Electron Paramagnetic Resonance Assessment Of Brain Tissue Oxygen Tension In Anesthetized Rats

The adequacy of cerebral tissue oxygenation (PtO2) is a central therapeutic end point in critically ill and anesthetized patients. Clinically, PtO2 is currently measured indirectly, based on measurements of cerebrovascular oxygenation using near infrared spectroscopy and experimentally, using positron emission tomographic scanning. Recent developments in electron paramagnetic resonance (EPR) oximetry facilitate accurate, sensitive, and repeated measurements of PtO2. EPR is similar to nuclear magnetic resonance but detects paramagnetic species. Because these species are not abundant in brain (or other tissues) in vivo, oxygen-responsive paramagnetic lithium phthalocyanine crystals implanted into the cerebral cortex are used for the measurement of oxygen. The line widths of the EPR spectra of these materials are linear functions of PtO2. We used EPR oximetry in anesthetized rats to study the patterns of PtO2 during exposure to various inhaled and injected general anesthetics and to varying levels of inspired oxygen. Rats anesthetized with 2.0 minimum alveolar anesthetic concentration isoflurane maintained the largest PtO2 (38.0 +/- 4.5 mm Hg). and rats anesthetized with ketamine/xylazine had the smallest PtO2 (3.5 +/- 0.3 mm Hg) at a fraction of inspired oxygen (FIO2) of 0.21, P < 0.05. The maximal PtO2 achieved under ketamine/xylazine anesthesia with FIO2 of 1.0 was 8.8 +/- 0.3 mm Hg, whereas PtO2 measured during isoflurane anesthesia with FIO2 of 1.0 was 56.3 +/- 1.7 mm Hg (P < 0.05). These data highlight the experimental utility of EPR in measuring PtO2 during anesthesia and serve as a foundation for further study of PtO2 in response to physiologic perturbations and therapeutic interventions directed at preventing cerebral ischemia.