A simultaneous recording of ventricular pressure and volume during cardiac cycle generates pressure-volume (P-V) loops which represent a cyclical nature of the pressure and volume relationships in a cardiac cycle. The end-systolic pressure–volume relationship (ESPVR) plot is obtained when aortic pressure is varied over several beats, and a line is fitted through end-systolic points of the recordings. The slope of the line corresponds to maximal ventricular elastance (E_max) during contraction. Implicit in this model is the fact that the performance of the ventricle (contractility) can be characterized as time-varying elastance which waxes and wanes during the heart cycle but is independent of instantaneous pressure and volume (Fig. 10.1). During a stable contractile state, the slope of the ESPVR line is a reproducible index of contractility. Since the heart’s energy expenditure is related to its mechanical work, a close relationship exists between its energetic demands and mechanical activity. The total work of the heart consists of the external work which the heart performs against the aortic pressure (afterload) and the internal work needed for excitation–contraction coupling. Suga showed that the total mechanical energy expended by the heart corresponds to the area under the ESPVR curve [3, 4]. It has been further demonstrated, purely on phenomenological grounds, that the left ventricular oxygen consumption per beat corresponds linearly with the systolic pressure–volume area (PVA) [5]. It is of note that at a given contractile state, the PVA/O₂ cost shows a remarkable stability under various pre- and afterload conditions. The PVA/O₂ cost is moreover independent of heart rate, the type of contraction, i.e., whether ejecting or isovolumic, and of cardiac output, as will be shown in Sect. 16.​6.