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IN CARDIAC SURGERY, BLOOD VOLUME ACCOUNTS FOR THE INCREASED RISK IN WOMEN

Toronto General Hospital, University Health Network, University of Toronto 200 Elizabeth Street, Toronto, Ontario M5G 2C4

BACKGROUND: The risk of death after cardiac surgery is higher in women than in men. The reason for this difference is still unclear. It has been known that women have a substantially smaller circulation blood volume (CBV) than men, which means that they become more anemic during surgery and receive red blood cell (RBC) transfusions more often. The objective of this study was to determine if these two variables account for the increased risk of death in women after cardiac surgery with CPB.

METHODS: After REB approval, data were prospectively collected on consecutive patients undergoing cardiac surgery with CPB from 1999 to 2003 in an academic hospital. Two multivariable logistic regression models were constructed to determine the independent predictors of in-hospital, all-cause mortality. Model 1 mimicked previous studies and assessed the relationship between gender and mortality by adjusting for multiple perioperative variables excluding those related to intraoperative anemia and RBC transfusion. Model 2 included the latter variables (measured as nadir hematocrit during CPB (nHct) and transfusion of 2 units of RBCs on the day of surgery) as well as the variables included in model 1. The relationships between gender and mortality in these models were examined.

RESULTS: Of the 9215 patients who underwent cardiac surgery with CPB, 169 (1.8%) died. The unadjusted risk of death was higher in women than men [68/2316 (2.9%) vs. 101/6899 (1.5%); P<0.0001 (chi-squared test)]. Compared to men, women were more anemic during surgery [mean nHct 20% vs. 24%; P<0.0001 (t-test)] and received 2 units of RBCs more often [52% vs. 22%; P<0.0001 (chi-squared test)]. In model 1, gender was independently related to risk of death, with the odds of death 1.6 times higher in women (95% C.I. 1.1 – 2.3; P=0.015). In model 2, however, gender was no longer independently related to risk of death (P=0.2); it was replaced by nHct (P=0.01) and RBC transfusion (P=0.001).

CONCLUSION: Female gender is not independent risk factor for death during cardiac surgery with CPB when factors related to perioperative anemia and RBC transfusion are adjusted for. The increased risk of death in women, therefore, is due to their smaller CBV.

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