Effect of exercise on left ventricular mechanical efficiency in conscious dogs

We studied the effect of exercise (7.2 to 8.0 km/h) on the efficiency of the conversion of metabolic energy to external work or stroke work (SW) by the left ventricle (LV).

Energy use was calculated from LV myocardial oxygen consumption per beat (MVO2). LV volume was calculated from orthogonal dimensions and coronary flow measured with ultrasonic flow probes. The total mechanical energy of the LV was calculated as the pressure-volume area (PVA). At rest, the MVO2-PVA point fell on the MVO2-PVA relation determined by steady-state changes in arterial pressure produced by graded infusions of phenylephrine. Exercise increased the slope (Ees) of LV end-systolic pressure-volume (PV) relation by 29%. During exercise, the MVO2-PVA point shifted to the right only slightly above the control MVO2-PVA relation by 0.007 +/- 0.005 mL O2.beat-1.100 g LV-1. Despite the increase in ventricular contractility with exercise, the PVA/MVO2 ratio was unchanged because of the marked increase in PVA. During exercise, the transmission of total mechanical energy to external work (SW/PVA) increased from 65 +/- 5% to 72 +/- 4% (P < .01) as the ratio of the arterial end-systolic elastance to Ees decreased from 1.1 +/- 0.2 to 0.8 +/- 0.1 (P < .05). Thus, LV mechanical efficiency (SW/MVO2 = SW/PVA.PVA/MVO2) improved from 12.9 +/- 1.5% to 14.3 +/- 1.1% (P < .05) during exercise.

Exercise increases the efficiency of conversion of metabolic energy to external work by the LV due to alteration in LV arterial coupling resulting in increased production of mechanical energy and enhanced transmission of mechanical energy to external work, which more than offsets any increased metabolic cost of the enhanced contractility.