The oxyhemoglobin dissociation curve before, during and after cardiac surgery

The oxyhemoglobin dissociation curve was quantified in 15 patients subjected to hypothermic cardiopulmonary bypass under opiate-benzodiazepine anesthesia using the alpha-stat approach to control blood acid-base status. The P50 was calculated from a single measurement of oxygen tension and hemoglobin saturation in blood obtained from the pulmonary artery or the venous line from the cardiopulmonary bypass circuit. In addition, the P50 was directly determined at the registered patient temperature. The P50 decreased from 3.87(+/- 0.15) kPa (mean, SEM) before anesthesia to 1.55(+/- 0.16) kPa during hypothermic (25.43 +/- 1.99 degrees C) cardiopulmonary bypass (p less than 0.001). On rewarming, the P50 increased to 4.89 +/- 0.27 kPa (at 36.14 +/- 0.14 degrees C, p less than 0.001 compared to the preinduction and hypothermic values). Eight hours after cardiopulmonary bypass the P50 returned to the preinduction value (3.72 +/- 0.22 kPa). The relationship between temperature and P50 is described by the regression equation: P50 = 0.22(+/- 0.02).Temperature--3.78(+/- 0.62). The correlation was 0.78 (p less than 0.001). It is concluded that (1) the leftward shift of the oxyhemoglobin dissociation curve during hypothermia may be detrimental to oxygen delivery and (2) the oxygen saturation of the venous blood should not be used indiscriminately to evaluate cellular oxygen status.