Theoretical maximal flow of the left ventricle is sensitive to change in ventricular afterload

Systolic mechanical behavior of the ventricular pump can be quantified by both maximal systolic elastance and theoretical maximal flow. How sensitive these two model parameters are to changes in loading conditions has never been examined, however. The aim of the study is to determine the roles of these two parameters in the perturbation of ventricular loads. The loading conditions are altered by the use of methoxamine, a specific alpha(1)-selective adrenergic agonist. Left ventricular pressure and ascending aortic flow waves in male Fischer 344 rats are measured by a high-fidelity pressure sensor and electromagnetic flow probe, respectively. Isovolumetric pressure of the left ventricle is estimated from the instantaneous pressure of an ejecting contraction by a curve-fitting technique. The two parameters that characterize systolic pumping mechanics of the left ventricle are inferred by making use of an elastance-resistance model. Results in this study show that the theoretical maximum flow, the amount of outflow generated by the ventricle if it were to eject under zero load condition, is sensitive to change in ventricular afterload. On the contrary, the maximal systolic elastance is independent of loading conditions in a given constant contractile state of the ventricle. It is believed that afterload-dependent behavior of the theoretical maximal flow is the major factor responsible for the poor predictive performance of the elastance-resistance model. (C) 1998 Academic Press.