Performance of brain tissue oxygen probes in vitro under varying physiological conditions
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Abstract
Patients with Traumatic Brain Injuries (TBI) commonly experience periods of brain tissue hypoxia which can result in secondary brain injuries and worse outcomes during recovery. Currently, the Integra® Licox® brain tissue oxygen monitor is an FDA-approved clinical tool that is used in clinical practice to monitor brain tissue oxygenation (PbtO2) to allow early intervention practices in the patients. The Licox probe is designed as a Clark electrode which measures the oxygen partial pressure (pO2) in aqueous solutions by reducing oxygen gas into water. The current measured created by the reduction reaction is proportional to the oxygen partial pressure. There have been many studies that have tested the ability of the Licox probe to accurately measure the pO2 value in vitro and there is a large debate about the probe’s reability and stability under changing conditions in the injured brain. TBI patients undergo major metabolic changes during the course of recovery which alters the chemical environment of the cerebrospinal fluid (CSF) fluid. In order to confidently rely on the Licox system for clinical usage an understanding of its behavior under varying conditions must be tested.
Our study focused on the performance of the Licox probe under varying physiologic conditions of salinity, pH, temperature, and hydrostatic pressure. We created a isolated experimental chamber where these conditions were manipulated in a controlled manner to observe the effects of these variables on in vitro Licox pO2 values.
The results in our study show that the Licox is not sensitive to physiologic variations in pH, salinity, and hydrostatic pressure. In saline solutions with pO2 values between 20-40mmHg, with other variables constant, as hydrostatic pressure was raised in the closed chamber from 0-70mmHg, there were no observable changes in the Licox pO2 readout. In solutions with pO2 values of 150mmHg with other variables constant, as salinity was increased there was no change in the Licox pO2 readout and the value remained at 143.66mmHg, 95% CI [143.01, 144.32]. Lastly, in solutions with pO2 value of 35mmHg, the pO2 value remained at 34.33mmHg 95% CI [34.0, 35.3] across the different pH values tested while other variables were held constant.
The Licox probe was found to be sensitive to temperature for solutions with pO2 values between 20-40mmHg. The Licox pO2 rose by every degree with 95% CI [0.780, 0.977] (p<0.001). In the 20mmHg pO2 solution, the rise in Licox pO2 value was also found to be statistically significant. The Licox pO2 also rose by every degree with 95% CI [0.864, 0.998] (p=0.0003).
The Licox probe measures pO2 with acceptable accuracy under certain conditions, however there are significant observable changes in Licox pO2 readouts under varying temperatures. These changes are especially of concern at the pO2
value of 20mmHg as many clinical intervention decisions are made at this critical threshold value. There is a possibility these changes in pO2 values may be explained by changes in dissolved oxygen concentrations which are simultaneously occurring in the solution due to fluctuations in temperature. In a clinical setting these results are important as they can help clinicans decide if the observed changes in pO2 values can be attributed to actual changes in brain tissue oxygenation rather than fluctuations caused by Licox probe calibrations or changes in total dissolved oxygen content.
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Attribution 4.0 International