Defining oliguria during cardiopulmonary bypass and its relationship with cardiac surgery-associated acute kidney injury

Background. While urine flow rate <= 0.5 ml kg(-1) h(-1) is believed to define oliguria during cardiopulmonary bypass (CPB), it is unclear whether this definition identifies risk for acute kidney injury (AKI). The purpose of this retrospective study was to evaluate if urine flow rate during CPB is associated with AKI. Methods. Urine flow rate was calculated in 503 patients during CPB. AKI in the first 48 h after surgery was defined by the Kidney Disease: Improving Global Outcomes classification. Adjusted risk factors associated with AKI and urine flow rate were assessed. Results. Patients with AKI [n=149 (29.5%)] had lower urine flow rate than those without AKI (P<0.001). The relationship between urine flow and AKI risk was non-linear, with an inflection point at 1.5 ml kg(-1) h(-1). Among patients with urine flow <1.5 ml kg(-1) h(-1), every 0.5 ml kg(-1) h(-1) higher urine flow reduced the adjusted risk of AKI by 26% (95% CI 13-37; P<0.001). Urine flow rate during CPB was independently associated with the risk for AKI. Age up to 80 years and preoperative diuretic use were inversely associated with urine flow rate; mean arterial pressure on CPB (when <87 mmHg) and CPB flow were positively associated with urine flow rate. Conclusions. Urine flow rate during CPB <1.5 ml kg(-1)h(-1) identifies patients at risk for cardiac surgery-associated AKI. Careful monitoring of urine flow rate and optimizing mean arterial pressure and CPB flow might be a means to ensure renal perfusion during CPB.