BACKGROUND

Continuous measurement of urinary PO (PuO) is being applied to indirectly monitor renal medullary PO. However, when applied to critically ill patients with shock, its measurement may be affected by changes in FiO and PaO and potential associated O diffusion between urine and ureteric or bladder tissue. We aimed to investigate PuO measurements in septic shock patients with a fiberoptic luminescence optode inserted into the urinary catheter lumen in relation to episodes of FiO change. We also evaluated medullary and urinary oxygen tension values in Merino ewes at two different FiO levels.

RESULTS

In 10 human patients, there were 32 FiO decreases and 31 increases in FiO. Median pre-decrease FiO was 0.36 [0.30, 0.39] and median post-decrease FiO was 0.30 [0.23, 0.30], p = 0.006. PaO levels decreased from 83 mmHg [77, 94] to 72 [62, 80] mmHg, p = 0.009. However, PuO 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 was 0.30 [0.21, 0.30] and median post-increase FiO was 0.35 [0.30, 0.40], p = 0.008. PaO levels increased from 64 mmHg [58, 72 mmHg] to 71 mmHg [70, 100], p = 0.04. However, PuO 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 and the variation in PuO values. In 9 Merino ewes, when comparing oxygen tension levels between FiO 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 and PaO within the context of usual care did not affect PuO. Our findings were supported by experimental data and suggest that PuO can be used as biomarker of medullary oxygenation irrespective of FiO.