The superiority of maximum fiber elastance over maximum stress-volume ratio as an index of contractile state.

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Abstract
The end-systolic pressure-volume relationship has been used as a load-independent measure of ventricular pump performance. Since comparisons of load between ventricles of different size and thickness may be more accurately made with measurements of wall stress than with those of pressure, the end-systolic stress-volume relationship at one or more levels of end-systolic stress has been used to estimate contractile function in hypertrophied ventricles. Linear regression of end-systolic stress against end-systolic volume for differently loaded beats has a slope termed maximum fiber elastance (fiber Emax) and a volume intercept termed Vo; the maximum stress-volume ratio (MSVR) for a single beat is an approximation of fiber Emax but assumes Vo = 0. However, the influence of preload, afterload, and inotropic state on these indexes has not been examined. We therefore studied the stress-volume relationship in seven open-chest dogs instrumented with ultrasonic crystals and micromanometers. Postextrasystolic potentiation (PESP) increased both the MSVR (5.7 +/- 1.83 vs 4.85 +/- 1.43 at control, p less than .05) and fiber Emax (13.93 +/- 3.24 vs 9.24 +/- 2.15 at control; p less than .05). Augmentation of preload by infusion of dextran, with the use of nitroprusside to maintain afterload relatively constant, did not significantly influence either the MSVR or fiber Emax. Vo was not significantly influenced by PESP, but was shifted to higher values by augmentation of preload (17.5 +/- 14.8 vs 13.9 +/- 11.1 ml at control; p less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

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