Reconsideration of normalization of Emax for heart size.

We previously proposed Emax Vd as a normalized form of Emax for heart size relatively independent of wall volume Vm, where Emax is the slope of the end-systolic pressure-volume line and Vd is its volume axis intercept. When Emax Vd remains constant, average circumferential stress for a specified average circumferential strain in the ventricular wall also remains relatively constant, despite changes in Vd/Vm around its normal value. Because accurate determination of Vd is difficult and stress for a given Emax Vd changes slightly with Vd/Vm, we investigated whether Vd could be replaced with Vm in a normalized Emax in this analysis. As the result, we obtained a function of Vd/Vm as the coefficient by which to multiply Emax Vd or Emax Vm to yield stress for a specified strain. Using this coefficient, one can easily calculate stress for any strain from Emax, Vd, and Vm in order to compare myocardial contractility among left ventricles of different sizes. The present study confirms the importance of Vd as an indispensable reference volume for normalization of Emax, as well as the low sensitivity of Emax Vd as a normalized Emax to changes in Vd/Vm. Only when Vd/Vm remains constant is Emax Vm proportional to Emax Vd and can replace Emax Vd.