Acid-base regulation, alpha-stat, and the emperor's new clothes.

Considerable time and effort have been expended to determine the most appropriate technique for management of a patient's acid-base status during hypothermic cardiopulmonary bypass. A critical question is whether to maintain plasma pH at 7.4 regardless of temperature (pH-stat) or to permit a relative alkalosis as the patient is cooled (alpha-stat). Until recently, there has been a remarkable lack of evidence in the literature for a consistent physiological benefit provided by one protocol over the other. The alpha-stat versus pH-stat controversy has taken on the characteristics of the emperor's new clothes, with alpha-stat winning by default because of theoretical arguments and because it was technically easier to perform. Part of the explanation is the realization that cellular mechanisms are capable of maintaining intracellular pH despite fluctuations in extracellular conditions. The prevailing plasma pH does have strong influence over cerebral blood flow, even to the point of overriding normal autoregulatory mechanisms. Recent evidence suggests that cerebral blood flow variations between alpha-stat and pH-stat conditions have important implications for patient outcomes.