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

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