Effect of Aspirin on Hemostasis and Thrombosis

Aspirin inhibits platelet function by acetylating platelet cyclo-oxygenase. When aspirin is administered in doses as low as 40–160 mg per day, it inhibits platelet cyclo-oxygenase activity by more than 80%. The effect of aspirin on platelet function is maintained for the life-span of the platelet and there is evidence that aspirin also acetylates platelets before they are released in the circulation and while they are still within megakaryocytes. Aspirin also inhibits the synthesis of PGI₂ by vascular wall cells but compared to the platelet, this vessel wall effect is relatively short-lived and requires slightly larger doses of aspirin. In vivo studies in rabbits indicate that very high doses of aspirin are thrombogenic. However, there is no evidence that aspirin is thrombogenic in man even when administered in high therapeutic doses.

The optimal antithrombotic dose of aspirin has not yet been determined. Clinically, impressive results have been obtained with low doses of aspirin (ranging from 100 to 300 mg per day) in preventing aorta coronary bypass thrombosis, in patients undergoing hemodialysis, and in patients with unstable angina. Aspirin is also effective in preventing stroke and death in patients with cerebral ischemia when administered in doses of approximately 1 gram per day. There are trends suggesting that aspirin is effective when administered in doses between 300 mg per day and 1500 mg per day in patients who have survived myocardial infarction.

The side-effects of aspirin are mainly gastrointestinal and are dose-related. Generalized bleeding is very uncommon and limited mainly to patients with other hemostatic abnormalities or due to the concomitant use of anticoagulant therapy.