Adverse effects of high dose aspirin on platelet adhesion to experimental autogenous vein grafts.

Prostacyclin (PGI2) production and platelet adhesion were studied in veins grafted into the arterial system of rabbits. Animal groups consisted of: no treatment; low dose aspirin (ASA) (0.5 mg . kg-1 X 24 h-1) plus dipyridamole (2 mg . kg-1 X 6 h-1); high dose ASA (40 mg . kg-1 X 24 h-1) plus dipyridamole (2 mg . kg-1 X 6 h-1); dipyridamole (2 mg . kg-1 X 6 h-1) alone. Results showed that vein grafts from animals treated with high dose ASA plus dipyridamole produced significantly less PGI2 than the other three groups (p less than 0.05 compared with the dipyridamole group; p less than 0.01 compared with the other two groups). In addition, there was significantly greater platelet deposition on the vein grafts from this high dose ASA group as compared to the low dose ASA group (p less than 0.05). By contrast, animals treated with dipyridamole alone had significantly less platelet deposition compared to both the control and high dose ASA groups (p less than 0.05). High dose ASA given to prevent thrombotic occlusion following coronary artery bypass grafting may, by reducing PGI2, result in enhanced platelet deposition. This in turn is likely to increase intimal hyperplasia as has been demonstrated previously with high dose ASA. Clinical studies, which have shown the early anti-thrombotic benefits of high dose ASA plus dipyridamole, have not measured graft intimal thickness. Since this process is an important cause of graft narrowing, ASA, in high dose, may adversely affect long-term graft survival.