To the Editor:
Silverton et al.1 nicely described their work with a prototype monitor that measures oxygen partial pressure in the urine at the exit from the urinary catheter. I commend the authors for further emphasizing the association between oxygen partial pressure in the urine and postoperative acute kidney injury (AKI) and stressing the potential benefit of real-time continuous kidney monitoring. There are, however, some important points of concern.
First, the authors found that mean urinary oxygen partial pressure threshold of 25 mmHg was associated with severe AKI. But the relationship between urinary oxygen partial pressure and the subsequent development of AKI might be more of a cumulative exposure response than a single threshold. A threshold of mean urinary oxygen partial pressure could not be used to elucidate the association between oxygen partial pressure in the urine and AKI. Maybe characterizing the hypo-urinary oxygen partial pressure exposure by time under the 25 mmHg threshold would be preferable.
Second, some variables that might affect the monitor accuracy of urine oximeter were not mentioned in the study. For example, loop diuretic of Henle can cause oxygen partial pressure in the urine to rise, but vasopressin (i.e., norepinephrine) can result in a reduction of urinary oxygen partial pressure.2
In short, I appreciate Silverton and colleagues for their great contributions to this important topic.
The author declares no competing interests.