Cardiac cooling increases Emax without affecting relation between O2 consumption and systolic pressure-volume area in dog left ventricle.

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Abstract
We studied the effects of cardiac cooling by 7 +/- 2 degrees C (SD) from 36 degrees C on both contractility index (Emax) and the relation between O2 consumption per beat (VO2) and systolic pressure-volume area (PVA) of the left ventricle in the excised cross-circulated dog heart preparation. PVA represents the total mechanical energy generated by a contraction. The VO2-PVA relation divides measured VO2 into unloaded VO2 and excess VO2. The slope of the VO2-PVA relation represents inversely the efficiency of the contractile machinery to convert chemical energy from the excess VO2 to total mechanical energy. Cooling is known to decrease myosin ATPase activity (Q10 of 2-3), which in turn is expected to increase the chemomechanical efficiency of cross bridges. Therefore, we expected an increase in the efficiency and hence a decreased slope of the VO2-PVA relation with cooling. The cooling increased Emax by 46 +/- 13% and the time to Emax by 45 +/- 27%. Pacing rate was constant or had to be slightly decreased to avoid arrhythmias with cooling. We found that neither the slope of the VO2-PVA relation nor unloaded VO2 significantly (p greater than 0.05) changed with the cooling. This result contradicts the expected increase in the efficiency with cooling. We conclude that cardiac cooling by 7 degrees C from 36 degrees C does not increase the efficiency of the contractile machinery in excised cross-circulated dog left ventricle.