Effect of mild hypothermia during cardiopulmonary bypass on erythrocytic hemoglobin oxygen delivery

Abstract

Objectives: To observe and define the degree of change in hemoglobin oxygen affinity induced by hypothermic extracorporeal circulation (ECC). Design: A prospective, nonrandomized, observational study. Setting: A single university medical center. Participants: Seventeen patients presenting for elective cardiac surgery. Interventions: Systemic hypothermia during ECC. Measurements and Main Results: During and after ECC, simultaneous arterial and mixed-venous whole-blood samples were obtained and immediately analyzed for gas tensions and hemoglobin saturation. Samples were obtained during the following times on ECC: initially after cardiopulmonary bypass onset during normothermia (37°C), after cooling to 32°C, and after rewarming to 37°C. A fourth sample was obtained 10 to 20 minutes after discontinuation of cardiopulmonary bypass. Extracorporeal pump flow and thermodilution-determined cardiac output were also recorded for calculation of oxygen delivery and consumption. Mixed-venous results were used to calculate in vivo the blood gas tension at which hemoglobin was 50% saturated with oxygen (P50). There were no differences in P50 for the 17 patients by analysis of variance (ANOVA) for repeated measures with paired t-test with Bonferroni correction. Furthermore, no change in P50 was observed during the course of cooling and rewarming in any individual patient's samples. Oxygen delivery decreased after hypothermia and rewarming from mild hypothermia; oxygen consumption was decreased after rewarming and markedly increased after discontinuation from ECC. Conclusion: Mild hypothermia to 32°C during ECC does not result in in vivo alterations in oxyhemoglobin dissociation and thus does not induce changes in oxygen delivery to peripheral tissues. However, oxygen usage appears to be markedly increased after cardiopulmonary bypass.