Urinary hypoxia: an intraoperative marker of risk of cardiac surgery-associated acute kidney injury

Background. Acute kidney injury (AKI) is common after cardiac surgery and profoundly affects postoperative mortality and morbidity. There are no validated methods to assess risk of AKI intraoperatively. Methods. We determined the association between postoperative AKI and intraoperative urinary oxygen tension (PO2), measured via a fiber optic probe in the tip of the urinary catheter, in 65 patients undergoing high- risk cardiac surgery requiring cardiopulmonary bypass (CPB). AKI was diagnosed by modified Kidney Disease: Improving Global Outcomes criteria. Results. Urinary PO2 fell during the operation, often reaching its nadir during rewarming or after weaning from CPB. Nadir urinary PO2 was lower in the 26 patients who developed AKI (mean +/- SD, 8.9 +/- 5.6 mm Hg) than in the 39 patients who did not (14.9 +/- 10.2 mm Hg, P = 0.008). Patients who developed AKI had longer periods of urinary PO2 <= 15 and 10 mmHg than patients who did not. Odds of AKI increased when urinary PO2 fell to <= 10 mmHg {3.60 [95% confidence interval (CI) 1.27-10.21]} or <= 5 mmHg [3.60 (95% CI 1.04-12.42), P = 0.04] during the operation. When urinary PO2 fell to <= 15 mmHg, for more than or equal to the median duration for all patients (4.8 min/h surgery), the odds of AKI were 4.85 (95% CI 1.64-14.40), P = 0.004. The area under the receiver-operator curve for this parameter alone was 0.69, and was 0.89 when other variables with P <= 0.10 in univariable analysis were included in the model. Conclusion. Low urinary PO2 during adult cardiac surgery requiring CPB predicts AKI, so may identify patients in which intervention to improve renal oxygenation might reduce the risk of AKI.