Factors that confound the prediction of renal medullary oxygenation and risk of acute kidney injury from measurement of bladder urine oxygen tension

Abstract

Aim: 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.

Methods: 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.

Results: 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 with reduced uPO₂ . During hyperoxic CPB high values of uPO₂ were often observed at low urine flow. Low urinary PO₂ during CPB (<10 mm Hg at any time during CPB) was associated with greater (4.5-fold) risk of AKI. However, low urine flow during CPB was not significantly associated with risk of AKI.

Conclusions: uPO₂ provides a robust estimate of mPO₂ , but this relationship is confounded by the simultaneous presence of systemic hyperoxia and low urine flow. Urine flow increases and uPO₂ decreases during CPB. Thus, CPB is probably the best time to use uPO₂ to detect renal medullary hypoxia and risk of post-operative AKI.

Keywords: acute kidney injury; cardiac surgery; cardiopulmonary bypass; hypoxia; sepsis.