Antithrombotic Therapy in Peripheral Arterial Occlusive Disease: Vascular reconstructions
In vascular reconstructions involving high-flow, low-resistance arteries greater than 6 mm in diameter (aor-toiliac, femoral, major visceral, renal, and proximal brachiocephalic vessels), thrombotic occlusion is unusual and 5- to 10-year patency rates in the range of 80 to 90% can be expected. Antithrombotic therapy is not indicated for such cases. An exception to this is axillo-femoral bypasses that are long, are vulnerable to thrombotic occlusion, and have low patency rates.
Reconstruction of small arteries with flow rates of less than 200 mL/min are prone to thrombosis, particularly if the bypass is long and crosses a flexion point at the knee or the groin. Such bypasses are more vulnerable to thrombotic occlusion than large-diameter, high-flow reconstructions, because an equivalent reduction in lumen from thrombus or anastomotic neointimal hyperplasia is much more likely to critically impair blood flow. Effective antithrombotic therapy would theoretically enhance patency and extend the functional longevity of small-vessel reconstructions.
There are six randomized trials of antiplatelet therapy in patients with peripheral arterial bypasses. Asthma inhalers read only In the two studies in patients undergoing prosthetic femoral-popliteal bypass, aspirin plus dipyridamole therapy was started preoperatively; both of these trials demonstrated a statistically significant reduction in prosthetic bypass occlusion. However, because these two studies had small numbers of patients in the treatment and control groups (level II studies), the results are not definitive. In contrast, a larger study (level I) by Kohler et al demonstrated no protective effect of aspirin plus dipyridamole. However, this study differed in that antiplatelet therapy was started postoperatively. In addition, only one third of the patients had prosthetic bypasses that are more vulnerable to thrombotic occlusion and would, therefore, be more likely to demonstrate a benefit of antiplatelet therapy. Based on the extensive experience in patients with saphenous vein aortocoronary bypass grafts, the timing of antiplatelet therapy is probably important because early perioperative events, such as platelet accumulation at sites of vascular injury, are important in causing thrombotic occlusion; this indicates that antiplatelet therapy needs to be started early. A large study (level I) by Clyne et al emphasizes this point and is helpful because it reconciles the differences between other trials in patients undergoing peripheral bypass. The treated patients in the study by Clyne et al received preoperative and intraoperative IV dipyridamole. Postoperatively, they were treated with aspirin plus dipyridamole for 6 weeks. There was a significant and marked reduction in occlusion among treated patients who had prosthetic reconstruction; in treated patients with saphenous vein reconstructions there was a nonsignificant trend suggesting benefit. Taken together, these studies suggest that antithrombotic therapy started before (but not after) surgery may improve patency of lower-extremity bypasses, particularly when a vascular prosthesis is implanted.