Dr. Silverton’s Research

Urine Oxygen: a Diagnostic and Therapeutic Tool for Hemorrhagic Shock

Perioperative acute kidney injury (AKI) is a concern for any anesthesiologist. While AKI increases morbidity and mortality, the problem is under-recognized because the diagnosis does not occur until 1-2 days post-surgery. This delay is because changes in serum creatinine take 24 – 48 hours to increase after injury has occurred. While the pathophysiology of AKI is multifactorial, surgical hemorrhage, anemia, and transfusion are likely significant contributors. A real-time monitor of end-organ oxygenation may result in earlier intervention, better end-organ perfusion, and decreased incidence of AKI. Multiple studies have proposed urinary oxygen concentration (PuO2) as a surrogate of kidney oxygenation. Preliminary data suggests that low PuO2 using a novel noninvasive monitor is associated with perioperative AKI and that simultaneous PuO2 and kidney tissue oxygen measurements are feasible in pigs. This trial seeks to investigate the following specific aims. Specific Aim 1: To establish noninvasive PuO2 as an early detection tool for kidney hypoxia during hemorrhage. Noninvasive PuO2 will be measured during controlled hemorrhage in pigs. During hemorrhage, it is hypothesized that changes in PuO2 will detect kidney hypoxia better than mean arterial pressure (MAP). Specific Aim 2: To determine whether a resuscitation protocol that includes PuO2 reduces kidney hypoxia compared to standard practice. After hemorrhage, pigs will be randomized to a resuscitation protocol driven by 1. MAP and urine output (UOP) or 2. MAP, UOP, and noninvasive PuO2. The hypothesis will be tested that the average kidney oxygen concentration will be higher in animals resuscitated using the PuO2 protocol compared to MAP and UOP alone. Furthermore, it is hypothesized that in this PuO2 group, there will be less kidney injury based on biomarkers and histology. Impact: By providing real-time data on critical end-organ oxygenation, the noninvasive device has the potential to reduce AKI and improve outcome.

Related Publications

Near-infrared spectroscopy for kidney oxygen monitoring in a porcine model of hemorrhagic shock, hemodilution, and REBOA
Natalie A Silverton, Lars R Lofgren, Kai Kuck, Gregory J Stoddard, Russel Johnson, Ali Ramezani, Guillaume L Hoareau 

Acute kidney injury is a common complication of trauma and hemorrhagic shock. In a porcine model of hemorrhagic shock, resuscitative endovascular balloon aortic occlusion (REBOA) and hemodilution, the authors hypothesize that invasive kidney oxygen concentration measurements would correlate more strongly with noninvasive near infra-red spectroscopy (NIRS) oxygen saturation measurements when cutaneous sensors were placed over the kidney under ultrasound guidance compared to placement over the thigh muscle and subcutaneous tissue. Using eight anesthetized swine who underwent hemorrhagic shock, 4 of which were resuscitated with intravenous fluids prior to the return of shed blood (Hemodilution protocol) and 4 of which underwent REBOA prior to resuscitation and return of shed blood (REBOA protocol). The authors find that kidney NIRS measurements were more closely related to thigh NIRS measurements than invasive kidney tissue oxygen concentration.

Improving urinary oxygen monitoring with a transit time algorithm: enhancing AKI detection in cardiac surgery
Ali Ramezani, Natalie Silverton & Kai Kuck

Acute kidney injury (AKI) affects 40–50% of cardiac surgery patients and is closely linked to renal medullary hypoxia. Although urinary oxygen partial pressure (PuO₂) offers real-time insight into renal oxygenation, variable urine transit times through the urinary catheter can impair measurement accuracy. The authors aimed to develop an algorithm that calculates transit time by modeling urine flow as discrete particles and to assess whether it improves PuO₂ estimation. The proposed algorithm models urine flow as discrete particles, tracking transit time through the urinary catheter. The transit time allows correcting oxygen measurements at the catheter exit, mitigating distortions from variable flow rates. Corrected measurements more accurately reflect the true oxygen levels entering the catheter across various flow conditions. By accounting for dynamic urine transit times, the proposed algorithm substantially enhances the accuracy of urinary oxygen monitoring. The authors determine that this improvement in estimating renal oxygenation may facilitate noninvasive detection of renal hypoxia and allow for timely interventions to reduce the incidence and severity of AKI in cardiac surgery patients.