Oral anticoagulation for stroke prevention is effective but underused
A series of clinical trials have shown the remarkable efficacy of anticoagulation with warfarin compared to placebo in reducing stroke risk in patients with AF (Copenhagen Atrial Fibrillation Aspirin and Anticoagulation, AFASAK ; Stroke Prevention in Atrial Fibrillation, SPAF [45]; Boston Area Anticoagulation Trial for Atrial Fibrillation, BAATAF; Canadian Atrial Fibrillation Anticoagulation, CAFA ; and Stroke Prevention in Nonrheumatic Atrial Fibrillation, SPINAF). In 2007, Hart and colleagues published a meta analysis of the aforementioned 5 primary prevention trials plus one study of secondary prevention and demonstrated a 62% relative risk reduction for stroke in patients anticoagulated with vitamin K-antagonists (warfarin). Additionally, a stroke causes higher costs than its prophylaxis, including expenses for drugs, monitoring, nursing, loss of working hours, hospital admissions and procedures due to bleeding complications. Nevertheless, this effect of anticoagulant therapy depends on achieving a protective therapeutic range. Patients with AF within the recommended target INR range of 2.0-3.0 survive longer and have reduced morbidity and gradually every 10% increase in time out of therapeutic range is associated with an increased risk of ischaemic stroke. However, the principal problem with anticoagulation is the variability of the effect of coumarin derivatives on the hemostatic system; patients may require very different doses (up to 10-fold differences) to reach the same level of anticoagulation, and the required dose may also vary over time in an individual patient. In the studies mentioned above, a relevant number of strokes occurred among patients randomized to receive warfarin, but with subtherapeutic INR levels below. As the trials were designed as intention-to-treat analyses, it is likely that the real efficacy of warfarin is underestimated.
Despite the proven benefit, OAC with warfarin or other vitamin K-antagonists remains underused in
clinical practice. The review of the literature identifies several barriers to the prescription of vitamin K-antagonists which are related to the patient, the physician, and the health care system. One of the strongest patient related predictors of warfarin withholding is age. The most important physician related reasons not to anticoagulate are 1) the perception of the benefit vs risk of therapy, insofar as the risk for embolism relative to hemorrhage is judged to be lower, and 2) the relative contraindication to therapy due to the lack of patient reliability or patient noncompliance as a reason for difficulties in monitoring the prothrombin ratio. Of the 596 (64.4%) eligible family physicians who participated in a representative survey in Australia, 15.8% reported having a patient with non-valvular atrial fibrillation experience an intracranial hemorrhage with anticoagulation and 45.8% had a patient with known nonvalvular atrial fibrillation experience a stroke without anticoagulation. When presented with a patient at „very high risk“ of stroke, only 45.6% of family physicians selected warfarin in the presence of a minor falls risk and 17.1% would anticoagulate if
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the patient had a treated peptic ulcer. Family physicians with less decisional conflict and longer-standing practices were more likely to endorse anticoagulation.
Concerns about the external validity of the aforementioned trials cannot be ruled out as an argument for withholding warfarin, as they excluded most of the potential candidates, with some trials excluding >90% of screened patients . The most common reasons for trial exclusion were age and relative contraindications to anticoagulation. Therefore, the results in favor of anticoagulation were obtained under ideal circumstances for patients with an average age at about 70 years. Consistently, there are physicians who are cautious with prescribing anticoagulants to patients who are older and/or frailer than the trial population. Finally, concerning the health care system, there may be a lack of resources and experienced personnel in the community to adopt practice recommendations and to provide anticoagulation therapy at rates similar to those of controlled clinical trials. There is no doubt that INR monitoring either through patient self-management, or by specialized anticoagulation clinics improves the quality of care and reduces the rate of complications.
Gage and colleagues reported on 597 hospitalized cases with a mean age of 80 years, which had chronic AF documented during their index admission. Overall, at discharge 34% of patients were prescribed warfarin, 21% were prescribed aspirin, and 45% were not prescribed any antithrombotic therapy. Patients aged ≥ 76 years were less likely to receive antithrombotic therapy than were younger patients (p<0.001), females were less likely than males to receive antithrombotic therapy (p=0.02), and patients treated in rural facilities were prescribed antithrombotic therapy less frequently than patients in metropolitan facilities (p=0.02). Outcome analysis in 463 cases demonstrated that patients who were prescribed antithrombotic therapy were significantly less likely to have an adverse outcome (death or hospitalization for an ischemic event; p=0.0001). Thirty-three suffered strokes or TIAs: 9 of these occurred in the 163 patients who were prescribed warfarin, 8 occurred in the 96 patients who were prescribed aspirin, and 16 occurred in the 204 patients who were prescribed neither therapy. The authors discussed two main reasons for the low use of antithrombotic therapy: inconvenience and physicians’ fear of hemorrhage. Physicians’s subjective judgement seems to attribute a greater negative value to hemorrhagic strokes than to ischemic ones, even when the health outcomes are the same. The emphasis on avoiding hemorrhagic strokes and other iatrogenic events may cause physicians and patients to choose therapy that minimizes side effects rather than therapy that maximizes benefit.
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