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ATRIAL FIBRILLATION, NON-VALVULAR, AND STROKE

Clinical Bottom Line:
If you have a patient with non-valvular atrial fibrillation:
  1. Identify validated risk factors:
  2. Clinical Features:
    1. hypertension? (BP >160 and/or >90 or chronic Rx)
    2. heart failure within 100 days? (orthopnea, dyspnea on exertion or edema responding to diuretics; S3 gallop + rales; CXR cardiomegaly or redistribution; or elevated LV or wedge at catheterization)
    3. prior arterial thromboembolism? (ischemic stroke of any cause, TIA, or systemic embolism).
  3. Echocardiographic Features:
    1. Global left ventricular dysfunction
    2. Left atrial size >2.5 cm/m2 by M-mode
  4. Translate this into an annual risk for thromboembolism (stroke plus peripheral arterial embolism; almost all will be strokes):

Risk
Factors		 Annual Risk
without Rx		 Absolute Risk Reduction NNT: 1 yr to prevent 1 event
Warfarin Aspirin Warfarin Aspirin
Clinical Only
None0.0250.0170.00960115
One0.0720.0480.0252140
2 or all 30.1760.1180.0628286
Clinical plus Electrocardiographic
None0.010.0070.004149286
1 or 20.060.040.0212548
3 to all 50.1860.1250.065816

* assumes that the 67% RRR from warfarin and the 35% RRR from aspirin apply to all sub-groups
**and remember that the NNH to produce a major haemorrhage from a year of Rx with warfarin (1-2%) is 50-100.

The Evidence:
Taken from:

  1. the placebo group in the SPAF (Stroke Prevention in Atrial Fibrillation) Trial1; so that they would get an estimate of the risks without treatment.
  2. the average efficacy from warfarin (Relative Risk Reduction of about 80% from both the SPAF2 and BATAF3 [Boston Area Anticoagulation Trial for Atrial Fibrillation] trials).
  3. the pooled estimate of the efficacy of aspirin from the SPAF (RRR=42%) and Scandinavian4 (RRR=16%) trials (Relative Risk Reduction of about 35%).

References:

  1. SPAF Investigators: Predictors of thromboembolism in atrial fibrillation: I. Clinical features in patients at risk. Ann Intern Med 1992:116:1-5. SPAF Investigators: Preliminary report of the SPAF study. NEJM 1990;322:863-8.
  2. SPAF Investigators: Stroke prevention in atrial fibrillation: final results. Circulation 1991;84:527-39.
  3. The BATAF Investigators: The effect of low-dose warfarin on the risk of stroke in patients with non-rheumatic atrial fibrillation. NEJM 1990;323:1505-11.
  4. Petersen P, Boysen G: Prevention of stroke in atrial fibrillation. NEJM 1990;323:482.

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