Figure 1. Schematic illustrations of the framework of the Emax(a contractility index)-PVA (systolic pressure volume area, a measure of total mechanical energy)-VO2(myocardial oxygen consumption per beat) relationship fully used in this study. A, Emaxand PVA in the left ventricular (LV) pressure-volume (P-V) diagram. Emaxis the slope of the line connecting the end-systolic P-V point and V0(see below). E sub max is alternatively defined as the slope (Ees) of the end-systolic pressure-volume relation (ESPVR) in a stable contractile state. PVA is the area surrounded by the ESPVR line, the end-diastolic P-V relation curve, and the systolic P-V trajectory. V0is a volume at which LV isovolumic peak pressure and, hence, PVA are zero. In an isovolumic contraction, P-V data points move up and down on the vertical systolic P-V trajectory as indicated by arrows, and PVA is composed only of potential energy. B, The linear VO2-PVA relationship in a volume loading run (see text) in a stable contractile state (thick solid line). The slope a of this relationship represents the oxygen cost of PVA. VO2consists of two components at the VO2intercept b of this relation: the PVA-dependent VO2corresponding to the energy use for crossbridge cycling, and the PVA-independent VO2(unloaded VO2) corresponding to the energy use for the total calcium handling in the excitation-contraction (E-C) coupling and for basal metabolism. C, Three volume loading VO2-PVA relationships in the baseline contractility (thick solid line) and in two altered contractilities (thin solid lines). They usually are parallel to each other, having the same oxygen cost of PVA (slope a). Only the PVA-independent VO2(b, indicated by arrows) increases or decreases as Emaxincreases or decreases, respectively. In an inotropism run at a constant LV volume, VO2-PVA data point (filled circle) deviating from a baseline data (empty circle) with changes in Emaxforms a new steeper relationship. This steeper VO2-PVA relationship is the composite VO2-PVA relationship (upward and downward relationships are shown as thick solid lines with arrow heads). D, The relationship between the PVA-independent VO2and Emax. The slope c of this relationship represents the oxygen cost of Emax, and the PVA-independent VO2intercept d indicates the PVA-independent VO2at zero Emaxnearly equal to basal metabolism.