Effect of hypothermia and cardioplegia on intramyocardial voltage and myocardial oxygen consumption

Intramyocardial voltage and myocardial oxygen consumption were measured in the fibrillating heart between the temperatures of 37 degrees C and 25 degrees C and in the arrested heart after infusion of potassium cardioplegic solution in 10 adult mongrel dogs. Electrical activity from the myocardium was recorded using specially designed plunge electrodes, and intramyocardial voltage was monitored by an in-line voltmeter. Myocardial oxygen consumption gradually decreased from 5.8 +/- 0.6 ml O2/min at 37 degrees C to 2.3 +/- 0.5 ml O2/min at 25 degrees C. In contrast, hypothermia did not cause a similar decrease in intramyocardial voltage which remained within a range of 1.8 +/- 0.5 mV to 2.4 +/- 0.5 mV between the temperatures of 37 degrees C and 25 degrees C. The infusion of potassium cardioplegic solution resulted in a dramatic decrease in voltage to 43 +/- 5 microV, and myocardial oxygen consumption fell to 0.5 +/- 0.3 ml O2/min. Our data demonstrated that the mean voltage of the fibrillating heart remains constant between the temperatures 37 degrees C and 25 degrees C and myocardial oxygen consumption decreases with hypothermia, which suggests that voltage does not correlate with the level of myocardial oxygen consumption. Myocardial oxygen consumption and intramyocardial voltage, however, decrease dramatically when cardioplegia is instituted.