Optimal Temperature Management During Cardiopulmonary Bypass: Warm, Cold, or Tepid?

Abstract

Hypothermia permitted the advent of cardiac surgery and is considered by many the mainstay of cerebral protection during cardiopulmonary bypass (CPB). How ever, some clinicians have questioned the importance of reduced temperatures during CPB and advocate "normo thermic" heart surgery. Hypothermia (mild, moderate, and profound) provides protection during periods of inadequate oxygen delivery by at least two mecha nisms. First, metabolic rate is directly related to tempera ture ; therefore, reduced temperatures increase toler ance to inadequate oxygen delivery. Q10 values (the ratio of metabolic rates at temperature X°C and tempera ture X°C - 10°C) are controversial and are reported as varying between 2.0 and 5.0. During profound hypother mia (temperature = 17°C), metabolic requirements are 10% to 15% of normothermic values. Second, reduced temperatures (even minimal reductions [34°C to 35°C]) attenuate the release of glutamate and other excitatory amines from ischemic neuronal cells. This phenomenon is thought to play an important role in hypothermic cerebral protection. Many investigators have assessed the impact of normothermic temperatures on cerebral outcomes in cardiac surgery patients. Although seem ingly conflicting conclusions are reported, this much is clear: cerebral temperatures in excess of 37°C exacer bate ischemic injury and even mild hypothermia re duces central nervous system damage in the ischemic brain.