Normalization of end-systolic pressure-volume relation and emax of different sized hearts.

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Abstract
A shift of the ventricular end-systolic pressure-volume (P-V) relation and a change in its slope Emax reasonably reflect a change in contractility in a given ventricle. However, comparison of Emax of different sized hearts may be difficult without an appropriate normalization. Emax of different sized hearts was normalized to the force-length (F-L) relation to unit mass of myocardium in the ventricular wall. The end-systolic P-V relation as (end-systolic pressure) = Emax (end-systolic volume Vi - Vd), where Vd = volume axis intercept of the end-systolic P-V relation line was formulated. Both thick wall sphere and cylinder models of the ventricle with a wall volume of Vm were used. Circumferential F-L relation of unit myocardium in different ventricular wall layers were formulated as functions of Emax, Vd and Vm. As long as the product of Emax and Vd remains constant, the F-L relation in the midwall layer and the average F-L relation in the wall apparently remain relatively unchanged regardless of wide changes in Vm and Vi. The elevation of the F-L relation curve, which represents myocardial contractility, was found to change in proportion to Emax Vd, largely independent of Vm and Vi, or the size of the ventricle.