Measurement of Aortic Compliance in Vivo

Abstract

Based on the lumped parameter Windkessel theory, compliance of the aorta of anesthetized dogs was calculated in vivo. The formula used was C=(SV-P_ST_S/R)/(P₂-P₁), where C=aortic compliance, SV=stroke volume, P_S=mean systolic pressure, T_S=duration of ejection period, R=total peripheral resistance, P₁=aortic pressure at the onset of ejection and P₂=aortic pressure at the end of ejection. Mean C with SD from 12 dogs weighing about 10 Kg. was 0.18±0.07 ml./mm.Hg. Aortic compliance was markedly increased on intravenous administration of a long active nitrite, N-ethoxy-carbonyl-3-morpholinosydnonimine (SIN-10), at a dose of 1 mg./Kg. (+237±90%, n=7, significant at P<0.001, 15 min. after injection), suggesting that this substance dilated smooth muscle in the wall of the aorta. Cardiac output and aortic pressure were both diminished upon SIN-10 administration. Though aortic compliance tended to increase as aortic pressure decreased, the increase in aortic compliance was much smaller (+41.2±45.3%, n=7, not significant at P=0.05), when a comparable decrease in aortic pressure was induced by electrical stimulation of the carotid sinus nerve.