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 (Emax) 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. Emax 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 micrograms.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 Emax 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 Emax 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.
Krams
R
R
Laboratory for Experimental Cardiology, Erasmus University Rotterdam, The Netherlands.
Duncker
D J
DJ
McFalls
E O
EO
Hogendoorn
A
A
Verdouw
P D
PD
D000818:Animals; D004195:Disease Models, Animal; D004280:Dobutamine / Q000494:pharmacology*; D004734:Energy Metabolism / Q000187:drug effects*; D005260:Female; D006321:Heart / Q000187:drug effects; D006339:Heart Rate / Q000187:drug effects; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects; D015428:Myocardial Reperfusion Injury / Q000378:metabolism*; D009206:Myocardium / Q000378:metabolism*; D010101:Oxygen Consumption / Q000187:drug effects; D013268:Stimulation, Chemical; D013318:Stroke Volume / Q000502:physiology; D013552:Swine
D004280:Dobutamine