Pmax, end systolic elastance, and Starling's law of the heart

Starling's law is a description relating the energy released by the contracting ventricle to the end diastolic fiber length. The peak systolic pressure during isovolumic contraction is an example of this relationship; Sunagawa and coworkers (12) termed that pressure Pmax. It may also be estimated during ejecting beats. Here we compare the variable Pmax at a given end diastolic volume with that of end systolic elastance as an indicator of ventricular function, changes in either being indicative of changes in the inotropic state of the heart. We specifically investigate 2 points on the end systolic pressure-volume relationship (ESPVR) plot: 1) the Pmax, EDV point, obtained from the concept of the maximum pressure, Pmax, the pressure an isovolumically contracting ventricle would produce at a given end diastolic volume (EDV); and 2) the end systolic pressure, end systolic volume point. Both points would be expected to shift proportionally with changes in the slope of ESPVR. Data were obtained from 2 published reports in humans in the literature and from a group of 5 dogs subjected to a wide range of preloads and afterloads. High correlations (>0.9) were obtained for changes in the slope of ESPVR and the Pmax, EDV points. The latter, a more readily obtainable variable from single ejecting beats, is proposed as a version of Starling's law, the changes in which are directly comparable with the changes in end systolic elastance. The changes in the end systolic pressure, end systolic volume points, were not as reliable (<0.7).