Effects of arterial and venous pressure alterations on transcapillary fluid exchange during raised tissue pressure

To assess local haemodynamic effects of raised tissue pressure per se and of arterial and venous pressure variations during raised tissue pressure, and to evaluate the vascular waterfall phenomenon. An isolated pump-perfused sympathectomised cat skeletal muscle enclosed in a plethysmograph. Hydrostatic capillary pressure (Pc), tissue volume alterations and blood flow were recorded at various arterial (PA) or venous (PV) pressure levels at a raised tissue pressure (Ptissue). Total vascular resistance (Rtot) and its three consecutive sections, arterial resistance (Rart), venular resistance (Rvenule), and venous outflow orifice resistance (Rorifice), were recorded. Increase in Ptissue increased Pc due to a marked increase in Rorifice and unchanged Rart and, when Ptissue > PV, by about 90% of the Ptissue increase. During the raised Ptissue, a PA increase (95 to 115 mmHg) increased Pc (by 3.1 +/- 1.1 mmHg) causing fluid filtration, and a PA decrease (95 to 75 mmHg) decreased Pc (by 2.4 +/- 0.5 mmHg) causing fluid absorption. Rart was unchanged, indicating impaired autoregulation. Increased PV had no haemodynamic effects when PV < Ptissue due to a gradual decrease in Rorifice towards zero when PV reached Ptissue. At PV > Ptissue blood flow and Pc increased gradually, causing fluid filtration. Tissue volume is increased by raised and decreased by lowered PA, the latter may be of use to decrease ICP in the injured brain. The results indicate that PEEP or head elevation will not influence ICP from the venous side if CVP < ICP. Finally, the "vascular waterfall phenomenon" was rejected as Rorifice is a normal variable fluid resistance.