Noninvasive urinary oxygen monitoring in predicting acute kidney injury: Silent messengers

Acute kidney injury (AKI) is a pervasive and formidable complication within the realm of critical care medicine, which affects a substantial proportion of patients admitted to intensive care units worldwide.[1] Despite concerted efforts to mitigate its impact, AKI remains a significant contributor to morbidity and mortality in critically ill patients, underscoring the urgent need for improved screening, diagnostic and prognostic tools. Traditionally, the diagnosis of AKI has relied on serum creatinine levels and urine output criteria. These often manifest after significant renal injury, limiting their utility in guiding early intervention strategies. Recognising the limitations of these conventional markers, researchers have sought to identify novel biomarkers and monitoring techniques capable of predicting AKI before irreversible damage ensues.[2] In this pursuit, attention has turned towards noninvasive urinary oxygen monitoring. Mechanism and Working Principle: Noninvasive urinary oxygen monitoring utilises urinary oxygen tension (PuO2) as a surrogate marker for renal medullary tissue oxygenation (tPO2). The underlying mechanism involves the physiological exchange of oxygen between the microvasculature of the renal medulla and urine within the collecting ducts. Within the renal medulla, oxygen is primarily delivered by the vasa recta, which participates in a countercurrent exchange system, allowing efficient oxygen delivery to the metabolically active medullary tissue while preserving the corticomedullary oxygen gradient. Oxygen diffuses from the vasa recta into the surrounding interstitium and then into the collecting ducts, where it equilibrates with PuO2.[3] Noninvasive urinary oxygen monitoring devices leverage this physiological phenomenon by measuring the oxygen tension of urine collected from the bladder or the ureter. By inserting sensors or probes into the urinary catheter or sampling urine externally, these devices can provide continuous or periodic measurements of PuO2. The measured PuO2 reflects the oxygen content of urine, which in turn reflects oxygen tension in the renal medulla. Clinical Studies: Various authors have used and reported urinary oxygen monitoring [Table 1].Table 1: Clinical studies reporting urinary oxygen monitoringChallenges and Considerations: Confounding variables such as urinary flow dynamics, systemic oxygenation, pre-existing renal disease and urine microbiota composition may introduce inaccuracies in oxygen measurements, necessitating validation studies to account for these factors and enhance the accuracy and reliability of urinary oxygen monitoring in predicting AKI risk. Future Directions: Large-scale multicentre clinical trials are warranted to validate the predictive performance of urinary oxygen monitoring in diverse patient populations across different clinical settings. Research efforts should prioritise the development of portable, user-friendly devices capable of real-time continuous monitoring of bladder oxygen tension in diverse clinical settings, with sensitivity and usability. Integrating PuO2 data into risk predictive models and risk stratification algorithms may improve the accuracy and precision of AKI risk prediction in critically ill patients. Developing and validating predictive models incorporating urinary oxygen monitoring and clinical, demographic and biomarker data can optimise risk assessment and patient management strategies. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.