Influence of ventricular contractility on non-work-related myocardial oxygen consumption

The relationship between myocardial oxygen consumption (MVO2) and the total pressure-volume area (PVA), which represents the total mechanical work performed during a cardiac cycle, has been shown to be linear and independent of loading conditions: MVO2 = aPVA + b. When inotropic state is enhanced, the MVO2-PVA relation shifts upward (increase in b), and when inotropic state is depressed the relation shifts downward (decrease in b). However, the quantitative relationship between contractility and b (the non-work-related myocardial oxygen consumption) determined over a wide range of contractilities is not known. In seven isolated blood perfused canine hearts, left ventricular (LV) contractility was increased by dobutamine and decreased with nifedipine or reduction of coronary blood flow. At each level of contractility, the end-systolic pressure-volume relationship (ESPVR) and the MVO2-PVA relation were determined. For each heart, the resulting values of b (ml O2/beat) were plotted as a function of Emax (mmHg/ml), an index of contractility defined as the slope of the ESPVR. There was a linear relation between Emax and b over a wide range of contractilities; on average, b (ml O2/beat) = 0.0036 Emax (mmHg/ml) + 0.0101 [r = 0.929-0.978 (95% confidence interval)], when Emax was varied over an average range of 2.8-9.6 mmHg/ml. These results suggest a common underlying determinant of contractility and non-work-related oxygen consumption.