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Urinary oxygen tension (uPO₂) may provide an estimate of renal medullary PO₂ (mPO₂) and thus risk of acute kidney injury (AKI). We assessed the potential for variations in urine flow and arterial PO₂ (aPO₂) to confound these estimates.

In 28 sheep urine flow, uPO₂, aPO₂ and mPO₂ were measured during development of septic AKI. In 65 human patients undergoing cardiac surgery requiring cardiopulmonary bypass (CPB) uPO₂ and aPO₂ were measured continuously during CPB, and in a subset of 20 patients, urine flow was estimated every 5 minutes.

In conscious sheep breathing room air, uPO₂ was more closely correlated with mPO₂ than with aPO₂ or urine flow. The difference between mPO₂ and uPO₂ varied little with urine flow or aPO₂. In patients, urine flow increased abruptly from 3.42 ± 0.29 mL min⁻¹ to 6.94 ± 0.26 mL min⁻¹ upon commencement of CPB, usually coincident …