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

BACKGROUND: Continuous measurement of urinary PO(2) (PuO(2)) is being applied to indirectly monitor renal medullary PO(2). However, when applied to critically ill patients with shock, its measurement may be affected by changes in FiO(2) and PaO(2) and potential associated O(2) diffusion between urine and ureteric or bladder tissue. We aimed to investigate PuO(2) 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. PaO(2) levels decreased from 83 mmHg [77, 94] to 72 [62, 80] mmHg, p = 0.009. However, PuO(2) 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. PaO(2) levels increased from 64 mmHg [58, 72 mmHg] to 71 mmHg [70, 100], p = 0.04. However, PuO(2) 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 PaO(2) and the variation in PuO(2) 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 ± 13.4 mmHg vs. 27.9 ± 15.4 mmHg, respectively, p = 0.6766) and this was similar to urinary oxygen values (27.1 ± 6.17 mmHg vs. 29.7 ± 4.41 mmHg, respectively, p = 0.3192). CONCLUSIONS: Changes in FiO(2) and PaO(2) within the context of usual care did not affect PuO(2). Our findings were supported by experimental data and suggest that PuO(2) can be used as biomarker of medullary oxygenation irrespective of FiO(2). SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s40635-022-00479-y.