Variation in blood oxygenation and cerebral pO₂ in a mouse model measured by EPR spectrometry

The partial pressure of oxygen (p02) is one of the most important parameters in many physiological and pathological processes. As a highly aerobic organ, the brain is very sensitive to changes in oxygen availability. The importance of a sufficient oxygen supply is reflected in the brain through many complex mechanisms that ensure adequate blood flow and oxygen delivery. Methods for investigating brain oxygenation are therefore of importance especially in pathological conditions. In the past, one of the primary methods for measuring pO2 in organs such as the brain, was to use small oxygen-sensitive electrodes. These had several disadvantages including; mechanical damage to the tissue, consumption of oxygen by the electrode itself and the inability to make repeated measurements from the same site. Over the past 10 years, oxygen measurements made in the brain using near infrared absorption or phosphorescence quenching have become more common. These methods have the advantages of being largely non invasive compared to using oxygen electrodes but can be limited to making measurements in tissue to a certain depth, or do not directly measure pO2. EPR oximetry has some advantages over other oximetric methods including; the ability to directly measure p02, the ability to obtain repeatable measurements at well resolved sites, and a high degree of accuracy and sensitivity across a range of oxygen levels.