The independent effects of anemia and transfusion on mortality after coronary artery bypass

Background: Both anemia and transfusions (Tx) are associated with mortality after cardiac operations. However, the relative contributions of anemia and Tx and their interaction on late mortality have not been determined.

Methods: 922 patients who underwent isolated coronary artery bypass grafting (CABG) were retrospectively studied. Anemia (A+) was defined as hemoglobin<12 g/dL for men and <11 g/dL for women. Patients who received (Tx+) and did not receive (Tx-) transfusions were compared; patient characteristics were controlled for by the use of Cox analysis and then by matching Tx+ to Tx- patients based on identical hemoglobin levels at admission and by propensity matching.

Results: 5.3% of Tx- patients died, compared with 11% of Tx+ patients (p=0.001). The interaction of anemia and Tx was associated with a greater hazard of dying. In particular, A+Tx+ (anemic, received transfusion) patients had a threefold hazard of death (2.918, 95% confidence interval=1.512-5.633, p=0.001) compared with A-Tx- (nonanemic, no transfusion) patients. A+Tx+ patients had twice the hazard of dying as did A+Tx- (anemic, no transfusion) (hazard ratio=2.087, 95% confidence interval=1.004-4.336, p=0.049). In populations matched by preoperative hemoglobin levels or by propensity scores, similar results were seen: a significant interaction between anemia and transfusion of red blood cells. A+Tx+ patients fared significantly worse than did the other three groups. Although there was no difference in mortality between A- patients who did or did not receive transfusions, A+T+ patients had triple the risk as A+T- patients, whereas A+Tx- patients had a similar risk of late mortality as A-Tx- patients.

Conclusions: The anemia-transfusion interaction was associated with an increased hazard of late mortality.