Nadir Hematocrit on Bypass and Rates of Acute Kidney Injury: Does Sex Matter?

Background. Reports have associated nadir hematocrit (Hct) on cardiopulmonary bypass with the occurrence of renal dysfunction. Recent literature has suggested that women, although more often exposed to lower nadir Hct, have a lower risk of postoperative renal dysfunction. We assessed whether this relationship held across a large multicenter registry. Methods. We undertook a prospective, observational study of 15,221 nondialysis-dependent patients (10,376 male, 68.2%; 4,845 female, 31.8%) undergoing cardiac surgery between 2010 and 2014 across 26 institutions in Michigan. We calculated crude and adjusted OR between nadir Hct during cardiopulmonary bypass and stage 2 or 3 acute kidney injury (AKI), and tested the interaction of sex and nadir Hct. The predicted probability of AKI was plotted separately for men and women. Results. Nadir Hct less than 21% occurred among 16.6% of patients, although less commonly among men (9.5%) than women (31.9%; p < 0.001). Acute kidney injury occurred among 2.7% of patients, with small absolute differences between men and women (2.6% versus 3.0%, p = 0.20). There was a significant interaction between sex and nadir Hct (p = 0.009). The effect of nadir Hct on AKI was stronger among male patients (adjusted odds ratio per 1 unit decrease in nadir Hct 1.10, 95% confidence interval: 1.05 to 1.13) than female patients (adjusted odds ratio 1.01, 95% CI: 0.96, 1.06). Conclusions. Lower nadir Hct was associated with an increased risk of AKI, and the effect appears to be stronger among men than women. Understanding of the mechanism underlying this association remains uncertain, although these results suggest the need to limit exposure to lower nadir Hct, especially for male patients. (C) 2015 by The Society of Thoracic Surgeons