Local vascular responses to elevation of an organ above the heart

Elevation of an organ above the heart reduces the arterial and venous hydrostatic pressures in proportion to the height of elevation. Intact autoregulation protects organs. such as the brain and skeletal muscle. from significant alterations in blood flow and hydrostatic capillary pressure due to the decrease in arterial inflow pressure during such a manoeuvre. However. the consequences of the decreased hydrostatic pressure on the venous side are far from clarified. The present study analyses the local haemodynamic effects of the decrease in arterial and venous hydrostatic pressures that occur during vertical elevation of an organ above the heart at atmospheric and raised tissue pressures (0, 10 and 30 mmHg). A sympathectomized cat skeletal muscle enclosed in a plethysmograph and perfused from the animal was used as the experimental model. The results show that elevation of the muscle above the heart at atmospheric tissue pressure created a variable vascular resistance starting at the venous outlet of the organ. and related to the difference between tissue pressure and venous outflow pressure. This resistance completely protects the organ from the hydrostatic pressure alterations on the venous side. The results also show that arterial pressure variations will exert the same haemodynamic influences on the organ as tissue pressure variations, except for the formation of the venous outflow resistance at raised tissue pressure. The application of these results to normal and injured organs. e.g. normal and injured skeletal muscle and brain, with various tissue pressures. is discussed.