The oxyhemoglobin dissociation curve before, during and after cardiac surgery

Abstract

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 °C) cardiopulmonary bypass (p<0.001). On rewarming, the P₅₀ increased to 4.89 ± 0.27 kPa (at 36.14 ± 0.14 °C, p<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 < 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.

Key Words:

• anesthesia
• hypothermia
• oxyhemoglobin dissociation.