Total Systemic Vascular Compliance Measured as Incremental Volume-Pressure Ratio

We developed a method to measure the compliance, defined as ΔVΔP, of the total systemic vascular bed (Ct) without stopping systemic flow. In 10 open-chest dogs, venous return (and cardiac output) was maintained at a constant level by inserting a perfusion pump (with no reservoir) between the caval veins and the right atrium. About 5% of the estimated total blood volume was withdrawn from the dog within 45 seconds. Resultant changes in central venous pressure were measured 10, 30, 60, 120, and 180 seconds after the volume change. The bled volume was then reinfused to repeat similar venous pressure measurements for another 180 seconds. The measurements were repeated after slightly increasing the blood volume or sectioning the vagi. The mean value of Ct measured 10 seconds after hemorrhage or reinfusion was 1.96 ± 0.10 (SE) ml/mm Hg/kg body weight before vagotomy. Paired t-tests indicated no significant difference between measurements of bleeding and reinfusion, measurements with different initial venous pressures or blood volume, or measurements before and after vagotomy. However, Ct increased with time during the 2-minute period by about 25% of the initial 10-second value (P<0.001) and remained constant thereafter.