The contribution of valves to saphenous vein graft resistance

Abstract

Saphenous vein resistance influences graft flow rates and may affect graft patency in lower limb revascularization. To quantitate specifically the contribution of saphenous vein valves to this resistance, 10 human saphenous veins (mean length 68 cm, diameter 0.42 mm, and 5.2 valves per vein) were perfused with water under carefully controlled pressure gradients designed to simulate different peripheral resistances in the outflow bed. The Reynolds number was maintained at 350 to 600, within the physiologic range for in vivo grafts. Veins were perfused under both venous (10 mm Hg) and arterial (100 mm Hg) mean pressures to determine the effects of distension on the overall resistance of the conduit. The valves were bisected according to Leather's techniques and flow was measured in both directions, antegrade (simulating “reversed” grafts) and retrograde (simulating “in situ” grafts). Data (mean ± standard error) were normalized to the baseline flow for each vein with intact valves and expressed as a percentage change. Data were analyzed by means of Student's t test (p < 0.05). Baseline antegrade flow with intact valves averaged 71.0 ± 3.0 ml/min at pressure gradients (ΔP) of 10 mm Hg and 95.0 ± 2.6 ml/min for ΔP = 20 mm Hg. After valve incision, antegrade flow (reversed) increased an average of 29% at both pressure gradients. Retrograde flow (in situ) through the bisected valves was only 19% greater than baseline antegrade flow and was significantly less than antegrade flow through bisected valves. The difference is explained by theoretic considerations of stenosis area and orifice shape. The increases in flow did not correlate with vein length or diameter, nor did flow change with different distension pressures. These results indicate that saphenous vein valves are an important source of flow resistance. Bisection of valves appears to decrease resistance much more than vessel distension by arterial pressures. It is possible that the improved patency of in situ grafts may reflect the hemodynamic benefit of valve incision more than other factors. These data also suggest that maximal flows are achieved by antegrade flow through saphenous veins with bisected valves.