Hemolysis Index and Cardiopulmonary Bypass Time as Predictors of Cardiac Surgery-associated Acute Kidney Injury: An Observational Cohort Study

Objectives: Acute kidney injury (AKI) is a common and serious complication of cardiac surgery, often linked to the use and duration of cardiopulmonary bypass (CPB). The Hemolysis Index (HI) has been proposed as a surrogate marker of hemolysis and a potential predictor of cardiac surgery-associated AKI (CS-AKI). This study was designed to evaluate the associations between CPB time and HI with the onset of CS-AKI. Design: An observational cohort study; retrospective analysis of prospectively collected data. Setting: A single-center, Italian university tertiary care hospital. Participants: Patients admitted to the cardiothoracic intensive care unit (ICU) following cardiac surgery between 2019 and 2023. Interventions: HI was measured at ICU admission. CS-AKI and its severity were defined using the Kidney Disease: Improving Global Outcomes creatinine-based criteria. Associations between HI, CPB duration, and CS-AKI were assessed using logistic regression analyses, adjusting for the Cleveland Clinic Score. Measurements and Main Results: A total of 1,195 patients were included in the analysis. Median age was 70 years (interquartile range [IQR] 62-75), and 310 patients (26%) were female. Median CPB duration was 110 minutes (IQR 80-140), and median HI at ICU admission was 25 (IQR 15-42). HI showed a moderate correlation with CPB duration (r = 0.367, p < 0.001). CS-AKI occurred in 103 patients (9%). In multivariable analysis, both HI and CPB time were independently associated with CS-AKI (odds ratio per 10-unit increase in HI 1.52, 95% confidence interval 1.04-2.18, p = 0.027; odds ratio per hour of CPB 1.32, 95% confidence interval 1.08-1.63, p < 0.001). Conclusions: HI and CPB time were independently associated with CS-AKI. The utility of incorporating these parameters into CS-AKI predictive models warrants further evaluation.