Effects of externally applied compression on blood flow in the human dependent leg

In healthy humans placed in a horizontal position, venous transmural pressure was increased from about 10 to 45 mmHg in the mid-calf region by lowering the leg below heart level. This increase in vascular transmural pressure evoked a vasoconstrictor response, as 133Xe-measured blood flow was reduced by 56% (P less than 0.01) in subcutaneous tissue and by 31% (P less than 0.01) in skeletal muscle, when recorded at the same mid-calf level. The tissue pressure in the dependent leg was elevated stepwise by inflation of a standard whole leg cuff. The vasoconstrictor response was still present when the cuff was inflated to 10 mmHg, but absent in both tissues when external pressures between 20-40 mmHg were applied. The results suggest that the observed absence of vasoconstriction can be ascribed to inhibition of the local veno-arteriolar reflex mechanism. External pressures exceeding 40 mmHg reduced blood flow in both tissues. External pressures of 60 mmHg reduced blood flow by 45% (P less than 0.01) in subcutaneous tissue and by 19% (P less than 0.02) in skeletal muscle. In conclusion, the vasoconstrictor response evoked in the dependent leg is qualitatively similar, but quantitatively different in the two tissues, possibly due to a difference in basal vascular tone. The vascular adjustments to external compression of the dependent leg is similar in the two tissues at low external pressures (0-40 mmHg), but different at high pressures, the latter probably due to a difference in vessel collapsibility in the compressed tissues.