Comparable efficiencies of chemomechanical energy transduction between beating and fibrillating dog hearts

We have recently proposed a mechanical index, equivalent pressure-volume (PV) area (ePVA), as a measure of the total mechanical energy during ventricular fibrillation (VF). ePVA, an analogue of the PV area (PVA) of a beating heart, is the area surrounded by the isobaric line drawn at the VF pressure, the end-systolic and end-diastolic PV relations of the beating state. In the present study, using a closed-air chamber system, we actually produced isobaric contractions, PVAs of which were identical with ePVAs during VF. Myocardial O2 consumption (VO2) during VF was measured and compared with the estimated value from VO2 of isobaric contraction with identical PVA and equivalent heart rate (eHR). eHR, an estimate of the contraction frequency of each myocyte during VF, was determined from unloaded VO2 in beating and fibrillating states. The efficiency of the energy conversion from VO2 for mechanical purposes to the total mechanical energy (contractile efficiency) during VF was calculated as the reciprocal of the slope of the VO2-ePVA relation. The estimated VO2 during VF agreed with measured VO2 (r = 0.96, regression coefficient = 1.13). The slope of the VO2-ePVA relation during VF was not different from that in the beating state in all hearts by analysis of covariance, and mean contractile efficiency during VF (51 +/- 23%) was not significantly different from that in the beating state (40 +/- 12%). We conclude that 1) ePVA is considered to represent the total mechanical energy during VF, and 2) contractile efficiency during VF is comparable to that in the beating state.