Slope of human left ventricular end-systolic force-length relation is independent of myocardial length

We have introduced a new contractility index (Ec), i.e., the slope of the left ventricular (LV) end-systolic force-length (Fes-Les) relation. To examine whether Ec was dependent on the LV wall myocardial length, 16 normal hearts of human subjects were evaluated to determine the LV end-systolic force-dimension (Fes-Des) and pressure-dimension (Pes-Des) relations (dimension denotes the distance between the LV septum and posterior wall). LV end-systolic pressures and dimensions were estimated simultaneously by intra-arterial cannulation and LV echocardiography. In seven subjects, the effect of a dobutamine infusion was also assessed. The Fes-Des relation was found to be nearly linear. Slopes and extrapolated dimension intercepts were obtained for the LV Fes-Des and Pes-Des relations [Ec, slope of LV Pes-Des relation (Es), and extrapolated dimension intercept of LV Fes-Des (Do), and of Pes-Des relation (D'o), respectively]. Es showed a hyperbolic relation to the baseline LV Des, whereas Ec was unrelated to it. The average variation for Ec (9.5%) was smaller than that for Es (22.5%). Dobutamine infusion increased Ec, Es, and D'o, whereas Do was not changed. Thus the assumption of linearity of the LV Fes-Les relation was found to be reasonable in normal human hearts. Do appears to provide a more accurate parameter than D'o for estimating the actual unstressed myocardial length, whereas Ec could possibly serve as an index of LV wall performance in the normal human heart that is independent of myocardial length and nearly constant between individuals.