DOBUTAMINE RESTORES THE REDUCED EFFICIENCY OF ENERGY-TRANSFER FROM TOTAL MECHANICAL WORK TO EXTERNAL MECHANICAL WORK IN STUNNED PORCINE MYOCARDIUM

Objective: In order to determine whether the relatively high oxygen consumption of stunned myocardium is related to decreased mechanical efficiency, myocardial oxygen consumption (MVO2) and its major determinants were studied in 10 open chest anaesthetised pigs. Methods: According to the time varying elastance concept, MVO2 is determined by contractility (E(max)) and total mechanical work (PLA), which is the sum of the external work (EW) and potential energy (PE). Mechanical efficiency (EW/MVO2) equals the product of EW/PLA (=efficiency of energy transfer or EET) and PLA/MVO2. E(max) is the slope of the end systolic pressure-segment length relationship, determined by gradually clamping the aorta. PLA is the area enclosed by the end systolic pressure-segment length relationship and the pressure-segment length trajectory. EW is the area of the pressure-segment length loop. Systemic haemodynamics, regional segment shortening, and MVO2 were determined at baseline, during stunning (two sequences of 10 min occlusion and 30 min of reperfusion), after a subsequent 50 beats.min-1 increase in heart rate by atrial pacing and additional infusion of 2 mug.kg-1.min-1 dobutamine. Results: Stunning decreased segment shortening from 18.2(SEM 1.9)% to 10.2(1.5)%, MVO2 from 4.16(0.27) X 10(-2) to 2.84(0.25) X 10(-2) mumol-beat-1.g-1, and E(max) from 47(9) to 23(3) mm Hg.mm-1 (all p<0.05). PLA decreased by 13(4)%, as EW decreased by 42(6)%, and PE tended to increase. Although EET decreased from 0.58(0.04) to 0.40(0.03) (p<0.05), there was no decrease in the mechanical efficiency, as an increase in PE caused an increase in PLA/MVO2 which compensated for the decrease in EET. Dobutamine infusion increased E(max) and EW per beat to 120(23)% and 67(8)% of baseline, respectively, while MVO2 [4.12(0.53) mumol-beat-1.g-1] and EET [0.57(0.04)] returned to baseline. Conclusions: In stunned myocardium, mechanical efficiency is not decreased despite a decrease in EET. The increase in EET after dobutamine may explain the lack of the excessive increase in MVO2.