Simultaneous measurement of rat brain cortex PtO2 using EPR oximetry and a fluorescence fiber-optic sensor during normoxia and hyperoxia.

Electron paramagnetic resonance (EPR) oximetry is a promising, relatively non-invasive method of monitoring tissue partial pressure of oxygen (PtO(2)) that has proven useful in following changes in PtO(2) under various physiologic and pathophysiologic conditions. Optimal utilization of the method will be facilitated by systematic comparisons with other available methods. Here, we report on the absolute values and changes of rat brain PtO(2) using EPR oximetry and the OxyLite, an oxygen monitor based on fluorescence quenching, at adjacent locations in the same brain. EPR oximetry utilizes an implanted oxygen-sensitive material and reports tissue PtO(2) at the surface of the material. OxyLite measures PtO(2) using the fluorescence lifetime of a chromophore fixed to the tip of an optical fiber that is inserted into tissue. Measurements were made at a depth of 2-3 mm into the cortex during normoxia and during breathing of carbogen (95% O(2):5% CO(2)) followed by a return to normoxia. We conclude that in this study (1) PtO(2) values reported by the two methods are similar but not exactly the same, (2) both methods can record a baseline and rapid changes in PtO(2), (3) changes in PtO(2) induced by increasing FiO(2) from 0.26 to 0.95 (carbogen) were similar by the two methods and (4) in some rats breathing carbogen, absolute values of PtO(2) were above the sensitive range of the OxyLite method.