, et al. Poor response to activated protein C as a prominent risk predictor of advanced atherosclerosis and arterial disease. Circulation 1999;99:614–19.


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[68] Ridker PM, Hennekens CH, Stampfer MJ, Manson JE, Vaughan DE. Prospective study of endogenous tissue plasminogen activator and risk of stroke. Lancet 1994; 343:940–3. [69] Ridker PM, Vaughan DE, Stampfer MJ, Manson JE, Hennekens CH. Endogenous tissue-type plasminogen activator and risk of myocardial infarction. Lancet 1993; 341:1165–8. [70] Chandler WL, Stratton JR. Laboratory evaluation of fibrinolysis in patients with a history of myocardial infarction. Am J Clin Pathol 1994; 102:248–52. [71] Juhan-Vague I, Pyke SD, Alessi MC, Jespersen J, Haverkate F, Thompson SG. Fibrinolytic factors and the risk of myocardial infarction or sudden death in patients with angina pectoris. ECAT Study Group. European Concerted Action on Thrombosis and Disabilities. Circulation 1996; 94:2057–63. [72] Chandler WL, Alessi MC, Aillaud MF, Henderson P, Vague P, Juhan-Vague I. Clearance of tissue plasminogen activator (TPA) and TPA/plasminogen activator inhibitor type 1 (PAI-1) complex: relationship to elevated TPA antigen in patients with high PAI-1 activity levels. Circulation 1997; 96:761–8. [73] Meade TW, Ruddock V, Stirling Y, Chakrabarti R, Miller GJ. Fibrinolytic activity, clotting factors, and long-term incidence of ischaemic heart disease in the Northwick Park Heart Study. Lancet 1993; 342:1076–9. [74] Anderson FA Jr, Wheeler HB, Goldberg RJ, Hosmer DW, Patwardhan NA, Jovanovic B, et al. A population-based perspective of the hospital incidence and case-fatality rates of deep vein thrombosis and pulmonary embolism. The Worcester DVT Study. Arch Intern Med 1991; 151:933–8. [75] Heijboer H, Brandjes DP, Buller HR, Sturk A, ten Cate JW. Deficiencies of coagulationinhibiting and fibrinolytic proteins in outpatients with deep-vein thrombosis. N Engl J Med 1990; 323:1512–16. [76] Thaler E, Lechner K. Antithrombin III deficiency and thromboembolism. Clin Haematol 1981;10: 369–90. [77] Allaart CF, Poort SR, Rosendaal FR, Reitsma PH, Bertina RM, Briet E. Increased risk of venous thrombosis in carriers of hereditary protein C deficiency defect. Lancet 1993; 341:134– 8. [78] Dahlback B, Carlsson M, Svensson PJ. Familial thrombophilia due to a previously unrecognized mechanism characterized by poor anticoagulant response to activated protein C: prediction of a cofactor to activated protein C. Proc Natl Acad Sci USA 1993; 90:1004–8. [79] Koster T, Rosendaal FR, de Ronde H, Briët E, Vandenbroucke JP, Bertina RM. Venous thrombosis due to poor anticoagulant response to activated protein C: Leiden Thrombophilia Study. Lancet 1993; 342:1503–6. [80] Svensson PJ, Dahlback B. Resistance to activated protein C as a basis for venous thrombosis. N Engl J Med 1994; 330:517–22. [81] Vandenbroucke JP, Koster T, Briët E, Reitsma PH, Bertina RM, Rosendaal FR. Increased risk of venous thrombosis in oral-contraceptive users who are carriers of factor V Leiden mutation. Lancet 1994; 344:1453–7. [82] De SV, Martinelli I, Mannucci PM, Paciaroni K, Chiusolo P, Casorelli I, et al. The risk of recurrent deep venous thrombosis among heterozygous carriers of both factor V Leiden and the G20210A prothrombin mutation. N Engl J Med 1999; 341:801–6. [83] Hirsh J, Lee AY. How we diagnose and treat deep vein thrombosis. Blood 2002; 99:3102–10. [84] Lee AY, Hirsh J. Diagnosis and treatment of venous thromboembolism. Annu Rev Med 2002; 53:15–33. [85] Kraaijenhagen RA, in’t Anker PS, Koopman MM, Reitsma PH, Prins MH, van den EA, et al. High plasma concentration of factor VIIIc is a major risk factor for venous thromboembolism. Thromb Haemost 2000; 83:5–9. [86] Kyrle PA, Minar E, Hirschl M, Bialonczyk C, Stain M, Schneider B, et al. High plasma levels of factor VIII and the risk of recurrent venous thromboembolism. N Engl J Med 2000; 343:457– 62.


