The effects of three hypotensive agents, sodium nitroprusside (SNP), nitroglycerin (NTG), and adenosine triphosphate (ATP), on blood flow distribution and vascular capacitance were examined in dogs anesthetized with sodium pentobarbital. To eliminate the modification by the baroreflex, carotid sinus was denervated and aortic and cardiopulmonary vagal fibers were sectioned. Total systemic circulation was divided into two parallel compartments, splanchnic (SP) and extra-splanchnic (ESP) vascular beds. Alteration of vascular capacitance was assessed by a change in systemic blood volume with constant cardiac output and constant venous pressure using a total heart-lung bypass. SNP- and ATP-induced hypotension caused blood flow redistribution from the SP to ESP beds, and this redistribution is greater (P < 0.01) with ATP than that with SNP. In contrast, NTG-induced hypotension did not significantly cause redistribution. Systemic blood volume was increased during NTG- (10.4 ± 2.2 ml/kg), and SNP-induced (4.8 ± 1.1 ml/kg) hypotension. The increase by NTG was significantly greater (P < 0.05) than that by SNP. In contrast, ATP-induced hypotension did not significantly change systemic blood volume. Since redistribution can result in a passive change in vascular capacitance, the differences in capacitance among SNP, NTG, and ATP can be explained in part by differences in redistribution of blood flow. Redistribution of blood flow from SP to ESP beds can increase venous return due to increasing the slope of the venous return curve. The results suggest that redistribution should be taken into consideration in evaluating the hemodynamic changes during induced hypotension.
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