Noninvasive Urine Oxygen Monitoring and the Risk of Acute Kidney Injury in Cardiac Surgery

Abstract

Background: Acute kidney injury (AKI) is a common complication of cardiac surgery. An intraoperative monitor of kidney perfusion is needed to identify patients at risk for AKI. The authors created a noninvasive urinary oximeter that provides continuous measurements of urinary oxygen partial pressure and instantaneous urine flow. They hypothesized that intraoperative urinary oxygen partial pressure measurements are feasible with this prototype device and that low urinary oxygen partial pressure during cardiac surgery is associated with the subsequent development of AKI.

Methods: This was a prospective observational pilot study. Continuous urinary oxygen partial pressure and instantaneous urine flow were measured in 91 patients undergoing cardiac surgery using a novel device placed between the urinary catheter and collecting bag. Data were collected throughout the surgery and for 24 h postoperatively. Clinicians were blinded to the intraoperative urinary oxygen partial pressure and instantaneous flow data. Patients were then followed postoperatively, and the incidence of AKI was compared to urinary oxygen partial pressure measurements.

Results: Intraoperative urinary oxygen partial pressure measurements were feasible in 86/91 (95%) of patients. When urinary oxygen partial pressure data were filtered for valid urine flows greater than 0.5 ml · kg-1 · h-1, then 70/86 (81%) and 77/86 (90%) of patients in the cardiopulmonary bypass (CPB) and post-CPB periods, respectively, were included in the analysis. Mean urinary oxygen partial pressure in the post-CPB period was significantly lower in patients who subsequently developed AKI than in those who did not (mean difference, 6 mmHg; 95% CI, 0 to 11; P = 0.038). In a multivariable analysis, mean urinary oxygen partial pressure during the post-CPB period remained an independent risk factor for AKI (relative risk, 0.82; 95% CI, 0.71 to 0.95; P = 0.009 for every 10-mmHg increase in mean urinary oxygen partial pressure).

Conclusions: Low urinary oxygen partial pressures after CPB may be associated with the subsequent development of AKI after cardiac surgery.

Figure 1:

Urinary oximeter used in this study, including flow sensor, oxygen sensor port, and temperature sensor as well as the direction of urine flow from the urinary catheter, through the device, to the collecting bag.

Figure 2:

Patient enrollment profile.

Figure 3:

An example of the intra-operative urinary oxygen partial pressure, mean arterial pressure (MAP), pulse oximetry (SpO2), and cerebral oximetry tracings from a patient who subsequently developed AKI. The black lines indicate the start and end of CPB. The grey box highlights a time period of both hypotension and hypoxemia after bypass. During this period there was also a decrease in both cerebral oximetry and urine oxygen. MAP = red; SpO2 = green; Right cerebral oximetry = blue; Left cerebral oximetry = purple; Urine Oxygen Partial Pressure = yellow.

Figure 4:

Left axis: the mean urinary oxygen partial pressure (PuO2) with 95% confidence intervals during the two intra-operative time periods (cardiopulmonary bypass and post cardiopulmonary bypass) for the patients who developed AKI (orange solid squares) and those who did not (blue solid circles). Right axis: daily serum creatinine measurements with 95% confidence intervals from baseline until post-operative day 7 for patients who developed AKI (orange open squares) and those who did not (blue open circles). P values from comparisons at specific time points for the AKI and non-AKI groups are reported if < 0.05. P values for serum creatinine were adjusted for multiple comparisons using Hommel’s procedure.