Importance of intraoperative oliguria during major abdominal surgery: findings of the Restrictive versus Liberal Fluid Therapy in Major Abdominal Surgery trial

Background: The association between intraoperative oliguria during major abdominal surgery and the subsequent development of postoperative acute kidney injury (AKI) remains poorly defined. We hypothesised that, in such patients, intraoperative oliguria would be an independent predictor of subsequent AKI. Methods: We performed a post hoc analysis of data from the Restrictive versus Liberal Fluid Therapy in Major Abdominal Surgery (RELIEF) trial of conservative vs liberal fluid therapy during and after major abdominal surgery. We used chi(2), logistic regression, and fractional polynomials to study the association between intraoperative oliguria defined as a urinary output <0.5 ml kg(-1) h(-)(1) and the development of postoperative AKI defined by the Kidney Disease Improving Global Outcomes consensus criteria. Results: We included 2444 of 2983 patients from the RELIEF trial in this study. A total of 889 patients (36%) met oliguric criteria intraoperatively. Oliguria occurred in 35% of those without AKI, and 44%, 48%, and 45% of those who developed postoperative AKI Stages 1-3, respectively (P<0.001 for trend). Intraoperative oliguria was associated with an increased risk of AKI, risk ratio: 1.38 (95% confidence interval: 1.14-1.44; P<0.001), but greater intensity of oliguria (urine output <0.3 ml kg(-1) h(-1)) did not increase this risk further. Most patients with oliguria did not develop AKI; the positive predictive value of oliguria was 25.5%, and the negative predictive value was 81.6%. Conclusions: Intraoperative oliguria, defined as urine output <0.5 ml kg(-1) h(-1), was relatively common and was associated with postoperative AKI. However, the predictive utility of oliguria for AKI was low, whilst its absence had a good predictive value for an AKI-free postoperative course.