Pressure-dependent tissue bleeding in cutaneous flap during nitroprusside infusion in the dog

The present study was designed to determine whether arterial pressure or blood flow contributes to the reduction in tissue bleeding (TB) during sodium nitroprusside (SNP) induced hypotension. We performed the experiment in dogs with sinus denervation and vagotomy to minimize reflex intervention. The Group 1 of Series I experiment consisted of 6 dogs with the measurements of systemic arterial pressure (SAP) and femoral arterial flow (Q). The SAP was deliberately lowered from a control level of 145.3 mmHg to 100, 80 and 60 mmHg with iv SNP infusion of various doses. The SNP-induced hypotension was accompanied by decreases in Q and vascular resistance. In Group 2 (n = 6), the SAP was decreased from control to 100 and 60 mmHg by SNP infusion and TB was collected from an incised skin flap over the medial thigh. The total amount of TB over a period of 20 min was decreased from 20.2 +/- 0.5 ml at 100 mmHg to 13.4 +/- 0.5 ml at 60 mmHg (P less than 0.001). This series of experiment demonstrated that a decrease in SAP from 100 mmHg to 60 mmHg reduced TB by 33.7%, while Q by only 17.4%. In the Series II experiment, we controlled the hindlimb Q and perfusion pressure (Pa) by a roller pump. At an initial constant Q (14.4 ml/min) the Pa averaged 99.7 mmHg and the TB amounted to 18.2 +/- 1.6 ml/20 min. SNP infusion decreased Pa to 58.6 mmHg and TB to 13.8 +/- 0.8 ml/20 min (P less than 0.01). The Q was then increased to 37.2 ml/min to elevate Pa (99.5 mmHg) during SNP infusion. Surprisingly, TB at this condition averaged 18.9 +/- 2.1 ml/20 min, a value not different from the control (P greater than 0.5). After the SNP infusion was stopped, Pa was elevated to 137.0 mmHg at the same Q (37.2 ml/min). The amount of TB at this condition was greatly increased to 48.6 +/- 2.9 ml/20 min (P less than 0.001). The results indicate that SNP decreases SAP, hindlimb Q, vascular resistance and TB in dogs with baroreceptor denervation. The data obtained from the studies of the constant Q perfusion with and without SNP suggest that the major determinant of tissue bleeding is blood pressure instead of blood flow.