Geometric and muscle physiological determinants of cardiac stroke volume as evaluated on the basis of model calculations.

Based on model calculations a mathematical approach has been developed which permits an analysis and approximative evaluation of the significance of geometric and various muscle physiological factors for cardiac stroke volume with respect to anatomical ventricular size. Despite increasing wall stress the stroke volume generally increases with growing anatomical heart size, reaching a maximum beyond which it falls off. On the basis of the model of a thick-walled sphere for the left ventricle, stroke-volume-radius relations have been calculated for three different types of chronic ventricular enlargement (constancy of wall thickness, wall volume, or of the ratio of wall thickness to inner radius) in particular. In all three cases stroke volume increases with chronic enlargement of the heart, at least to a certain extent. Thus, stroke volume can be augmented with increasing anatomical heart size under constant contractile conditions despite decreasing ejection fraction. This fact has to be considered in the assessment of ventricular function. Nomograms are obtained by varying contractility, wall thickness, or end-systolic pressure while keeping the other contractile conditions constant, thus permitting the evaluation of the effect of therapeutic measures in the presence of cardiac dilatation.