ASSESSMENT OF CEREBRAL PO(2) BY EPR OXIMETRY IN RODENTS - EFFECTS OF ANESTHESIA, ISCHEMIA, AND BREATHING GAS

This report describes experiments designed to assess and illustrate th e effectiveness of a new method for the measurement of cerebral inters titial pO(2) in conscious rodents. It is based on the use of low frequ ency electron paramagnetic resonance (EPR) spectroscopy with lithium p hthalocyanine as the oxygen sensitive probe. Magnetic resonance imagin g was used to document placement of the probe in the brain, and to ass ess potential cerebral changes associated with the placement. The tech nique provided accurate and reproducible measurements of localized pO( 2) in the brains of conscious rodents under a variety of physiological conditions and for time periods of at least 2 weeks. Using this appro ach we quantitated the depressing effects on cerebral pO(2) of three r epresentative anesthetics, isoflurane, ketamine/xylazine, and sodium p entobarbital. The effects of changing the content of oxygen in the bre athing gas was investigated and found to change the cerebral pO(2). In experiments with gerbils, crystals of lithium phthalocyanine were imp lanted in each side of the brain and using a one-dimensional magnetic field gradient, simultaneous measurement of pO(2) values from normal a nd ischemic (ischemia induced by unilateral ligation of a carotid arte ry) hemispheres of the brain were obtained. These results demonstrate that EPR oximetry with lithium phthalocyanine is a versatile and usefu l method in the measurement of cerebral pO(2) under various physiologi cal and pathophysiological conditions.