VENTRICULAR PERFORMANCE Efficiency of energy transfer from pressure - volume area to external mechanical work increases with contractile state and decreases with afterload in the left ventricle of the anesthetized closed - chest dog

We studied the effects of ventricular end-systolic elastance (Ees) and effective arterial elastance (Ea) on the efficiency of energy transfer from pressure-volume area (PVA) to external mechanical work (EW) in the left ventricle of anesthetized closed-chest dogs. PVA represents the total mechanical energy generated by ventricular contraction, which is an intermediate form of energy between oxygen consumption, the total energy input, and EW, the effective energy output. PVA and EW were determined from ventricular pressure and volume, which were continuously measured with a volumetric conductance catheter. Measurements of Ees were obtained by transiently increasing afterload by an inflation of a Fogarty catheter in the thoracic descending aorta. Ea was determined as the ratio of end-systolic pressure to stroke volume. The EW/PVA efficiency of a steady-state contraction increased from 55% to 64%, with a 58% increase in Ees after dobutamine. Ees, which was smaller than Ea before dobutamine, became nearly equal to Ea after dobutamine, maximizing EW for a given end-diastolic volume. EW/PVA efficiency decreased with an abrupt increase in afterload before and after dobutamine. The sensitivity of the decrease in the EW/PVA efficiency to an increase in end-systolic pressure was significantly less after than before dobutamine. We could account for all these changes in EW/PVA efficiency by the relative changes in Ees and Ea in the pressure-volume diagram. Circulation 77, No. 5, 1116-1124, 1988. MECHANICAL EFFICIENCY of the heart as the ratio of external mechanical work (EW) to myocardial oxygen consumption (V02) varies between 0 and 30% as a function of ventricular loading conditions and contractile state.1-4 We have recently found that this efficiency can be divided into two steps: the efficiency of energy transfer from V02 to pressure-volume area (PVA) as the first step, and the efficiency of energy transfer from PVA to EW as the second step,4-6 as shown in figure 1, A. PVA, which represents the total mechanical energy generated by ventricular contraction, is an intermediate form of energy between the first and second steps.4 6 PVA is defined as the area circumscribed by the end-systolic and end-diastolic pressure-volume relations and the systolic pressure-volume From the Department of Cardiovascular Dynamics, National Cardiovascular Center Research Institute, Osaka, Japan. Supported in part by a grant-in-aid (61480102) for Scientific Research from the Ministry of Education, Science, and Culture, and a Research grant (60C-3) for Cardiovascular Diseases from the Ministry of Health and Welfare of Japan. Address for correspondence: Takashi Nozawa, M.D., National Cardiovascular Center, 5 Fujishirodai, Suita, Osaka, Japan. Received April 8, 1987; revision accepted Jan. 7, 1988. 1116 trajectory, as shown in the diagram5-8 in figure 1, B. We have already extensively studied the efficiency of energy transfer from V02 to PVA and found it a function of loading conditions and contractile state in excised hearts as well as hearts in situ.6-8 The loadand contractility-dependent overall efficiency from V02 to EW could be better understood by the quantitative study of the efficiency of energy transfer from PVA to EW as a function of afterload and contractile state. In this study we wanted to determine whether we could theoretically predict the changes in EW/PVA efficiency when contractility and afterload were altered. The theoretical predictability of this efficiency would greatly facilitate the quantitative understanding of the changes in cardiac mechanical efficiency under normal and pathologic conditions. To this end, we used a volumetric conductance catheter9 to study in situ left ventricles of anesthetized closed-chest dogs, the feasibility of which has already been established.'10' 1l To analyze experimental results, we used Ees and Ea. Ees, end-systolic ventricular elastance, is the slope of the end-systolic pressure-volume relation (ESPVR),12-14 CIRCULATION by gest on A ril 6, 2017 http://ciajournals.org/ D ow nladed from LABORATORY INVESTIGATION-VENTRICULAR PERFORMANCE