Effect of Different Aspirin Doses on Arterial Thrombosis after Canine Carotid Endarterectomy: A Scanning Electron Microscope and Indium-111-labeled Platelet Study

Although it is widely accepted that aspirin inhibits platelet aggregation in arterial thrombosis, the appropriate dosage of aspirin remains quite controversial. The purpose of this study was to determine the effect of different doses of aspirin (0.5 mg/kg vs. 10 mg/kg) on mural thrombus formation after carotid endarterectomy. Eighteen hours after oral aspirin administration, 20 endarterectomies were performed on mongrel dogs with the use of the operating microscope. Blood flow was then restored for 3 hours and the vessels were prepared for investigation with the scanning electron microscope. Ten endarterectomies were also performed on unmedicated dogs as controls. Five minutes before vessel unclamping, autologous indium-111-labeled platelets were administered intravenously, and the endarterectomized portions of the vessels were studied with a gamma counter system after harvesting. Group 1, the control group, revealed extensive mural thrombus consisting of platelet aggregates, fibrin, red blood cells, and white blood cells. Six of the 10 vessels in Group 2, premedicated with 0.5 mg of aspirin per kg, demonstrated varying amounts of mural thrombus. Group 3 (10 vessels), premedicated with 10 mg of aspirin per kg, revealed a platelet monolayer completely covering the exposed vessel wall media, with scattered white blood cells and infrequent fine fibrin strands overlying the platelet surface. The mean (± SD) radioactivity per group expressed as counts/minute/mm2 was: Group 1-2055.3 ± 1905.5, log = 7.253 ± 0.926; Group 2-1235.6 ± 1234.3, log = 6.785 ± 0.817; Group 3-526 ± 433.06, log = 5.989 ± 0.774. Data analysis revealed significantly less (99% confidence interval) platelet deposition in Group 3 vessels when compared to Groups 1 and 2. This study demonstrates that a single dose of aspirin, 10 mg/kg, allows a platelet monolayer to form and simultaneously dramatically inhibits thrombus formation at the endarterectomy site. It also shows that a low dose of aspirin (0.5 mg/kg) is ineffective. The data suggest that 10 mg of aspirin per kg ingested before carotid endarterectomy would be quite beneficial in the clinical situation. (Neurosurgery 14:198-203, 1984)