EFFECTS OF ARTERIAL AND VENOUS-PRESSURE ALTERATIONS ON TRANSCAPILLARY FLUID EXCHANGE DURING RAISED TISSUE PRESSURE

Objective: 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. Design: An isolated pump-perfused sympathectomised cat skeletal muscle enclosed in a plethysmograph. Interventions: Hydrostatic capillary pressure (P-c), tissue volume alterations and blood flow were recorded at various arterial (P-A) or venous (P-v) pressure levels at a raised tissue pressure (P-tissue). Total vascular resistance (R(tot)) and its three consecutive sections, arterial resistance (R(art)), venular resistance (R(venule)), and venous outflow orifice resistance (R(orifice)), were recorded. Results: Increase in P-tissue increased P-c due to a marked increase in R(orifice) and unchanged R(art) and, when P-tissue>P-v, by about 90% of the P-tissue increase. During the raised P-tissue a P-A increase (95 to 115 mmHg) increased P-c (by 3.1+/-1.1 mmHg) causing fluid filtration, and a PA decrease (95 to 75 mmHg) decreased P-c (by 2.4+/-0.5 mmHg) causing fluid absorption. R(art) was unchanged, indicating impaired autoregulation. Increased P-v had no haemodynamic effects when P-v<P-tissue due to a gradual decrease in R(orifice) towards zero when P-v reached P-tissue. At P-v>P-tissue blood flow and P-c increased gradually, causing fluid filtration. Conclusions: Tissue volume is increased by raised and decreased by lowered P-A, 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 R(orifice) is a normal variable fluid resistance.