AJP - Heart and Circulatory Physiology, Vol 247, Issue 2 283-H294, Copyright © 1984 by American Physiological Society
Time constant of isovolumic pressure fall: new numerical approaches and significance
G. Martin, J. V. Gimeno, J. Cosin and M. I. Guillem
The effects of left ventricular diastolic pressure changes on the values of
the time constant (T) of isovolumic pressure fall are controversial.
Normally, T is calculated either by linear regression of 1n left
ventricular pressure (LVP) vs. time (TL) or by using an exponential model
with asymptote (PB, extrapolated base-line pressure to which LVP would fall
if decay continued indefinitely). This study, in intact dogs, has been
designed to revise the effects that drugs that alter load (angiotensin and
nitroprusside, without and with autonomic blockade) and inotropism
(isoproterenol and propranolol) might have on the relaxation rate using
different numerical methods. We found that 1) the upward or downward
translation of the LVP curve had an important effect on the value of TL
calculated by the semilogarithmic method; 2) when a model with asymptote
was used, the simultaneous changes of T and PB, were not related to the
dose of the drug; 3) the values of TL and the -dP/dt values extrapolated to
15 mmHg from a model with PB, were much more sensitive to beta-agonist or
antagonist drugs than to others whose action was through load alteration.
We feel that some of the earlier studies on relaxation rate determinants
should be revised as possibly some of the conclusions have been based on
artifacts introduced by the method chosen for the computation of the
relaxation parameters.
