Estimation of the Left
Ventricular Relaxation Time Constant τ Requires Consideration of
the Pressure Asymptote
S. F. J. LANGER^{1}, H. HABAZETTL^{1,2},
W. M. KUEBLER^{2}, A. R. PRIES^{1}
^{1}Institute of Physiology, Campus Benjamin Franklin,
Charité – University of Medicine Berlin, and ^{2}Institute
of Anesthesiology, Deutsches Herzzentrum Berlin, Berlin, Germany
Received March 12, 2004
Accepted December 30, 2004
Online available February 16, 2005
Summary
The left ventricular isovolumic pressure decay, obtained by
cardiac catheterization, is widely characterized by the time
constant τ (tau) of the exponential regression p(t)= P_{¥}+(P0–P_{¥})exp(–t/τ).
However, several authors prefer to prefix P_{¥}=0
instead of coestimating the pressure asymptote empirically;
others present τ values estimated by both methods that often
lead to discordant results and interpretation of lusitropic
changes. The present study aims to clarify the relations between
the τ estimates from both methods and to decide for the more
reliable estimate. The effect of presetting a zero asymptote on
the τ estimate was investigated mathematically and empirically,
based on left ventricular pressure decay data from isolated
ejecting rat and guinea pig hearts at different preload and
during spontaneous decrease of cardiac function. Estimating τ
with preset P_{¥}=0
always yields smaller values than the regression with
empirically estimated asymptote if the latter is negative and
vice versa. The sequences of τ estimates from both methods can
therefore proceed in reverse direction if τ and P_{¥}
change in opposite directions between the measurements. This is
exemplified by data obtained during an increasing preload in
spontaneously depressed isolated hearts. The estimation of the
time constant of isovolumic pressure fall with a preset zero
asymptote is heavily biased and cannot be used for comparing the
lusitropic state of the heart in hemodynamic conditions with
considerably altered pressure asymptotes.
Key words
Ventricular function • Ventricular relaxation • Relaxation time
constant • Rat • Guinea pig
