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

Abstract

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 FiO2 and PaO2 and potential associated O2 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 FiO2 change. We also evaluated medullary and urinary oxygen tension values in Merino ewes at two different FiO2 levels. Results In 10 human patients, there were 32 FiO2 decreases and 31 increases in FiO2. Median pre-decrease FiO2 was 0.36 [0.30, 0.39] and median post-decrease FiO2 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 FiO2 was 0.30 [0.21, 0.30] and median post-increase FiO2 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 FiO2 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 FiO2 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 FiO2.

Renal arterial infusion of tempol prevents medullary hypoperfusion, hypoxia, and acute kidney injury in ovine Gram‐negative sepsis

“…All sheep received adequate maintenance fluids (~2000 mL for ~40 kg sheep), which we have previously reported to be sufficient for maintaining central venous pressure and a hyperdynamic circulatory state 46 . We provide further evidence to substantiate measurement of bladder urinary oxygenation as a reliable estimate of renal medullary hypoxia during septic AKI and in response to therapies that alter medullary tissue oxygenation, which aligns with our recent findings in critically ill humans with and without sepsis 47‐49 …”

“…46 We provide further evidence to substantiate measurement of bladder urinary oxygenation as a reliable estimate of renal medullary hypoxia during septic AKI and in response to therapies that alter medullary tissue oxygenation, which aligns with our recent findings in critically ill humans with and without sepsis. [47][48][49] We acknowledge several limitations. Gram-negative sepsis may not fully recapitulate the sepsis-related pathophysiological changes that occur in response to other infections.…”

“… 46 We provide further evidence to substantiate measurement of bladder urinary oxygenation as a reliable estimate of renal medullary hypoxia during septic AKI and in response to therapies that alter medullary tissue oxygenation, which aligns with our recent findings in critically ill humans with and without sepsis. 47 , 48 , 49 …”

Renal medullary oxygenation during laparoscopic vs open surgery: the impact of blood pressure management—a pilot randomized controlled trial

“…This could have provided additional information on kidney function concerning PuO 2 . However, we previously investigated these questions in pre-clinical and human previous studies [24,26,38]. Finally, the PuO 2 difference at incision between usual and high MAP targets among…”

“…We have shown in previous studies that PuO 2 levels are a reliable marker of renal medullary oxygenation [24][25][26] and can be used as a real-time biomarker of acute renal stress. We also demonstrated a strong association between low PuO 2 levels and acute kidney injury in cardiac surgery patients [27].…”