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Attila Magyar,1,2 Martin Wagner,2–4 Phillip Thomas,1,2 Carolin Malsch,2 Reinhard Schneider,3 Stefan Störk,4,5 Peter U Heuschmann,2,4,6 Rainer G Leyh,1 Mehmet Oezkur1,21Department of Cardiovascular Surgery, University Hospital Würzburg, Würzburg, Germany; 2Institute of Clinical Epidemiology and Biometry, University of Würzburg, Würzburg, Germany; 3Division of Nephrology, Department of Medicine I, University Hospital Würzburg, Würzburg, Germany; 4Comprehensive Heart Failure Center, University of Würzburg, Würzburg, Germany; 5Division of Cardiology, Department of Medicine I, University Hospital Würzburg, Würzburg, Germany; 6Clinical Trial Center Würzburg, University Hospital Würzburg, Würzburg, Germany Background: Acute kidney injury (AKI) is a serious complication after cardiac surgery that is associated with increased mortality and morbidity. Heme oxygenase-1 (HO-1) is an enzyme synthesized in renal tubular cells as one of the most intense responses to oxidant stress linked with protective, anti-inflammatory properties. Yet, it is unknown if serum HO-1 induction following cardiac surgical procedure involving cardiopulmonary bypass (CPB) is associated with incidence and severity of AKI. Patients and methods: In the present study, we used data from a prospective cohort study of 150 adult cardiac surgical patients. HO-1 measurements were performed before, immediately after and 24 hours post-CPB. In univariate and multivariate analyses, the association between HO-1 and AKI was investigated. Results: AKI with an incidence of 23.3% (35 patients) was not associated with an early elevation of HO-1 after CPB in all patients (P=0.88), whereas patients suffering from AKI developed a second burst of HO-1 24 hours after CBP. In patients without AKI, the HO-1 concentrations dropped to baseline values (P=0.031). Furthermore, early HO-1 induction was associated with CPB time (P=0.046), while the ones 24 hours later lost this association (P=0.219). Conclusion: The association of the second HO-1 burst 24 hours after CBP might help to distinguish between the causality of AKI in patients undergoing CBP, thus helping to adapt patient stratification and management. Keywords: acute kidney injury, cardiac surgery, heme oxygenase-1, cardiopulmonary bypass