Is τ a preload-independent measure of isovolumetric relaxation?

Abstract

Several studies have been performed in patients with a variety of myocardial disease that have identified a prolongation of τ. However, it is not clear whether prolongation of τ represents abnormal myocardial physiology of the effect of excessive load associated with a particular disease process. Accordingly, we evaluated the effect of τ of an isolated decrease in preload induced by inferior vena cava occlusion before the appearance of reflex changes in six patients designated as normal by catheterization criteria. A computer-based digitization routine identified cardiac contractions in all patients early after inferior vena cava occlusion where left ventricular end-diastolic pressure decreased (18.3 ± 6.3 to 9.3 ± 5.8, p < 0.05) while left ventricular systolic pressure (113.3 ± 13.8 to 111.8 ± 14.0, p = NS) and heart rate (66.0 ± 10.0 to 65.9 ± 10.3, p = NS) did not change. After this alteration in preload, no change in τ from baseline, as calculated by the logarithmic [T(L)], derivative [T(D)], or method of Mirsky [T( 1/2 )], was noted: T(L), 47.4 ± 6.5 to 44.6 ± 7.6; T(D), 39.3 ± 8.1 to 39.8 ± 8.4; T( 1/2 ), 33.0 ± 4.0 to 31.8 ± 4.6; all p = NS. The baseline pressure extrapolated from isovolumetric relaxation did not change in these preload beats compared with baseline (+4.26 ± 6.20 to -0.80 ± 4.87, p = NS). Subsequent beats were identified where left ventricular systolic pressure showed a numeric decrease compared with baseline (113.3 ± 13.8 to 100.8 ± 14.3, p = NS) despite no change in heart rate (66.0 ± 10.0 to 66.8 ± 10.5, p = NS). The extrapolated baseline pressure decreased in these subsequent beats compared with baseline (+4.26 ± 6.20 to -3.90 ± 4.26, p < 0.05) despite no change in τ (T(L), 47.4 ± 6.5 to 41.6 ± 8.4; T(D), 39.3 ± 8.1 to 40.8 ± 11.1; T( 1/2 ), 33.0 ± 4.0 to 31.7 ± 4.2; all p = NS). Visual display of the individual natural log of left ventricular pressure versus time values used to calculate T(L) were consistent with exponential pressure decay as an appropriate model for isovolumetric relaxation. This study demonstrates that τ is a preload independent measure of isovolumetric relaxation.