Bladder urine oxygen tension for assessing renal medullary oxygenation in rabbits: experimental and modeling studies

Oxygen tension (Po 2 ) of urine in the bladder could be used to monitor risk of acute kidney injury if it varies with medullary Po 2 . Therefore, we examined this relationship and characterized oxygen diffusion across walls of the ureter and bladder in anesthetized rabbits. A computational model was then developed to predict medullary Po 2 from bladder urine Po 2 . Both intravenous infusion of [Phe 2 ,Ile 3 ,Orn 8 ]-vasopressin and infusion of N G -nitro-l-arginine reduced urinary Po 2 and medullary Po 2 (8–17%), yet had opposite effects on renal blood flow and urine flow. Changes in bladder urine Po 2 during these stimuli correlated strongly with changes in medullary Po 2 (within-rabbit r 2 = 0.87–0.90). Differences in the Po 2 of saline infused into the ureter close to the kidney could be detected in the bladder, although this was diminished at lesser ureteric flow. Diffusion of oxygen across the wall of the bladder was very slow, so it was not considered in the computational model. The model predicts Po 2 in the pelvic ureter (presumed to reflect medullary Po 2 ) from known values of bladder urine Po 2 , urine flow, and arterial Po 2 . Simulations suggest that, across a physiological range of urine flow in anesthetized rabbits (0.1–0.5 ml/min for a single kidney), a change in bladder urine Po 2 explains 10–50% of the change in pelvic urine/medullary Po 2 . Thus, it is possible to infer changes in medullary Po 2 from changes in urinary Po 2 , so urinary Po 2 may have utility as a real-time biomarker of risk of acute kidney injury.