Predicting oxygen tension along the ureter

Continuous measurement of bladder urine oxygen tension (PO₂) is a method to potentially detect renal medullary hypoxia in patients at risk of acute kidney injury (AKI). To assess its practicality, we developed a computational model of the peristaltic movement of a urine bolus along the ureter and the oxygen exchange between the bolus and ureter wall. This model quantifies the changes in urine PO₂ as urine transits from the renal pelvis to the bladder. The model parameters were calibrated using experimental data in rabbits, such that most of the model predictions are within ±1 SE of the reported mean in the experiment, with the average percent difference being 7.0%. Based on parametric experiments performed using a model scaled to the geometric dimensions of a human ureter, we found that bladder urine PO₂ is strongly dependent on the bolus volume (i.e., bolus volume-to-surface area ratio), especially at a volume less than its physiological (baseline) volume (<0.2 mL). For the model assumptions, changes in peristaltic frequency resulted in a minimal change in bladder urine PO₂ (<1 mmHg). The model also predicted that there exists a family of linear relationships between the bladder-urine PO₂ and pelvic urine PO₂ for different input conditions. We conclude that it may technically be possible to predict renal medullary PO₂ based on the measurement of bladder urine PO₂, provided that there are accurate real-time measurements of model input parameters.