Red Cell Transfusion-Associated Hemolysis in Cardiac Surgery: An Observational Cohort Study

Red cell viability is impaired during storage, resulting in excess hemolysis during storage and after transfusion. As a result, transfusions may oversaturate the hemoglobin clearance pathways, resulting in cell-free hemoglobin and iron toxicity in susceptible patients, such as those undergoing cardiac surgery with cardiopulmonary bypass. To explore this hypothesis, we assessed the relationship of red cell transfusions with cell-free hemoglobin and transferrin saturation levels in a consecutive cohort of cardiac surgical patients.

Laboratory measures of hemolysis were obtained in consecutive cardiac surgical patients 15 to 30 minutes after bypass. Multivariable regression models controlling for important confounders were constructed to determine the independent relationship of red cell transfusions during bypass with cell-free hemoglobin and transferrin saturation levels post-bypass, analyzed as continuous variables (linear regression) and categorized at the 90th percentiles (logistic regression).

Of the 543 included patients, 82 (15.1%) received red cell transfusions during bypass (median 1; interquartile range 1-2 units). Cell-free hemoglobin was detected in all patients (mean 11.3; standard deviation ± 9.3; 90th percentile 18 μmol/L), and transferrin saturations were relatively high (mean 41 ± 19%; 90th percentile 66%). After controlling for confounders, transfusions were not associated with cell-free hemoglobin (P > .25 in linear and logistic regression) but were directly associated with transferrin saturation levels (P < .001 in linear and logistic regression). Transfused patients had a 6.2-fold (95% confidence interval: 2.4-16.1) risk-adjusted increase in the odds of having high (>66%) transferrin saturation levels.

The findings support the hypothesis that transfusion-related adverse events may be in part caused by the excessive hemolysis of transfused red cells, which can lead to acute iron overload and related toxicity. This suggests that strategies aimed at avoiding or mitigating transfusion-related acute iron overload may improve the safety of red cell transfusions.