Sex-Specific Differences in Perioperative Blood Management After Cardiac Surgery: The Role of Haemodilution

Background/Objectives: Female patients undergoing cardiac surgery receive red blood cell (RBC) transfusions more frequently than male patients, but the mechanisms underlying this disparity remain incompletely understood. Haemodilution during cardiopulmonary bypass (CPB), in the context of lower physiological total blood volume (TBV) and uniform fluid management, may contribute to sex-specific differences in haemoglobin (Hb) trajectories and transfusion exposure. This study investigated whether sex-specific differences in haemodilution contribute to perioperative RBC transfusion exposure in patients undergoing elective cardiac surgery with CPB. Methods: In this secondary analysis of a prospective, single-centre observational cohort study, we included 174 adults undergoing elective coronary artery bypass grafting and/or valve surgery. TBV was estimated using the Nadler formula. Perioperative Hb concentrations, fluid administration and RBC transfusions were recorded from baseline to postoperative day 3. Multivariable regression and mixed-effects models were used to evaluate sex-specific differences in haemodilution and transfusion exposure. The study was registered at ClinicalTrials.gov (NCT05033236). Results: At baseline, Hb concentrations were lower in female than in male patients (12.0 vs. 13.5 g dL−1, p < 0.001). During CPB, female patients exhibited lower Hb concentrations together with a greater relative increase in TBV, consistent with more pronounced haemodilution. RBC transfusions occurred more frequently in female patients both intraoperatively (42.3% vs. 12.3%, p < 0.001) and overall (59.6% vs. 36.1%, p = 0.005). After adjustment for Hb and TBV relative to sex-specific thresholds, CPB duration and surgical procedure type, female patients had 3.9-fold higher odds of receiving at least one RBC unit (odds ratio 3.9, bootstrap 95% CI 1.5–13.3; p = 0.003). Conclusions: Female patients undergoing cardiac surgery experience disproportionate haemodilution and higher transfusion exposure. A dilution-aware, TBV-informed interpretation of Hb concentration may support more individualised and equitable transfusion decision-making.