Transfusions during cardiopulmonary bypass: better when triggered by venous oxygen saturation and oxygen extraction rate

During cardiopulmonary bypass (CPB), red blood cell transfusions may be required to correct dilutional anemia. The decision-making process for transfusions is usually based on the level of hemoglobin. This study investigates the hypothesis that oxygen-derived variables (mixed venous oxygen saturation, SvO(2), and oxygen extraction rate, O2ER) may be more reliable predictors of the efficacy of the transfusion. Thirty-six patients for 41 transfusion episodes during CPB were retrospectively analyzed. For each patient, oxygen-derived variables, including SvO(2) and O2ER, were measured before and after the transfusion. No changes in pump flow were allowed between the two measurements. The efficacy of transfusion was defined as an increase in SvO(2) of at least 5%. We identified 11 transfusion episodes leading to an efficacious SvO(2) increase. Factors associated with the efficacy of the transfusion were a low SvO(2) and a high O2ER. No association was found with hemoglobin values, unless for a trend for efficacy of transfusion in patients with very low (<6 g/dL) hemoglobin values. Cut-off values of 68% for SvO(2) and 39% for O2ER were predictive for the efficacy of red blood cell transfusions, with a high accuracy (c-statistics 0.856 and 0.848, respectively) and negative and positive predictive values exceeding 82%. In conclusion, SvO(2) and O2ER are better than the hemoglobin value for guiding the decision-making process of red blood cell transfusions to correct hemodilutional anemia during CPB.