Background The relation between left ventricular (LV) oxygen consumption (M hivin Vo-2) and pressure-volume area (PVA) developed in isolated hearts provides a powerful method to understand cardiac energetics. We investigated application of this relation to the intact circulation, determining its response to steady-state and transient load alterations and enhanced contractility in conscious animals. Methods and Results Eight dogs were instrumented to measure LV pressure (micromanometer), LV volume (three sonomicrometers), and left circumflex and anterior descending coronary artery flows (ultrasonic flowmeter). Data were acquired after recovery from the surgery with the animals awake and unsedated. After administration of hexamethonium and atropine, steady-state loading conditions were changed with phenylephrine or nitroprusside in four to five steps before and during the infusion of dobutamine (6 to 10 mu-g cntdot kg-1 cntdot min-1). M hivin Vo-2 and PVA obtained under steady-state conditions were linearly correlated both before and during dobutamine. The M hivin Vo-2-PVA relation obtained on a beat-to-beat basis during transient caval occlusion was less linear and not coincident with the steady-state relation. Dobutamine shifted the steadystate M hivin Vo-2-PVA relation upward in all hearts, increasing the M hivin Vo-2 axis intercept of the M hivin Vo-2-PVA relation (P lt .01). This intercept correlated with ventricular contractility assessed by the slope (E-es) of the LV end-systolic pressure-volume relation determined by caval occlusion (r=.76, P lt .05). The slope of the M hivin Vo-2-PVA relation increased with dobutamine in seven of eight animals, with the inverse of the slope (representing contractile efficiency) being 31+-6% during control and 24+-6% after dobutamine (P=.06). Conclusions M hivin Vo-2 and PVA are linearly related during steady-state alterations in loading conditions in conscious dogs but not on a beat-by-beat basis during transient caval occlusion. Increase in contractility by dobutamine produces an upward shift of the M hivin Vo-2-PVA relation.