Studies of the effects of hypothermia on regional myocardial blood flow and metabolism during cardiopulmonary bypass. III. Effects of temperature, time, and perfusion pressure in fibrillating hearts.

This study compares the effects of normothermic and hypothermic spontaneous fibrillation at perfusion pressures of 100 and 50 mm. Hg on the adequacy and distribution of coronary blood flow. During normothermia (37 degrees C.), subendocardial oxygen delivery decreased 45 per cent ( p less than 0.01) and left ventricular flow became redistributed away from the subendocardium (endo-epi flow ratio fell from 1.2 to 0.8) when perfusion pressure was lowered to 50 mm. Hg; abnormal glycolysis (lactate washout) became evident when perfusion pressure was raised to 100 mm. Hg and ischemia was demonstrated by histochemical stains. Hypothermia (28 degrees C) reduced myocardial oxygen uptake 52 per cent (p less than 0.01) at 100 mm. Hg perfusion pressure; left ventricular flow, distribution, and metabolism did not change from control values. Lowering perfusion 50 mm Hg caused a pronounced reduction in subendocardial oxygen delivery (63 per cent, p less than 0.01); abnormal glycolysis developed and histochemical ischemia was seen. These studies show that lowering perfusion pressure in normothermic fibrillating hearts impairs oxygen delivery to the left ventricular subendocardium. While hypothermia significantly reduces left ventricular oxygen requirements, the ventricle is not protected against subendocardial ischemia if perfusion pressure falls to levels frequently used during clinical open-heart surgery.