Consideration for the risk of bleeding on warfarin
Patients should be assessed for their risk of major bleeding before initiating either ASA- or warfarin-based treatment. Many factors must be considered in estimating the bleeding risk for a patient on warfarin therapy. Most studies examining the use of warfarin have not included patients with a history of intracerebral hemorrhage, recent severe bleeding in the preceding 3-6 months, or previously
demonstrated intolerance or bleeding while on warfarin.
Recent studies suggest that cognitive impairment is also associated with increased bleeding risk.
Advanced age (≥80 years) is another reported significant risk factor for bleeding. A recent prospective cohort study of anticoagulation in elderly patients who were initiated on warfarin therapy, revealed a 4.7% risk/year of major hemorrhage in patients 65-80 years old, compared with 13.1% in older patients. 8 Furthermore, the same study suggested an increased risk of bleeding with increasing
CHADS 2 scores (2.0%-4.3% for CHADS 2 <3 and >19% for patients with CHADS 2 ≥3).
These data on the risk of bleeding may lead to a negative opinion on the use of anticoagulation therapy in the elderly with elevated CHADS 2 scores. Fortunately, the BAFTA trial provides some further guidance for this population. BAFTA randomized elderly patients from referring physicians who were willing to have their patients treated with either ASA or warfarin. These patients had an annual
rate of 1.9% for major hemorrhage while on warfarin, compared with 2.0% while on ASA. An increased CHADS 2 score was not associated with a higher risk of major hemorrhage in this study. However, the risk of stroke with AF increased substantially with age (2.8%/year
for patients 75-79 years of age vs 5.6%/year for those ≥85 years of age). Therefore, the benefits of anticoagulation appear to outweigh the risks for most elderly patients with AF in this randomized controlled study.
Finally, there is good evidence indicating that the highest risks of bleeding with oral anticoagulation occur during the first 6 months of therapy. Patients who have demonstrated tolerance to anticoagulation in the past are at a significantly lower risk of major hemorrhage while on warfarin compared with new users of the drug. Selecting a prophylactic regimen In multiple trials, anticoagulation with warfarin or other vitamin K antagonists has clearly and consistently demonstrated superiority to ASA or any other antiplatelet regimen for the purposes of preventing cardioembolic events. The target international normalized ratio (INR) should be 2.0 to 3.0, unless another concomitant indication for anticoagulation warrants a higher INR. Warfarin confers a relative risk reduction of approximately 66% in preventing stroke and other cardioembolic events over not using either warfarin or ASA. Compared with the use of ASA, warfarin confers a relative risk reduction of slightly >50%.
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In determining a strategy for stroke prophylaxis, it is important to recognize that the absolute benefits from anticoagulation are dependent on the baseline risk of embolic events. Patients with a CHADS 2 score of 0 and none of the moderate risk factors have a low annual risk of stroke and do not warrant anticoagulation with warfarin.
The AHA has specifically issued a level III recommendation to reinforce the notion that young patients (<60 years old) with AF, but no other risk factors for thromboembolism, should not be offered warfarin as prophylaxis. 7 It is reasonable to consider ASA as stroke prophylaxis in this group, especially if they have other potential indications for ASA, such as an increased risk for coronary events.
If there is no specific reason against using warfarin, it is recommended that most patients with a CHADS 2 score of ≥1 be offered oral anticoagulation. The risk of a stroke is increased sufficiently to offset the increased risk of bleeding with warfarin in most of these patients, unless a strong concern for bleeding exists. There is a large group of people with AF whose CHADS 2 scores lie in the intermediate-risk range (scores 1-2); >60% of all thromboembolic strokes in patients not taking warfarin appear to occur in this subgroup. The AHA guidelines recommend either ASA or oral anticoagulation for all patients with a
CHADS 2 score of 1 and oral anticoagulation for all patients with a score of ≥2. 7 If oral anticoagulation is contraindicated, then ASA is recommended as an alternative. In comparison, the Canadian Cardiovascular Society (CCS) 9 recommends warfarin therapy for all patients with
a CHADS 2 score ≥1, unless the only CHADS 2 risk factor for stroke is diabetes, in which case the CCS is more liberal and recommends either warfarin or ASA.
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Historically, the elderly have been undertreated with anticoagulation, especially given that they are at an increased risk of bleeding, experiencing higher rates of falls and malignancies, have increased susceptibility to intracerebral bleeding, and more polypharmacy (ie, increased risks of drug interactions). However, they are also at increased risk of stroke in the setting of AF. The BAFTA trial demonstrated a significant reduction in its primary outcome measure (stroke, intracranial hemorrhage or other
arterial embolism) with warfarin over ASA in patients >75 years old (1.8% per year vs 3.8% per year). It remains to be seen whether the next AHA guidelines update will recommend that age >75 alone (eg, CHADS 2 score 1) is associated with a sufficiently high stroke risk to prefer warfarin over ASA.
In the absence of any of the CHADS 2 risk factors, the presence of ≥1 moderate risk factors prompted the AHA guidelines to provide a weaker recommendation (class IIa) for the use of ASA or warfarin, but both are mentioned as reasonable options. CCS guidelines for these more moderate risk factors are similar, with the exception that it does not list female sex as a risk factor.
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