The Brain Uses Mostly Dissolved Oxygen During Profoundly Hypothermic Cardiopulmonary Bypass

Background. During profoundly hypothermic cardiopulmonary bypass, cerebral venous oxygen saturation increases (eg, to 98% at 15°C). We reanalyzed results of clinical studies to learn why. Methods. One hundred sixty-eight cerebral oxygen transport measurements were available from 96 infants and children undergoing profoundly hypothermic cardiopulmonary bypass during repair of congenital heart defects. Results. Dissolved oxygen accounted for 2% to 17% of arterial oxygen content, depending on the arterial oxygen partial pressure and hemoglobin concentration. The fraction of the cerebral metabolic rate for oxygen obtained from dissolved oxygen depended on pump flow, temperature, hemoglobin concentration, and arterial oxygen partial pressure (all p < 10−3). For “full-flow” cardiopulmonary bypass, temperatures less than 18°C, and arterial oxygen partial pressure measurements more than 180 mm Hg, the mean ± standard deviation of the fraction of cerebral metabolic rate for oxygen obtained from dissolved oxygen equaled 77% ± 19%. Conclusions. Dissolved oxygen satisfies most of the brain’s oxygen requirements during profound hypothermic cardiopulmonary bypass. This result reflects four properties of profound hypothermic cardiopulmonary bypass: (1) increases in hemoglobin’s oxygen affinity due to profound hypothermia (which impairs oxygen transfer from hemoglobin to cerebral tissue), (2) use of hemodilution, (3) use of high arterial oxygen partial pressure, and (4) low cerebral metabolic rate of oxygen.