Vascular responses to hypercapnia in anesthetized dogs

To evaluate the vascular responses to systemic acute mild hypercapnia (Pa(CO)(2) = 65 mmHg), we determined the vascular compliance with the relation between the change in circulating blood volume and the change in central venous pressure during and after fluid infusion in dogs anesthetized with halothane in normocapnia and hypercapnia. Circulating blood volume was measured continuously by (51)Cr-labeled erythrocyte dilution method together with hemodynamic variables. Small reduction in vascular compliance (8.1 +/- 1.0 ml.mmHg(-1).kg(-1) in normocapnia, 5.8 +/- 0.5 ml.mmHg(-1).kg(-1) in hypercapnia), large reduction in delayed compliance, which were quantitated by computer simulation using Maxwell's viscoelastic model, and significant increase in blood volume in central circulation were observed in hypercapnia. The essential change in hypercapnia was concluded as the vasoconstriction in capacitance vessels. Simultaneously, the reduction of total peripheral resistance (1.09 +/- 0.08 mmHg.min.kg.ml(-1) in normocapnia, 0.98 +/- 0.07 mmHg.min.kg.ml(-1) in hypercapnia) with no change in transvascular filtration coefficient (0.14 +/- 0.02 ml.mmHg(-1).min(-1).kg(-1)) suggests the increase in shunt flow in peripheral circulation.