Problems with use of the end systolic pressure-volume slope as an indicator of left ventricular contractility: an alternate method.

The use of end-systolic elastance as a parameter of left ventricular contractility is based on a theory put forward by Sagawa and is limited by an assumed linearity of the model for a time varying compliance. A major problem is the contractility-dependent curvilinearity of the end-systolic pressure-volume relationship (ESPVR), which may result in intercept volumes that are below a passive unstretched volume. Based on the experimental data of Burkhoff et al., we demonstrate the reasons for the lack of correlation of the slope (k) of the end-systolic pressure (P(es))-volume (V(es)) relationship with ventricular contractility. A parabolic relation, P(es) = a x V(es)2 + b x V(es) + c, was used for approximation of the ESPVRs, as proposed by Burkhoff et al., and the slope (k = 2 x a x V(es) + b), together with the volume axis intercepts (V0), calculated for the tangents for all P(es)/V(es) points of the ESPVRs. The results demonstrate the volume range dependence of the slope and V0 of linear regression lines. However, intercepts of the ESPVRs tangents with a midrange constant pressure line (e.g., 80 mmHg or 100 mmHg) justify the use of the shift of the end-systolic pressure-volume relationship as a parameter of left ventricular contractility. This method appears to be valid over a much wider range of end-systolic volumes than is the use of the slope alone.