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[87] Van HV, Van der LI, Bertina RM, Rosendaal FR. High levels of factor IX increase the risk of venous thrombosis. Blood 2000; 95:3678–82. [88] Meijers JC, Tekelenburg WL, Bouma BN, Bertina RM, Rosendaal FR. High levels of coagulation factor XI as a risk factor for venous thrombosis. N Engl J Med 2000; 342:696–701. [89] Bombeli T, Jutzi M, De Conno E, Seifert B, Fehr J. In patients with deep-vein thrombosis elevated levels of factor VIII correlate only with von Willebrand factor but not other endothelial cellderived coagulation and fibrinolysis proteins. Blood Coagul Fibrinolysis 2002; 13:577–81. [90] Folsom AR, Cushman M, Heckbert SR, Rosamond WD, Aleksic N. Prospective study of fibrinolytic markers and venous thromboembolism. J Clin Epidemiol 2003; 56:598–603. [91] Tsai AW, Cushman M, Rosamond WD, Heckbert SR, Tracy RP, Aleksic N, et al. Coagulation factors, inflammation markers, and venous thromboembolism: the longitudinal investigation of thromboembolism etiology (LITE). Am J Med 2002; 113:636–42. [92] Kamphuisen PW, Eikenboom JC, Vos HL, Pablo R, Sturk A, Bertina RM, et al. Increased levels of factor VIII and fibrinogen in patients with venous thrombosis are not caused by acute phase reactions. Thromb Haemost 1999; 81:680–3. [93] Swiatkiewicz A, Jurkowski P, Kotschy M, Ciecierski M, Jawien A. Level of antithrombin III, protein C, protein S and other selected parameters of coagulation and fibrinolysis in the blood of the patients with recurrent deep venous thrombosis. Med Sci Monit 2002; 8:CR263–CR268. [94] den Heijer M, Koster T, Blom HJ, Bos GM, Briet E, Reitsma PH, et al. Hyperhomocysteinemia as a risk factor for deep-vein thrombosis. N Engl J Med 1996; 334:759– 62. [95] den Heijer M, Rosendaal FR, Blom HJ, Gerrits WB, Bos GM. Hyperhomocysteinemia and venous thrombosis: a meta-analysis. Thromb Haemost 1998; 80:874–7. [96] Stein JH, Rosenson RS. Lipoprotein Lp(a) excess and coronary heart disease. Arch Intern Med 1997; 157:1170–6. [97] Salomaa V, Matei C, Aleksic N, Sansores-Garcia L, Folsom AR, Juneja H, et al. Soluble thrombomodulin as a predictor of incident coronary heart disease and symptomless carotid artery atherosclerosis in the Atherosclerosis Risk in Communities (ARIC) Study: a case-cohort study. Lancet 1999; 353:1729–34. [98] Weiss EJ, Bray PF, Tayback M, Schulman SP, Kickler TS, Becker LC, et al. A polymorphism of a platelet glycoprotein receptor as an inherited risk factor for coronary thrombosis. N Engl J Med 1996; 334:1090–4. [99] Carter AM, Ossei-Gerning N, Grant PJ. Platelet glycoprotein IIIa PLA polymorphism in young men with myocardial infarction. Lancet 1996; 348:485–6. [100] Moshfegh K, Wuillemin WA, Redondo M, Lammle B, Beer JH, Liechti-Gallati S, et al. Association of two silent polymorphisms of platelet glycoprotein Ia/IIa receptor with risk of myocardial infarction: a case-control study. Lancet 1999; 353:351–4. [101] Gonzalez-Conejero R, Lozano ML, Rivera J, Corral J, Iniesta JA, Moraleda JM, et al. Polymorphisms of platelet membrane glycoprotein Ib associated with arterial thrombotic disease. Blood 1998; 92:2771–6. [102] Ridker PM. Fibrinolytic and inflammatory markers for arterial occlusion: the evolving epidemiology of thrombosis and hemostasis. Thromb Haemost 1997; 78:53–9. [103] Tracy RP. Epidemiological evidence for inflammation in cardiovascular disease. Thromb Haemost 1999; 82:826–31. [104] Mansfield MW, Grant PJ. Fibrinolysis and diabetic retinopathy in NIDDM. Diabetes Care 1995; 18:1577–81. [105] de Lange M, Snieder H, Ariens RA, Andrew T, Grant PJ, Spector TD. The relation between insulin resistance and hemostasis: pleiotropic genes and common environment. Twin Res 2003; 6:152–61. [106] Lane DA, Grant PJ. Role of hemostatic gene polymorphisms in venous and arterial thrombotic disease. Blood 2000; 95:1517–32.


