Duration of acute kidney injury impacts long-term survival after cardiac surgery

Background: Acute kidney injury (AKI) after cardiac surgery is associated with worse outcomes. However, it is not known how adverse long-term consequences vary according to the duration of AKI. We sought to determine the association between duration of AKI and survival. Methods: Medical records of 4,987 cardiac surgery patients from 2002 through 2007 with serum creatinine (SCr) collection at a medical center in northern New England were reviewed. Acute kidney injury was defined as at least a 0.3 (mg/dL) or at least a 50% increase in SCr from baseline and further classified into AKI Network stages. Duration of AKI was defined by the number of days AKI was present and categorized as no AKI and AKI for 1 to 2, 3 to 6, and at least 7 days. Results: Thirty-nine percent of patients exhibited AKI. Long-term survival was significantly different by AKI duration (p < 0.001). The proportion of patients with AKI duration, adjusted hazard ratio, and 95% confidence interval for mortality (no AKI as referent) were as follows: 1 to 2 days (18%; adjusted hazard ratio, 1.66; 95% confidence interval, 1.32 to 2.09), 3 to 6 days (11%; adjusted hazard ratio, 1.94; 95% confidence interval, 1.51 to 2.49), <7 days (9%; adjusted hazard ratio, 3.40; 95% confidence interval, 2.73 to 4.25). This graded relationship of duration of AKI with long-term mortality persisted when patients who died during hospitalization were excluded from analysis (p < 0.001). Propensity-matched analysis confirmed results. Conclusions: The duration of AKI after cardiac surgery is directly proportional to long-term mortality. This AKI dose-dependent effect on long-term mortality helps to close the gap between association and causation, whereby AKI stages and AKI duration have important implications for patient care and can aid clinicians in evaluating the risk of in-hospital and postdischarge death.