EFFECT OF EXERCISE ON LEFT-VENTRICULAR MECHANICAL EFFICIENCY IN CONSCIOUS DOGS

Background 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 s troke work (SW) by the left ventricle (LV). Methods and Results Energy use was calculated from LV myocardial oxygen consumption per beat (MV o(2)). LV volume was calculated from orthogonal dimensions and coronar y flow measured with ultrasonic flow probes. The total mechanical ener gy of the LV was calculated as the pressure-volume area (PVA). At rest , the MVo(2)-PVA point fell on the MVo(2)-PVA relation determined by s teady-state changes in arterial pressure produced by graded infusions of phenylephrine. Exercise increased the slope (E(es)) of LV end-systo lic pressure-volume (PV) relation by 29%. During exercise, the MVo(2)- PVA point shifted to the right only slightly above the control MVo(2)- PVA relation by 0.007+/-0.005 mL O-2.beat(-1).100 g LV(-1). Despite th e increase in ventricular contractility with exercise, the PVA/MVo(2) ratio was unchanged because of the marked increase in PVA. During exer cise, 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 ar terial end-systolic elastance to E(es) decreased from 1.1+/-0.2 to 0.8 +/-0.1 (P<.05). Thus, LV mechanical efficiency (SW/MVo(2)=SW/PVA.PVA/M Vo(2)) improved from 12.9+/-1.5% to 14.3+/-1.1% (P<.05) during exercis e. Conclusions Exercise increases the efficiency of conversion of meta bolic energy to external work by the LV due to alteration in LV arteri al coupling resulting in increased production of mechanical energy and enhanced transmission of mechanical energy to external work, which mo re than offsets any increased metabolic cost of the enhanced contracti lity.