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3 Commonly used anticoagulant and antiplatelet drugs Aspirin, heparin, and warfarin Andrew D Blann Introduction As discussed in Chapter 1, because atherothrombosis is the precipitating event in myocardial infarction (MI), stroke, and critical limb ischemia [1], agents to reduce the risk of these events are desirable. The active components of a thrombus are platelets and fibrin, and, while often present, red and white blood cells may be described as minor participants, although erythrocytes can enhance platelet aggregation by releasing adenosine diphosphate (ADP). It follows that effective antithrombotic therapy should be directed towards both platelets and the coagulation pathway that concludes with fibrin formation. Aspirin, heparin, and warfarin have been, and remain, the most commonly used drugs for reducing both the short-term and long-term risk of thrombosis, be it arterial or venous. However, despite commonality of final clinical effect, these drugs have radically different modes of action. Aspirin (as acetylsalicylic acid, or a derivative) is the primary antiplatelet agent in clinical use. Its value in the secondary prevention of atherothrombotic disease and in high-risk patients (such as those with atrial fibrillation or undergoing orthopedic surgery) is undoubted [2]. Heparin and warfarin are anticoagulants and both are (at least) very effective in reducing the risk of venous thrombosis. The former is an essential cofactor for antithrombin, but also has activity in reducing the function of other coagulation proteins [3, 4], while the latter reduces the risk of thrombosis by inhibiting the synthesis of key coagulation proteins [5]. However, while these three agents are the mainstay of current clinical practice, newer drugs currently in (limited) practice, in trial, and in development may supersede the “old guard.” In Chapter 15, Jilma discusses agents operating with different mechanisms, such as the platelet ADP-receptor blocker Clopidogrel [6], direct antithrombins such as Ximelagratan [7] and Hirudin [8], and the synthetic pentasaccharide Fondaparinux [9]. In acute coronary syndromes, thrombosis can also be inhibited by drugs that interfere with the platelet receptor for fibrinogen, the gpIIb/IIIa receptor complex [10]. These issues will be revisited in other chapters.

Commonly used anticoagulant and antiplatelet drugs

37

Aspirin Historical perspective Elwood et al. [11] indicate that aspirin appears to have been the first “modern” drug to be used therapeutically, at around 400 BC (e.g. Egyptian physicians making poultices from dried myrtle leaves) although the beneficial action of willow bark has been recognized for centuries. Although isolated by Hoffman and marketed by Bayer in the late nineteenth century, and synthesized some 40 years later, firm ideas regarding the mechanisms of the antiplatelet activity of this analgesic and antipyretic did not emerge until relatively recently. The 1960s were a great decade for fundamental research in platelets and aspirin, with the development by Born and O’Brien of their techniques for assessing platelet aggregation in vitro. Quick reported prolongation of the bleeding time after aspirin ingestion [12], while O’Brien [13], Weiss [14], and others such as Evans et al. [15] demonstrated additional effects of salicylates on platelets and alluded to possible mechanisms for this such as impaired responses to stimulants. Shortly afterwards, Vane, Smith, and Willis showed that these steps involved the enzyme cyclooxygenase (COX) [16, 17]. These and other studies lead directly to the first clinical trial of aspirin that, interestingly, failed to prove a benefit [18], However, a subsequent metaanalysis of early trials [19] provided a basis for its current use [2, 20]. Mode of action Platelets can be activated via a number of routes, such as by the physical effects of increased shear forces present in turbulent blood, and via binding of specific receptors by agonists such as ADP, prostanoids, serotonin, epinephrine, thromboxane A2 (TBXA2), thrombin, and collagen [21]. Other membrane components not intimately involved in direct signalling include structural molecules such as gpIb-V-IX that binds von Willebrand factor (vWF) and gpIa-IIa, the collagen adhesion receptor. Occupancy of signalling receptors by respective ligands (e.g. gpVI by collagen) induces the activity of intracellular second messenger systems (such as tyrosine kinases, protein kinase C, phospholipase C, and G proteins), cyclic adenosine monophosphate (cAMP) [22], and the mobilization of intracellular calcium [23]. Increased cytoplasmic calcium results in the liberation of arachidonic acid from membrane phospholipids by phospholipase A2, and steps that lead to activation of gpIIb/IIIa. Arachidonic acid is converted by COX to labile endoperoxide prostaglandin intermediates (PGG2 and PGH2) and thence by thromboxane synthase to TBXA2. These events bring about structural changes in the gpIIb/IIIa receptor complex (the most abundant platelet adhesion receptor (perhaps 50,000–80,000 copies per cell), also known as integrin αIIbβ3) that allow it to bind to its major ligand, fibrinogen [24]. Major changes such as shape change and the degranulation of dense and alpha granules follow. Many of the contents of these granules, such as ADP, fibrinogen, calcium, serotonin, and TBXA2, themselves contribute to additional activation by positive feedback [25], and these lead


