A new method for estimating skeletal muscle capillary pressure.

Venous (Pc,vo) and arterial occlusion capillary pressures were simultaneously compared with isogravimetric capillary pressure (Pci) in isolated rat hindquarters and canine gracilis muscles perfused with blood or an artificial plasma. Arterial or venous pressure transients following rapid occlusion of arterial inflow or venous outflow, respectively, were analyzed for the inflection point between rapid and slow components. This transition point was assumed to represent the beginning of discharge of blood stored in (arterial occlusion) or the addition of blood to (venous occlusion) skeletal muscle microvessels and was defined as the effective capillary pressure. In all preparations, Pc,vo was identical to Pci. Arterial occlusion pressures were the same as Pci and Pc,vo in artificial plasma-perfused preparations but were significantly greater (P less than 0.01) than Pci and Pc,vo obtained in blood-perfused preparations. This inequality between arterial occlusion pressure and Pci may be related to a critical closure of small precapillary vessels or the non-Newtonian behavior of blood. In addition, venous occlusion pressures were highly correlated (r = 0.95, P less than 0.01) to calculated capillary pressures obtained following simultaneous equivalent elevations of arterial and venous pressure. These results indicate that the primary sites of vascular compliance and fluid filtration reside at or very near one another in the skeletal muscle microcirculation and that the more easily determined venous occlusion capillary pressure is an adequate measure of the effective capillary pressure in skeletal muscle.