Author + information
- †Institute of Cardiovascular Science, University College London, London, United Kingdom
- ‡Department of Epidemiology, Imperial College, St Mary's Campus, London, United Kingdom
- §The Stern Cardiovascular Foundation, Munford, Tennessee
- ↵∗Reprint requests and correspondence:
Dr. Srijita Sen-Chowdhry, Institute of Cardiovascular Science, University College London, Paul O’Gorman Building, 72 Huntley Street, London WC1E 6DD, United Kingdom.
Old habits die hard, as do old clinical precepts. The war against the thrombus in clinical practice has long been based on the twin paradigms of white versus red thrombi and Virchow's triad. Platelet-predominant white thrombi form in high-flow arterial streams. Red thrombi, rich in fibrin and trapped erythrocytes, form in low-pressure environments in the venous circulation and atria (1). Virchow's triad offers a 3-way breakdown of the predisposition to thrombosis into abnormalities affecting the vessel wall, blood flow, and/or blood constituents (2).
Originally described in venous thromboembolism (VTE), the scope of Virchow's triad has expanded with the recognition that it can be usefully applied to both atrial fibrillation (AF) and arteriopathy. For arterial thrombi, the foremost determinant is endothelial injury/dysfunction associated with atherosclerosis, hypertension, inflammation, or trauma; but turbulent blood flow at bifurcations or stenotic regions and platelet hyperactivity also contribute. For venous thrombi, cognate risk factors are vascular injury (trauma, surgery, vasculitis, sepsis), venous stasis, and hypercoagulable states, whether genetic (e.g., factor V Leiden mutation) or acquired (e.g., supplemental estrogen). Susceptibility to intracardiac thrombus is increased in AF by atrial endothelial dysfunction; stasis of blood, particularly in the atrial appendages; and local inflammation coupled with a systemic prothrombotic state (3). Stroke risk in AF is widely estimated with the CHADS2 (Congestive heart failure, Hypertension, Age ≥75 years, Diabetes, Stroke/thromboembolism) or CHA2DS2-VASc (plus Vascular disease, Age 65 to 74 years, female Sex) scores; the clinical modifiers represented therein likely operate, at a mechanistic level, via Virchow's triad (2,4).
The white/red thrombus paradigm provides the rationale for pharmacological management of thrombosis. Antiplatelet agents are the mainstay of therapy for coronary/peripheral artery disease, whereas the coagulation pathway is the target in VTE and AF. In AF, for example, aspirin lowers the risk of ischemic stroke by 21%, whereas vitamin K antagonist (VKA) therapy achieves a 67% reduction, unfortunately at the cost of doubling the intracranial hemorrhage rate (4). Among other drawbacks, dose requirements for VKA therapy show substantial interindividual and intraindividual variation, influenced by age, genetic variants in CYP2C9 and VKORC1, diet, gut flora, drug interactions, and liver function (4). Despite regular monitoring, time spent in the narrow therapeutic range is ∼60% (5). Dual antiplatelet therapy predictably fails to alter plasma indices of thrombogenesis, but was established by the ACTIVE (Atrial fibrillation Clopidogrel Trial with Irbesartan for prevention of Vascular Events) trials as an acceptable middle course. Stroke risk was 40% lower with VKAs than aspirin plus clopidogrel, which in turn offered 27% reduction over aspirin alone (1,6).
The question then arises: if the red/white thrombus dichotomy is so clear cut, why does aspirin confer even a modest reduction in the stroke risk in AF and why should addition of clopidogrel offer incremental benefit? One plausible albeit orthodox response invokes the common association of AF with vascular disease and its risk factors, such as hypertension. The reduction in vascular events observed with aspirin use in known arteriopathy is in the same range (20% to 30%) as that in AF. The implication is that the recognized benefit of aspirin in vascular disease accounts for much of its apparent benefit in AF (1).
By the same token, anticoagulation might be expected to have limited role in vascular disease. Whether the remit of VKA therapy can be extended becomes particularly relevant in the common scenario of dual pathology: coexisting AF and vascular disease. Diabetes and hypertension, key risk factors for vascular disease, are also CHADS2 modifiers; and vascular disease per se—including myocardial infarction (MI), peripheral artery disease, and complex aortic plaque—increases thromboembolic risk in AF, as reflected in the upgrading of CHADS2 to the CHA2DS2-VASc score (4). These patients often need anticoagulation and adding antiplatelet agents augments their bleeding risk. Could VKAs confer sufficient protection from vascular events to be used as the sole therapy in this population?
In the setting of recent acute coronary syndrome (ACS) and/or percutaneous coronary intervention, the answer is probably no. VKA therapy is relatively effective in preventing secondary events unless stenting is involved, in which case, even when combined with aspirin, it is inferior to regimens incorporating a P2Y12 receptor inhibitor (clopidogrel, prasugrel, ticlopidine, ticagrelor) (4). The current recommendation is aspirin plus P2Y12 receptor inhibitor for 12 months; adherence may be particularly important after deployment of drug-eluting stents owing to delayed re-endothelialization. Yet omission of anticoagulation also appears suboptimal, increasing mortality (hazard ratio [HR]: 3.43) and major cardiac events in a study of 426 patients with AF and recent stenting (7). An argument exists for triple therapy—aspirin, P2Y12 inhibitor, and VKA—but potential benefits may be offset by up to 16% annual incidence of hemorrhage (8). An alternative strategy involves conjunctive use of VKA and P2Y12 receptor inhibitor, which, in a trial of 573 patients, attenuated bleeding risk versus triple therapy (HR: 0.36) without increasing major cardiovascular events (9). Although the WOEST (What is the Optimal antiplatElet and anticoagulant therapy in patients with oral anticoagulation and coronary StenTing) trial was underpowered to exclude excess stent thrombosis, a subsequent registry study supported the noninferiority of VKA plus clopidogrel versus triple therapy (10). Dropping the aspirin appears, then, to hold promise after recent ACS/revascularization.
