Ventricular stroke work and efficiency both remain nearly optimal despite altered vascular loading

Recent clinical and animal studies have suggested that ventricular- vascular coupling normally operates at either optimal ventricular efficiency (EFF = stroke work/myocardial oxygen consumption) or stroke work (SW) and that efficiency in particular is compromised by cardiac dysfunction. These distinctions between coupling states at maximal work vs. efficiency are largely based on theoretical models. To date, there are few direct experimental data defining optimal conditions for each parameter, respectively, in the same heart or tests of whether changes from these conditions must produce significant declines in both parameters. Therefore, 10 isolated blood-perfused canine hearts were studied at varying contractilities, with the heart ejecting into a simulated three-element Windkessel model of arterial impedance. For a given inotropic state [indexed by the slope of the end-systolic pressure-volume relationship (E(es))], myocardial oxygen consumption and SW were measured over a broad range of afterload resistances. The latter was indexed by the effective arterial elastance (E(a)) and ventricular-vascular interaction expressed by the ratio of E(a) to E(es) (E(a)/E(es)). On average, maximal SW occurred at E(a)/E(es) = 0.80 ± 0.16, whereas EFF was maximal at E(a)/E(es) = 0.70 ± 0.15 (P < 0.01). However, these differences were small, and both SW and EFF were ≥90% of their respective optima over a broad overlapping range of E(a)-to-E(es) ratios (0.3-1.3, corresponds with ejection fractions ranging from ~40 to 80%). These data show that both SW and efficiency are nearly maximal under many conditions of ventricular-vascular interaction. This suggests that precise optimization of either parameter is unlikely to be a major target of cardiovascular homeostasis under many circumstances.