Effect of sex on nadir hematocrit and rates of acute kidney injury in coronary artery bypass

Objective: Findings from a large multicenter experience showed that sex influenced the relationship between low nadir hematocrit and increased risk of acute kidney injury after cardiac surgery. We explored whether sex-related differences persisted among patients undergoing isolated coronary artery bypass grafting. Methods: We undertook a prospective, observational study of 17,363 patients without dialysis (13,137 male: 75.7%; 4226 female: 24.3%) undergoing isolated coronary artery bypass grafting between 2011 and 2016 across 41 institutions in the Perfusion Measures and Outcomes registry. Odds ratios between nadir hematocrit and stage 2 or 3 acute kidney injury were calculated, and the interaction of sex with nadir hematocrit was tested. The multivariable, generalized, linear mixed-effect model adjusted for preoperative and intraoperative factors and institution. Results: Median nadir hematocrit was 22% among women and 27% among men (P < .001). Women were administered a greater median net prime volume indexed to body surface area (407 vs 363 mL/m(2)) and more red blood cell transfusions (55.5% vs 24.3%; both P < .001). Acute kidney injury was higher among women (6.0% vs 4.3%, P < .001). There was no effect of sex on the relationship between nadir hematocrit and acute kidney injury (P = .67). Low nadir hematocrit was inversely associated with acute kidney injury (adjusted odds ratios per 1-unit increase in nadir hematocrit 0.96; 95% confidence interval, 0.93-0.98); this effect was similar across sexes and independent of red blood cell transfusions. Conclusions: We found no sex-related differences in the effect of nadir hematocrit on acute kidney injury after isolated coronary artery bypass grafting. However, the strong inverse relationship between anemia and acute kidney injury across sexes suggests the importance of reducing exposure to low nadir hematocrit.