Hypoxia as a biomarker of kidney disease

All established (e.g., serum creatinine, albuminuria) and emerging (e.g., neutrophil gelatinase-associated lipocalin, cystatin C) biomarkers of kidney disease suffer from the disadvantage that they are markers of damage to the kidney or loss of renal function. Tissue hypoxia is believed to be an initiating factor, in both chronic kidney disease (CKD) and acute kidney injury (AKI), so may provide a physiological biomarker for early diagnosis of both conditions. Currently blood oxygen dependent magnetic resonance imaging (BOLD MRI) appears to have little diagnostic value in human CKD. On the other hand, the measurement of urinary oxygen tension (PO₂) has potential as a biomarker of risk of AKI in a hospital setting because: (i) Hypoxia in the renal medulla plays a central role in AKI of multiple causes; (ii) The vasa recta are closely associated with collecting ducts in the medulla so that pelvic urinary PO₂ would be expected to equilibrate with medullary tissue PO₂; (iii) The PO₂ of urine in both the renal pelvis and the bladder varies in response to stimuli that would be expected to alter medullary tissue PO₂;and (iv) New fibre-optic methods make it feasible to measure bladder urine PO₂ in patients with a bladder catheter. But translation of this approach to hospital practice requires: (i) A quantitative understanding of the impact of oxygen transport across the epithelium of the ureter and bladder on urinary PO₂ measured from the bladder,(ii) confirmation that changes in urinary PO₂ parallel those in medullary PO₂ in physiology and pathology, and (iii) Studies of the prognostic utility of urinary PO₂ in hospital settings associated with risk of AKI, such as in patients undergoing cardiac surgery with cardiopulmonary bypass, those at risk of sepsis, and those undergoing imaging procedures requiring administration of radiocontrast agents.