Histologic, morphometric, and biochemical evolution of vein bypass grafts in a nonhuman primate model. II. Modification of early changes by platelet inhibition with aspirin and dipyridamole.

The objective of this study was to determine the early influence of platelet inhibition on the histologic, morphometric, and biochemical evolution of vein bypass grafts in a nonhuman primate model. Cephalic vein grafts were interposed bilaterally in the femoral arteries of 15 stump-tailed macaque monkeys fed a diet that sustains plasma cholesterol levels of approximately 225 mg/dl. All animals received in combination aspirin, 80 mg/day, and dipyridamole, 50 mg/day. Grafts were excised from five animals for analysis on each of postoperative days 3, 7, 14, 30, 60, and 90. In animals subjected to platelet inhibition, cholesterol content in the graft was 170 +/- 52 micrograms/100 mg at 90 days, 205% of the level in ungrafted vein (p less than 0.01). This change was small in comparison with the increase to 686% of ungrafted vein observed in our study of control animals. In stepwise regression analysis, cholesterol content of grafts was best predicted by prevalence of foam cells (r2 = 0.82), and the proportion of intima as a fraction of total wall area was best predicted by the presence of macrophages (r2 = 0.69). Platelet inhibition did not decrease the extent of intimal hyperplasia. The prevalence of adherent platelets (r = -0.72) and the amount of fibrin (r = -0.78) correlated inversely with the amount of endothelium present during the first 14 days. The strength of these correlations declined with time, despite persistent lack of endothelium in some areas. Medial fibrosis occurred to the same extent as in control grafts, as did the early appearance of platelet factor VIII and fibronectin and the lack of vasa vasorum at 3 days followed by reappearance at 7 days. These data demonstrate that platelet inhibition dramatically reduces lipid uptake by grafts in the first 90 days but has less influence over histologic or morphometric changes.