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Cardiopulmonary bypass (CPB) is a procedure in which the blood is redirected away from the heart to a machine where it is oxygenated and pumped back into the body. In many cases, the temperature of the blood is also lowered to bring the temperature of the body to a level far below the normal 37 degrees centigrade (hypothermia). This procedure is used in both open heart and brain surgery. Hypothermia is thought to have a protective effect on these tissues, facilitating recovery. The physiological and biochemical consequences of hypothermia are poorly understood. To further investigate this, 43 sheep were anesthetized and instrumented for CPB; body temperature was then reduced to either 26 or 18 degrees centigrade. Brain and heart metabolism were monitored by measuring changes in the phosphorus-31 nuclear magnetic resonance spectrograph, a radiological technique that tracks the metabolism of high-energy phosphorous-containing compounds and intracellular pH (acidity). Hypothermia increased the phosphocreatine/adenosine triphosphate ratio (an index of high-energy phosphorous metabolism) in both heart and brain; lower temperatures caused larger effects in the brain (the only organ where the effects of two temperatures were measured). Intracellular pH was also increased in both heart and brain during hypothermia. Both the increased energy levels in heart and brain and the increased intracellular pH would be expected to exert protective effects, and may partially explain the beneficial effect of hypothermia on postoperative recovery. (Consumer Summary produced by Reliance Medical Information, Inc.)