Prolonged paradoxical effect of aspirin on platelet behaviour and bleeding time in man

A single high dose of aspirin(3.9 g orally) was administered to subjects and the time course of the effects on bleeding time, platelet aggregation and platelet production of thromboxane A 2 was followed over a period of one week. Two hours after treatment the bleeding time remained unchanged although marked inhibition of the platelet functions and thromboxane A 2 release were detected. Twenty-four and 72 hours after aspirin the bleeding time was substantially increased, returning to normal levels 1 week after treatment. This suggests that shortly after aspirin treatment the production of prostacyclin and thromboxane A2 were blocked. As the recovery of the endothelial cyclo-oxygenase proceeded faster than that of the platelet enzyme the balance prostacyclin/thromboxane A 2 was changed in favour of prostacyclin 24 and 72 h after the treatment, thus explaining the increased bleeding time and its slow recovery over one week.