The effect of arterial pH on whole body oxygen uptake during hypothermic cardiopulmonary bypass in man.

To test the hypothesis of Rahn and Reeves that the optimal arterial pH during hypothermia follows the alpha-stat concept, we studied 50 patients during coronary artery bypass grafting or valve replacement (or both) and aortic occlusion. Intravenous anesthesia was produced by high-dose fentanyl. Thiopentone 3 mg.kg-1 body weight and pancuronium 0.1 mg.kg-1 body weight were given at the start of cardiopulmonary bypass to prevent shivering. When the temperature of venous blood reached 25 degrees C, pump flow was reduced to 1.8 L.m-2.min-1. In 17 patients (group I), arterial pH was changed in a randomized order either from high to low or from low to high by adjusting the carbon dioxide fraction of the gas flow to the bubble oxygenator. At the end of 10-minute periods with stable arterial pH, oxygen uptake was calculated by multiplying pump flow and the arteriovenous oxygen difference. In 33 other patients (group II), arterial pH was kept constant during the period of stable hypothermia at 25 degrees C, and two to five determinations of oxygen uptake were performed in each patient. The carbon dioxide fraction in the gas and arterial pH varied between the patients. In group I, oxygen uptake was 31.4 ml.m-2.min-1 at pH 7.31 and 40.3 ml.m-2.min-1 at pH 7.52-a significant difference (p less than 0.001). In group II, oxygen uptake was positively correlated to arterial pH (r = 0.52, p less than 0.01). Thus the results were in line with the alpha-stat concept of acid-base management during hypothermia.