Association of gender and lowest hematocrit on cardiopulmonary bypass with acute kidney injury and operative mortality in patients undergoing cardiac surgery.

BACKGROUND: Nadir hematocrit on cardiopulmonary bypass (CPB) is a known risk factor for worse outcomes after cardiac surgery. Whether women, because of lower nadir hematocrit on CPB, are more prone to worse outcomes than men after cardiac surgery remains unknown.
METHODS: We evaluated 13,734 patients (31.3% women) undergoing cardiac surgery (6/1/2001 to 06/30/2011) to study the association of hematocrit on CPB and gender with postoperative acute kidney injury (AKI) stage 2-3 (increase in creatinine at least twice the baseline), and operative mortality.
RESULTS: Women were older (68 ± 12 vs 65 ± 12 years, p < 0.001), with more comorbidities. Baseline (37.4% ± 4.4% vs 39.8% ± 4.6%, p < 0.001) and nadir (24.5% ± 3.5% vs 27.4% ± 3.6%, p < 0.001) hematocrit were lower, whereas the hematocrit drop on CPB (baseline and nadir) was greater in women (12.9% ± 4.35 vs 12.4% ± 4.2%, p < 0.001). Observed AKI stage 2-3 and mortality rates were significantly higher in women than in men (5.8% vs 4.9%, p = 0.025 and 4.3% vs 3.4%, p = 0.009, respectively). While nadir hematocrit was inversely related to AKI stage 2-3 and death in both genders, the subgroup of patients with severe hemodilution (nadir hematocrit ≤ 22%) demonstrated a nonsignificant higher rate of AKI stage 2-3 in men (9.2% vs 7.8%) and a significant higher mortality in men (11% vs 7.6%) compared with women.
CONCLUSIONS: Both men and women undergoing cardiac surgery on CPB were prone to the deleterious effects of hemodilution on renal function and death. Yet, despite greater hemodilution, women had a lower relative risk of AKI and death than men at lower nadir hematocrit values on CPB suggesting better tolerance to hemodilution in women.