AJP - Heart and Circulatory Physiology, Vol 244, Issue 3 417-H422, Copyright © 1983 by American Physiological Society
Myocardial relaxation. VI. Effects of beta-adrenergic tone and asynchrony on LV relaxation rate
A. S. Blaustein and W. H. Gaasch
We studied the effect of left ventricular (LV) asynchrony and alterations
in beta-adrenergic tone on the systolic load (pressure) dependency of LV
isovolumic relaxation rate in anesthetized dogs. The time constant (T) of
isovolumic exponential pressure decline was used as an index of relaxation
rate. Variably afterloaded LV contractions resulted in a progressive
increase in LV end-systolic pressure from 124 +/- 6 in the control beat to
176 +/- 11 mmHg in the third beat and a progressive lengthening of T from
19 +/- 2 to 30 +/- 4 ms. The direct relation between LV end-systolic
pressure and T was nearly linear (r = 0.98), and the slope (k) of this
relation was taken to reflect the systolic load dependency of T.
Administration of isoproterenol (n = 6) produced a decrease in k from 0.11
+/- 0.02 to 0.08 +/- 0.02 (P less than 0.05); with propranolol (n = 6), k
increased from 0.08 +/- 0.02 to 0.27 +/- 0.04 (P less than 0.01). Right
ventricular epicardial pacing (n = 6) produced an asynchronous LV
contraction and an increase in k from 0.09 +/- 0.02 (atrial pacing) to 0.25
+/- 0.04 (P less than 0.01). These studies confirm the dependency of LV
relaxation rate on systolic loads and indicate that this form of
load-dependent relaxation can be modified by alterations in beta-adrenergic
tone and LV asynchrony. The observed alterations suggest the importance of
temporal dispersion of the contraction-relaxation sequence as a mechanism
responsible for disturbed relaxation.
