Stroke in postmyocardial infarction patients can cause death or disability. The aspirin dosage needed to prevent strokes in high-risk patients is uncertain because studies of the relationship between aspirin dosage and outcomes are inconsistent. O'Connor and associates used the Coumadin Aspirin Reinfarction Study (CARS) data to compare stroke prevention with 80 mg of aspirin and 160 mg of aspirin.

CARS provided comparison data on the efficacy of various combinations of aspirin and aspirin alone in preventing additional ischemic cardiac or brain vascular events. The results indicated that warfarin (1 or 3 mg) plus low-dose aspirin (80 mg) was no more efficacious than 160 mg of aspirin alone. Because the lower-dose warfarin used in this study was not found to affect coagulation profiles, comparison between the combination of 80 mg aspirin plus warfarin and 180 mg aspirin is effectively a comparison of the efficacy of 80 mg aspirin and 180 mg aspirin. Using an end point of ischemic stroke confirmed by computed tomographic scan, an analysis of the 80 mg aspirin plus 1 mg warfarin and the 180 mg aspirin groups was performed.

Although there was no difference in the frequency of nonfatal stroke between the two groups, the time to first ischemic stroke was higher in the group taking the combination of 80 mg of aspirin plus 1 mg of warfarin. The advantage demonstrated by 160 mg of aspirin was greatest in the higher risk groups of male patients, patients who suffered a Q-wave myocardial infarction, and patients who were older than 70 years of age. There was no difference in hemorrhage rates between the two dosages of aspirin.

Although this finding was determined by secondary analysis of the data, and the lower stroke rate was of borderline clinical significance, the authors conclude that many post-myocardial infarction patients are underdosed for prevention of stroke by being given 80 mg of aspirin or are not being given aspirin at all. A dosage of 160 mg per day is the minimum effective dosage for prevention of stroke in postmyocardial infarction patients.