Effects of venous pressure and posture on skin capillary perfusion

Various vasoconstriction mechanisms after a change in posture are involved in controlling skin blood flow and capillary transmural pressure to maintain adequate transcapillary exchange. The role of venous orthostatic pressure in the regulation of capillary nutritive perfusion, however, is less clear. We investigated capillary perfusion in 30 healthy subjects using intravital capillary microscopy of the big toenail-fold. Measurements were made in the supine and the sitting positions before and after incremental elevation of venous resistance, and thus the pre- to post-capillary pressure difference, by inflating a cuff around the ankle. On dependency, median capillary density rose (P < 0.0001), whereas red blood cell perfusion decreased (P < 0.0001). Cuff inflation of 30-60 mmHg in the supine position induced similar phenomena. Repeatedly, the same capillaries were found to be recruited after an increase in transmural pressure. We conclude that post-capillary pressure appears to be an important factor in the regulation of capillary perfusion, because an increase in venous resistance mimics the effects of dependency. The findings in this study support the theory of 'minimum perfusion pressure' for each capillary to be perfused with erythrocytes