38

ultimately to aggregation and (if present) adhesion (e.g. via vWF) to components of the subendothelium such as collagen, laminin, and vitronectin (Figure 3.1). Aspirin acts by irreversibly acetylating serine residue 530 (although some refer to serine 529) of the COX molecule, an enzyme present in platelets, endothelial cells, and in gastric mucosa. This amino acid is part of the active site for the substrate arachidonic acid, which it therefore cannot bind, resulting in reduced product, that is intracellular TBXA2. As this latter molecule is a powerful stimulant of addition cell activation, use of aspirin leads

Figure 3.1 Strategies to reduce the risk of thrombosis. This figure summarizes the relationships between the coagulation pathway (top right), platelets (center), and the blood vessel wall (bottom). Opportunities to inhibit thrombosis are in boxes, interactions as arrows. Warfarin acts on various soluble coagulation proteins, heparin(s) by promoting the activity of antithrombin and in inhibiting factor


39

Xa, and direct thrombin inhibitors (such as Ximelagratan) by interfering with thrombin. Clopidogrel blocks the ADP receptor on the platelet surface whereas aspirin enters the platelets to inhibit intracellular signalling. Finally, gpIIb/IIIa inhibitors (such as ReoPro) prevent fibrinogen binding. The figure also illustrates the fact that numerous pathways, as yet, have no well developed specific inhibitors close to the market. There include platelet signalling via the collagen and thrombin receptors, and molecules that minimize the interactions between vWF (arising from a damaged endothelium), the platelet, and the subendothelium (TBX: thromboxane; vWF: von Willebrand factor; ADP: adenosine diphosphate). to a reduction in platelet aggregation via this particular pathway. However, a weakness of aspirin is that it has little effect on other routes (e.g. the phospholipase-dependent pathway) to platelet activation. Thus, it fails to prevent thrombin-induced platelet aggregation and only partially inhibits that induced by ADP and high doses of collagen [26, 27]. Nevertheless, reduced TBXA2 has additional benefits as vasoconstriction is also reduced, providing an additional mechanism to minimized thrombosis. An additional theoretical disadvantage of aspirin is the reduction in levels of prostacyclin [16, 17], which effectively acts as an inhibitor of platelet aggregation and vasodilation. However, as the nucleate endothelium has the ability to synthesize new COX, fresh prostacyclin can be produced by this cell, and in any case it is clear that the beneficial inhibition of TBXA2 exceeds the possibly deleterious loss of prostacyclin. Aspirin may also have non-platelet effects that are desirable, such as inhibiting nuclear transcription initiators such as NFκb (implicated in the promotion of various genes with pro-inflammatory activity) [28], in protecting low-density lipoprotein cholesterol from oxidative modification [29], and in improving endothelial dysfunction in atherosclerosis [30], However, the precise value of these mechanisms in vivo and any possible contribution to a reduction in thrombosis is unclear. The drug itself is cleared rapidly from the circulation by the liver, but its effect on platelet function can (in theory) persist for up to 10 days (i.e. the maximum lifetime of the platelet), although in practice the effect is lost after 5–7 days, dependent on the dose, as fresh, aspirin-free platelets are shed by the megakaryocyte [14]. The daily


