Relationship of cerebral and myocardial intracellular pH to blood pH during hypothermia

The regulation of tissue pH during hypothermia is important for cellular homeostasis. The present study was undertaken to determine the relationship between blood pH and intracellular pH in the brain and heart during hypothermia in sheep and to compare these data with those in humans. Intracellular pH (pH(i)) was determined by ³¹P nuclear magnetic resonance (NMR) spectroscopic data collected from the heart and brain of sheep during cardiopulmonary bypass (CPB). Alpha-stat and pH-stat blood pH management schemes were compared. When the blood pH was held constant (pH stat), the pH(i) of the heart increased from 7.01 ± 0.01 at 37°C to 7.18 ± 0.02 at 26°C, and the pH(i) of the brain increased from 7.04 ± 0.02 at 37°C to 7.23 ± 0.02 at 26°C and to 7.32 ± 0.04 at 20°C. Alpha-stat pH management resulted in similar increases in pH to that found with pH-stat [heart: 7.00 ± 0.02 at 37°C to 7.19 ± 0.03 at 26°C; brain: 7.07 ± 0.02 at 37°C to 7.29 ± 0.02 at 26°C, and 7.32 ± 0.03 at 20°C]. The tissue pH of the heart in humans showed similar findings during blood pH-stat regulation. This study revealed that both the brain and heart pH(i) in heterotherms are regulated independently of blood pH, following alpha-stat principles, and thus maintain a pH gradient between the blood and tissues during hypothermia despite wide variations of blood pH. Therefore, known variations in brain and heart function between blood pH management schemes cannot be explained on the basis of tissue pH(i).