Metabolism of the heart and brain during hypothermic cardiopulmonary bypass

The alterations in tissue metabolism induced by hypothermic cardiopulmonary bypass are not completely known. Phosphorus-31 nuclear magnetic resonance spectroscopy was used to determine the effect of hypothermic cardiopulmonary bypass on energy states and intracellular pH of the heart and brain. Sheep were instrumented for cardiopulmonary bypass and had a radiofrequency coil placed over either the heart or skull. The animals were placed in a 4.7-T magnet at 37 °C and spectra obtained. The animals were cooled on cardiopulmonary bypass to either 26 °C (n = 17) or 18 °C (n = 14) for brain studies and to 26 °C (n = 12) for heart studies. Hypothermia increased the phosphocreatine/adenosine triphosphate ratio in the heart (2.38 ± 0.23 versus 3.18 ±0.37, 37 ° versus 26 °C, respectively, p = 0.03). The brain phosphocreatine/adenosine triphosphate ratio increased from 1.70 ± 0.09 at 37 °C to 2.00 ± 0.12 at 26 °C (p = 0.009) and 2.10 ± 0.07 at 18 °C (p = 0.0001). Intracellular pH increased during hypothermia (heart: 7.05 ± 0.02 to 7.18 ± 0.02, 37 ° versus 26 °C, p = 0.0001; and brain: 7.07 ± 0.02 versus 7.32 ± 0.02, 37 ° versus 18 °C, p = 0.0001). The adenosine triphosphate resonance position is known to be sensitive to magnesium binding as well as temperature and was shifted upfield (p < 0.01) in both the heart and brain. This effect could be totally explained by the temperature dependence of this process. It is concluded that deep hypothermia increases the energy state and the intracellular pH of both the heart and the brain. The elevated intracellular pH and the increase in tissue energy state may partially explain the beneficial effects of hypothermia on organ tolerance to ischemia.