40

administration of a low dose of aspirin, such as 20–50 mg o.d., may result in 90–98% inhibition of TBXA2 production within 3–7 days as inhibition of COX is cumulative and requires several doses for full effect [31]. Clinical effectiveness The minimum dose with the most consistent clinical benefit in numerous early clinical trials is often quoted as 160–325 mg o.d. with no apparent extra benefit at higher doses of 500–1,500 mg o.d. [2, 32, 33]. However, a more recent commentary concluded that 100 mg o.d. is adequate for the prevention of MI in subjects at high risk [34] while others recommend 75 mg o.d. [35]. One of the more recent meta-analyses of 65 trials of the effects of aspirin on vascular events in high-risk patients (excluding those with acute stroke) reported a significant effect, with a 23% odds reduction [2]. Further exact aspirinspecific extrapolations (e.g. to different subgroups of patients) are difficult as the metaanalysis considered all antiplatelet drugs (such as dipyridamole and ticlopidine) together. Nevertheless, as the great majority of participants took aspirin, it is widely believed that this drug protects those with a history of MI, acute MI, a history of stroke or transient ischemic attack, acute ischemic stroke, or other high-risk groups such as atrial fibrillation from a major vascular event [35]. Other meta-analyses have shown a benefit in subjects with unstable angina and non-ST segment elevation [33]. The International Stroke Trial in subjects with acute ischemic stroke showed the benefit of aspirin and recommended that it be started as soon as possible after onset [36]. Although aspirin is (weakly) effective in reducing the risk of venous thromboembolism (VTE) after orthopedic surgery [37, 38], there is a scarcity of head-to-head data with anticoagulants, and it plays no part in current guidelines [39, 40]. Side effects In general, low- to medium-dose aspirin is well tolerated in the majority of patients, with the principal adverse effects being gastrointestinal (GI) and hemorrhagic stroke. The former, leading to dyspepsia and bleeding, seems likely to be due to a reduction in prostacyclin, which normally offers protection. However, this damaging effect becomes significant at very high levels of aspirin, such as 900–1,300 mg o.d. The risk of GI effects has been shown to rise with increasing dose and with regular or recent use [41–43]. Two forms of COX are well recognized in man: constitutive COX-1 and inducible COX-2 [44]. It is popularly hypothesized that COX-1 is the “good” isoform generating prostaglandins for “house-keeping” functions such as GI mucosal integrity and homeostatic functions, whereas COX-2 is said to mediate inflammation and pathophysiology [45, 46]. Based on this, COX-2 inhibitors have been developed that aim to spare the GI tract, and that do not impair platelet function [47]. However, concerns have been raised, such as the possibility that inhibition of COX-2 activity may augment myocardial cell death, and that COX-2 inhibitors are prothrombotic and increase the risk of MI [48, 49]. Therefore, the value of these agents in minimizing thrombosis in cardiovascular disease is unclear. Clearly, more serious is the risk of intracranial hemorrhage, causing stroke. A metaanalysis of 16 placebo-controlled trials of aspirin for cardiac and other indications found


41

that aspirin increased the absolute risk of cerebral hemorrhage by 12 events per 30,000 person-years of follow-up. This risk can be minimized, but not eliminated, by administrating the lowest effective aspirin dose. It follows, therefore, that the possible value of aspirin in reducing thrombosis may be weighed against the risk of bleeding, with the greatest value being in those at highest risk [35, 50]. Despite the positive attributes of aspirin described here, several commentators have expressed the concern that it may not be broadly valuable [51]. Although recruiting patients with hypertension, the HOT trial reported no protective effect of aspirin on death, cardiovascular death, all cardiovascular events, all strokes, all MIs or silent MIs [52]. Aspirin, however, did protect against overt MI but at the cost of an increase in fatal and nonfatal major bleeds, minor bleeds, and total bleeds. There is also some evidence that dual use of aspirin and angiotensin-converting enzyme inhibitors in some subgroups of patients may be of dubious value [34, 51, 53], although this may not be true for lowdose aspirin. Nevertheless, aspirin seems to be effective in reducing the risk of colorectal cancer [54]. Other antiplatelet drugs Persantin (dipyridamole) is perhaps (historically speaking, being introduced in 1959) the second-best known antiplatelet agent. Like the quinolinone cilostazol, it acts by inhibiting phosphodiesterases, thereby preventing the degradation of cAMP to AMP [55]. The consequences of high cAMP include low intracellular calcium, which in turn minimizes cell activation, and a further bonus is in potentiating the action of prostacyclin. Despite this, in a significant proportion of cases, evidence-based medicine cannot support the widespread use of this agent in cardiological practice, although its full value in cerebrovascular disease is still unclear [2, 56, 57]. Non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen also function as COX inhibitors, but as most are competitive inhibitors, differing in potency, duration, and pharmacokinetics, comparison is difficult and the overall value of NSAIDs in preventing thrombosis has been questioned [58, 59]. However, there may be a place for non-aspirin drugs in those who are intolerant of aspirin due to contraindications such as GI sensitivity, or are resistant to aspirin (present in 5–10% of patients with stable coronary artery disease) [60, 61]. Combinations of different antiplatelet drugs As aspirin is effective in inhibiting only some of the metabolic pathways leading to platelet activation, the opportunity to combine it with a drug operating via a different route seems attractive. Meta-analysis indicates that addition of dipyridamole to aspirin produces no significant further reduction in vascular events compared with aspirin alone [2]. However, ADP-receptor blockade seems more fruitful. Rupprecht et al. demonstrated the synergistic and accelerated ex vivo platelet inhibitory effects of ticlopidine plus aspirin in patients after stent implantation compared with either monotherapy alone [62]. In the CURE trial of the combination of aspirin and clopidogrel, a primary outcome occurred in 11.4% of patients on aspirin alone, compared to 9.3% of patients on both agents (p75 • Previous stroke or TIA

