Two-port analysis of systemic venous and arterial impedances.

Hemodynamic properties of the systemic vasculature were measured in eight anesthetized dogs using two-port impedance analysis. Blood pressures and flows were measured at the aortic root and the caval-atrial junction. Impedances were computed from 0.05 to 20 Hz to characterize the systemic vasculature. Pseudorandom variations in flow were produced with an extracorporeal perfusion system. Impedance measurements were made at carotid baroreceptor pressures of 50, 125, and 200 mmHg. A six-parameter lumped-element model best fitted the measured impedance spectra. At 125 mmHg, the mean parameter values were venous inertance, 13.5 g.kg.cm-4; venous and arterial compliances, 0.769 and 0.0214 ml.mmHg-1.kg-1; venous and arterial characteristic impedances, 0.028 and 0.084 mmHg.kg.min.ml-1; and arterial-to-venous small-vessel resistance, 0.706 mmHg.kg.min.ml-1. Regression analysis showed significant dependence of small-vessel resistance on baroreceptor pressure. The other parameters were not dependent on carotid sinus pressure, which is consistent with baroreflex control of venous unstressed volume but not compliance. We conclude that two-port impedance analysis is a useful tool for studying venous hemodynamics and the dynamic coupling between the veins and the right heart.