Effects of changes in inspired oxygen fraction on urinary oxygen tension measurements

Background: Continuous measurement of urinary PO2 (PuO2) is being applied to indirectly monitor renal medullary PO2. However, when applied to critically ill patients with shock, its measurement may be affected by changes in FiO(2) and PaO2 and potential associated O-2 diffusion between urine and ureteric or bladder tissue. We aimed to investigate PuO2 measurements in septic shock patients with a fiberoptic luminescence optode inserted into the urinary catheter lumen in relation to episodes of FiO(2) change. We also evaluated medullary and urinary oxygen tension values in Merino ewes at two different FiO(2) levels. Results: In 10 human patients, there were 32 FiO(2) decreases and 31 increases in FiO(2). Median pre-decrease FiO(2) was 0.36 [0.30, 0.39] and median post-decrease FiO(2) was 0.30 [0.23, 0.30], p = 0.006. PaO2 levels decreased from 83 mmHg [77, 94] to 72 [62, 80] mmHg, p = 0.009. However, PuO2 was 23.2 mmHg [20.5, 29.0] before and 24.2 mmHg [20.6, 26.3] after the intervention (p = 0.56). The median pre-increase FiO(2) was 0.30 [0.21, 0.30] and median post-increase FiO(2) was 0.35 [0.30, 0.40], p = 0.008. PaO2 levels increased from 64 mmHg [58, 72 mmHg] to 71 mmHg [70, 100], p = 0.04. However, PuO2 was 25.0 mmHg [IQR: 20.7, 26.8] before and 24.3 mmHg [IQR: 20.7, 26.3] after the intervention (p = 0.65). A mixed linear regression model showed a weak correlation between the variation in PaO2 and the variation in PuO2 values. In 9 Merino ewes, when comparing oxygen tension levels between FiO(2) of 0.21 and 0.40, medullary values did not differ (25.1 & PLUSMN; 13.4 mmHg vs. 27.9 & PLUSMN; 15.4 mmHg, respectively, p = 0.6766) and this was similar to urinary oxygen values (27.1 & PLUSMN; 6.17 mmHg vs. 29.7 & PLUSMN; 4.41 mmHg, respectively, p = 0.3192). Conclusions: Changes in FiO(2) and PaO2 within the context of usual care did not affect PuO2. Our findings were supported by experimental data and suggest that PuO2 can be used as biomarker of medullary oxygenation irrespective of FiO(2).