American College of • Age >75 years Chest Physicians • History of Consensus (2001) hypertension • Previous stroke or TIA or systemic embolism • LV dysfunctiona • Rheumatic mitral valve disease, prosthetic valve • >1 moderate risk factor

a Impaired left ventricular function included recent congestive heart failure or left ventricular fractional shortening ≤25% by M-mode echocardiography. b Moderate to severe left ventricular systolic dysfunction on echocardiography or recent congestive heart failure. TIA: transient ischemic attack; SBP: systolic blood pressure; LV: left ventricle, Data from Reference [16],

A practical stratification scheme is listed in Table 6.6. According to this, “low-risk” AF patients, with an annual stroke rate of about 1%, are considered to be those aged below 65 years, with no history of hypertension, diabetes, structural heart disease, or systemic thromboembolism, and can be adequately managed with aspirin alone. The use of warfarin instead, would only prevent 4 strokes per year per 1,000 patients treated (NNT=250, Table 6.7). On the other hand, the “high-risk” group, with an annual stroke rate of more than 8% if untreated, consists of hypertensives and/or diabetic patients aged more than 75 years, or patients of any age with a previous history of thromboembolism, structural heart disease, or evidence of left ventricular systolic dysfunction. The beneficial effect of warfarin (over aspirin) in this case is overwhelming, preventing up to 40 strokes per year for every 1,000 patients treated (NNT=25, Table 6.7). The AF patients who do not fit to the criteria of either one of the two previous cases represent the

Antithrombotic therapy for atrial fibrillation

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“moderate risk” group with an annual stroke rate estimated at 4%. Clear-cut evidence is lacking here and they can be managed with either aspirin or warfarin. Clinical judgment as well as patient preferences may have a role in choosing the appropriate scheme in this case.

Table 6.6 Practical risk stratification scheme in nonvalvular atrial fibrillation—suggested anticoagulation. Risk to be reassessed regularly. (A) High risk (annual risk of CVA: 8–12%) • All patients aged 75 years and over with diabetes and/or hypertension • All patients with previous TIA, CVA, or systemic embolism • All patients with clinical evidence of valve disease, heart failure, thyroid disease, and/or impaired left ventricular function on echocardiographya Treatment: warfarin (target INR 2.0–3.0) if no contraindications and possible in practice (B) Moderate risk (annual risk of CVA: 4%) • All patients over 65 years not in high-risk group • All patients under 65 years with clinical risk factors: diabetes, hypertension, peripheral vascular disease, ischemic heart disease Treatment: either warfarin (INR 2.0–3.0) or aspirin (300 mg). In view of insufficient clear-cut evidence treatment may be decided on individual cases. Referral and echocardiography may help (C) Low risk (annual risk of CVA: 1%) • All patients under 65 years with no history of hypertension, diabetes, systemic embolism, or other clinical risk factors Treatment: aspirin (300 mg o.d.) a Echocardiocardiography not needed for routine risk assessment but refines clinical risk stratification in case of impaired left ventricular function and valve disease. A large atrium per se is not an independent risk factor on multivariate analysis. Data from Reference [32].

Table 6.7 Effect of risk stratification on stroke reduction by warfarin compared with aspirin or no antithrombotic treatment in atrial fibrillation. Risk stratification

Primary prevention

Annual stroke rate without any therapy (%)

Annual stroke rate with aspirin therapy (%)

Treatment with warfarin instead of aspirin Number needed to treat for 1 year to prevent 1 stroke (N)

Number of strokes saved yearly per 1,000 given warfarin (N)


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• Low risk

1

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