Posing a distinct challenge are patients with AF and stable coronary artery disease (in whom ≥12 months have elapsed after ACS/revascularization or there is no history of either). Anticoagulation remains optimal, but the risk/benefit ratio of add-on antiplatelet therapy alters owing to the now minimal threat of stent thrombosis and declining, although persistent, concerns over plaque instability. The 2011 American Heart Association/American College of Cardiology Foundation guidelines recommend VKA plus aspirin (11). A recent study of 8,700 patients with AF and stable coronary disease, however, found no difference in myocardial infarction/coronary death or thromboembolic risk between VKA therapy, VKA plus aspirin therapy, or VKA plus clopidogrel therapy (12).
In this issue of the Journal, Lamberts et al. (13) investigate the burden, impact, and optimal management of AF in >37,000 registry patients with both vascular disease and heart failure. The mean CHA2DS2-VASc score of 5 confirms a high-risk stratum; the mean HAS-BLED (Hypertension, Abnormal renal/liver function, Stroke, Bleeding history or predisposition, Labile international normalized ratio, Elderly [age >65 years], Drugs/alcohol concomitantly) score was 2.1, underscoring the need to achieve a delicate balance.
Besides being observational and retrospective in design, registry studies have limited available clinical details; we do not know how AF was picked up in these patients. Given real-world variations in practice, this may have ranged from 12-lead electrocardiography prompted by symptomatic deterioration to the more proactive approach of periodic ambulatory rhythm monitoring. Although the 29,660 patients without an AF diagnosis had improved survival, undiagnosed silent AF is a concern in this population, leading clinicians to seek surrogate markers such as left atrial dimensions. Even assuming, however, that some of these patients had paroxysmal AF, VKA therapy afforded no advantage over antiplatelet therapy in lowering thromboembolic risk (HR: 1.06; 95% confidence interval: 0.86 to 1.31). This is heartening because it implies that the burden of silent AF in these patients may not be sufficient to warrant anticoagulation. The WARCEF (Warfarin versus Aspirin in Reduced Cardiac Ejection Fraction) trial also found no difference in all-cause mortality between VKA versus aspirin in 2,305 patients with left ventricular systolic dysfunction and sinus rhythm, but this may have been because the benefits of VKA therapy in reducing ischemic stroke were offset by an increase in major hemorrhage among older patients (14).
Lamberts et al. (13) subdivided their AF sample into prevalent (pre-existing) and incident (new onset) categories but found similar thromboembolic risk. VKA therapy was more effective than antiplatelet therapy in attenuating this risk; combining the 2 afforded no advantage but did increase bleeding. This was no surprise; antiplatelet agents are included as additional protection against coronary events, not thromboembolism. But then came the pivotal finding: adding an antiplatelet agent to VKA therapy offered no incremental benefit in reducing the risk of MI/coronary death. Nor was there any significant difference between VKA-only therapy and VKA plus antiplatelet therapy in the combined outcome of thromboembolism and MI (HR: 1.00; 95% confidence interval: 0.89 to 1.14) (13).
Despite the large sample in this study, some of the reported confidence intervals are wide, suggesting possible underpowering. Nevertheless, it adds to accumulating evidence that an optimal risk/benefit balance in some patients with AF and vascular disease may be achieved by VKA therapy alone. In a clinical setting, decision making will be mutual and individualized, taking into account not only CHA2DS2-VASc and HAS-BLED scores but also coronary anatomy and, in heart failure patients, the extent of viable at-risk myocardium.
Returning to the red/white clot paradigm, however, the apparent efficacy of VKA therapy in preventing coronary events requires some explaining. If we assume the distinction in the thrombotic process is absolute, then the answer may lie in the pleiotropic properties of many drugs. Statin therapy, for example, may effect the reported 22% reduction in VTE risk by modulating endothelial function, inflammatory responses, and thrombogenesis (2). Similarly, VKAs have beneficial effects on myocardial contractility, oxygen consumption, platelet adhesiveness, and inflammatory reactions (15).
Yet it may also be time to replace our perception of a white/red thrombus dichotomy with a continuous spectrum. Carotid plaques and adverse cardiovascular events are more prevalent among VTE patients with idiopathic disease than those with transient acquired risk factors. At a mechanistic level, cytokine release, leukocyte recruitment, platelet activation, and fibrin turnover are common to both arterial and venous thrombosis. Antiplatelet therapy has been shown to reduce the incidence of VTE, just as warfarin can forestall coronary events (2,4).
One weapon can deal 2 blows in the war against the thrombus, which is good news for reducing iatrogenic bleeding. In patients with stable coronary disease, heart failure, and AF, VKAs can sometimes be that weapon. The impact of this finding may diminish in years to come as VKA use in nonvalvular AF declines in favor of alternatives that selectively inhibit factor Xa (rivaroxaban, apixaban and edoxaban) and thrombin (dabigatran). As direct-acting oral anticoagulants reduce the rate of intracranial hemorrhage by 50%, the risk/benefit ratio will shift and demand revisiting of the issues (5). Still relevant will be the recognition underpinning the current spate of antithrombotic studies: that optimal management of patients with apparent dual pathology is best guided by observational and empirical data to complement the oversimplified precepts of old.
The authors are grateful to Prof. William McKenna and Dr. Sripurna Das for constructive comments.
↵∗ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
Dr. Sen-Chowdhry is supported by the British Heart Foundation. Dr. Gordon is a speaker for Boehringer Ingelheim on Pradaxa.
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