Time constant of isovolumic pressure fall: determinants in the working left ventricle

Abstract

The influence of hemodynamics and pharmacologic agents on the time course of isovolumic pressure fall was studied in the working dog, right-heart-bypass preparation. The time constant (T) was determined for the exponential portion of the left ventricular pressure (LVP) fall following maximum negative dP/dt. No significant changes in T occurred as a result of variation in cardiac input between 1.5 and 4.0 l/min at constant peak LVP and heart rate (n = 8) or variation in peak LVP between 85 .+-. 3 and 192 .+-. 3 mmHg at constant cardiac input and heart rate (n = 7). Abrupt changes in heart rate from 110 to 180 beats/min at constant stroke volume and peak LVP shortened T from 27 .+-. 4 to 23 .+-. 3 ms. (P < 0.02). A significant increase in T occurred after administration of propranolol and norepinephrine decreased T and acetylstrophanthidin after propranolol decreased T. To dissociate possible effects of increased shortening velocity on T from effects of increased inotropy, mean velocity of circumferential fiber shortening (VCF) was estimated in 9 hearts. Lowering peak LVP from 160 .+-. 5 to 83 .+-. 3 mmHg resulted in increased mean VCF from 1.19 .+-. 0.27 to 1.54 .+-. 0.31 diameters/s (P < 0.01); T did not change significantly, 42 .+-. 4 vs. 45 .+-. 4 ms (not significant). Norepinephrine caused a similar increase in mean VCF from 1.59 .+-. 0.16 to 2.00 .+-. 0.22 diameters/s (P < 0.05), but T shortened, 36 .+-. 2 to 27 .+-. 2 ms (P < 0.01). In the working preparation, T appears independent of stroke volume, peak LVP and fiber shortening velocity within broad physiological ranges. Large increases in heart rate result in small but significant shortening of T. T is altered by pharmacologic agents thought to influence the active cardiac relaxing system and/or the inotropic state.