Intraoperative Anemia Mediates Sex Disparity in Operative Mortality After Coronary Artery Bypass Grafting

BACKGROUND Women undergoing coronary artery bypass grafting (CABG) have higher operative mortality than men. OBJECTIVES The purpose of this study was to evaluate the relationship between intraoperative anemia (nadir intraoperative hematocrit), CABG operative mortality, and sex. METHODS This was a cohort study of 1,434,225 isolated primary CABG patients (344,357 women) from the Society of Thoracic Surgeons Adult Cardiac Surgery Database (2011-2022). The primary outcome was operative mortality. The attributable risk (AR) (the risk-adjusted strength of the association of female sex with CABG outcomes) for the primary outcome was calculated. Causal mediation analysis derived the total effect of female sex on operative mortality risk and the proportion of that effect mediated by intraoperative anemia. RESULTS Women had lower median nadir intraoperative hematocrit (22.0% [Q1 -Q3: 20.0%-25.0%] vs 27.0% [Q1 -Q3: 24.0%-30.0%], standardized mean difference 97.0%) than men. Women had higher operative mortality than men (2.8% vs 1.7%; P < 0.001; adjusted OR: 1.36; 95% CI: 1.30-1.41). The AR of female sex for operative mortality was 1.21 (95% CI: 1.17-1.24). After adjusting for nadir intraoperative hematocrit, AR was reduced by 43% (1.12; 95% CI: 1.09-1.16). Intraoperative anemia mediated 38.5% of the increased mortality risk associated with female sex (95% CI: 32.3%-44.7%). Spline regression showed a stronger association between operative mortality and nadir intraoperative hematocrit at hematocrit values <22.0% (P < 0.001). CONCLUSIONS The association of female sex with increased CABG operative mortality is mediated to a large extent by intraoperative anemia. Avoiding nadir intraoperative hematocrit values below 22.0% may reduce sex differences in CABG operative mortality.