# Renal Metabolic Rate of Oxygen in Response to Hypoxia Challenges by Means of Quantitative MRI in Humans

## TL;DR

This study uses MRI to measure kidney oxygen use in healthy people during low oxygen conditions, showing the method can detect metabolic changes non-invasively.

## Contribution

The study introduces a non-invasive MRI method to assess renal oxygen metabolism during hypoxia in humans.

## Key findings

- Renal blood flow and arteriovenous oxygen difference remained stable during hypoxia.
- Quantitative MRI oximetry detected preserved renal metabolic rate of oxygen during hypoxemia.
- The method shows potential for monitoring early kidney disease.

## Abstract

In early kidney disease, tissue hypoxia occurs due to an imbalance between ATP supply and demand. Whole‐organ renal metabolic rate of oxygen (rMRO2) is therefore a potential biomarker for assessing renal function. This study evaluated the sensitivity of a quantitative MRI method to detect within‐subject changes in metabolic parameters during hypoxic gas challenges. Ten healthy adults were imaged at 3 T (5 female, ages 23–53 years) while undergoing mild and moderate hypoxia (PETO2 62 and 52 mmHg, respectively). The utilized MRI sequence simultaneously quantified blood flow rate (BFR) and venous oxygen saturation (SvO2) at the left renal vein, yielding, together with arterial oxygen saturation (SaO2) obtained by pulse oximetry, whole‐organ rMRO2 by invoking Fick's Principle. Repeated‐measures ANOVA was used to test differences in metabolic parameters between baseline and hypoxic conditions. SaO2 at baseline was 99% ± 1%, while renal SvO2 was 92% ± 3%. During progressive hypoxemia, the drop in SvO2 (mild 83% ± 4%, moderate 76% ± 5%, p < 0.01) paralleled the drop in SaO2 (mild 90% ± 1%, moderate 84% ± 2%), such that the arteriovenous difference in oxygenation (AVDO2) was constant when compared to baseline (p = 1). Renal BFR did not vary significantly between baseline (410 ± 65 mL/min) and hypoxemic conditions (mild, moderate of 430 ± 56 and 440 ± 48 mL/min, p > 0.34). Thus, rMRO2 did not significantly change during hypoxemia (baseline, mild, and moderate of 140 ± 50, 180 ± 80, and 170 ± 90 (μmol O2/min)/100 g, respectively, p = 1). In conclusion, the results demonstrate the method's sensitivity in detecting within‐subject changes in metabolic parameters in response to graded hypoxia. Quantitative MRI oximetry may be a feasible tool to assess and longitudinally monitor early metabolic changes in kidney disease.

This study provides experimental evidence in healthy adults of the kidney's physiological response to short‐term, acute hypoxemia, using non‐invasive a quantitative MRI technique. During hypoxemia, findings suggest preserved renal blood flow, coupled with preservation of the arteriovenous difference in oxygen saturation, subsequently resulting in a maintained whole‐organ renal metabolic rate of oxygen. Quantitative MRI‐oximetry may be a promising tool for advancing clinical understanding of renal oxygenation and for detecting early metabolic changes in kidney disease.