Autoregulation of cardiac output by passive elastic characteristics of the vascular capacitance system.

After a change in cardiac output, the magnitude of potential blood volume redistribution was investigated in 10 dogs anesthetized with chloralose. All of the venous return was pumped into a reservoir, using servocontrolled pumps to maintain fixed superior and inferior vena cava pressures. The cardiac output was set at various levels by pumping from the reservoir into the right atrium. Changes in reservoir volume were assumed to reflect the changes in vascular blood volume. After measuring the control responses, cardiovascular reflexes were blocked with hexamethonium. Reducing the cardiac output, for example, from 110 to 80 ml/(min.kg) with reflexes intact, caused a 9.2-ml/kg transfer of blood from the dog to the reservoir. With reflexes blocked, the same change in cardiac output caused 6.8 ml/kg of the blood to be transferred. Under the control conditions, throughout the range of 50-140 ml/(min.kg), an increase or decrease of cardiac output of 1 ml/(min.kg) elicited a 0.304 +/- 0.086 (mean +/- SD) ml/kg change in dog blood volume; with reflexes blocked, the flow sensitivity was 0.239 +/- 0.062 ml/kg. Thus, only 21% of the total blood volume redistribution was attributable to active reflex responses. Deterioration of the preparation may have attenuated the magnitude of active reflex activity. Neither the systemic vascular compliance of 1.80 +/- 0.35 ml/mm Hg.kg nor the fraction of venous return from the superior vena cava of 26.5 +/- 4.6% was significantly changed by reflex blockade.(ABSTRACT TRUNCATED AT 250 WORDS)