The Annals of Thoracic Surgery, Vol 30, 472-481, Copyright © 1980 by The Society of Thoracic Surgeons
Intramyocardial pH during elective arrest of the heart: relative effects of hypothermia versus potassium cardioplegia on anaerobic metabolism
GJ Wilson, JM Robertson, FJ Walters, DJ Steward and DC MacGregor
Using an intramyocardial pH needle probe (21 gauge) to monitor myocardial
metabolism during ischemia, we determined the effect of potassium
cardioplegia at both moderate and deep hypothermia. Five groups of 5 dogs
each were placed on cardiopulmonary bypass and the pH probe was inserted
approximately 10 mm into the left ventricular free wall. Cardiac ischemia
was achieved by cross-clamping the ascending aorta at 37 degrees C (Group
1), 27 degrees C (Group 2), or 17 degrees C (Group 3). In the remaining two
groups, aortic cross-clamping was followed by the infusion of 600 to 800 ml
of potassium cardioplegic solution adjusted to cardiac temperatures of 27
degrees C (Group 4) or 17 degrees C (Group 5). In each group, myocardial
temperature was maintained constant, electrical and mechanical activity
observed, and pH recorded until a plateau was reached or for 3 hours. Our
results show a progressive and significant decrease in the metabolic rate
with reduction in temperature over the 37 degrees to 17 degrees C range. By
abolishing contractile activity, potassium cardioplegia markedly reduces
the rate of hydrogen ion accumulation at 27 degrees C, but at 17 degrees C
the additive effect of cardioplegia is much less pronounced. These
observations support the principle of reducing contractile activity to a
minimum during elective arrest of the heart but indicate that potassium
cardioplegia does little to further reduce the rate of anaerobic
metabolism, as shown by the measurement of intramyocardial pH, under
conditions of deep hypothermia.
