Simultaneous NIR-EPR Spectroscopy of Rat Brain Oxygenation

Abstract

Changes in cerebral oxygenation were simultaneously monitored by electric paramagnetic resonance (EPR) oximetry and near-infrared spectroscopy (NIRS). The tissue oxygen tension (t-pO₂) was measured with an L-band (1.2 GHz) EPR spectrometer with an external loop resonator and the concentration of oxyhemoglobin [HbO₂] and deoxyhemoglobin [Hb] were measured with a full-spectral NIRS system. Mean cerebral hemoglobin saturation (SmcO₂) was calculated from the absolute [HbO₂] and [Hb]. Six adult male rats were implanted with lithium phthalocyanine (LiPc) crystals into the left cerebral cortex. The change in oxygenation of the brain was induced by altering the inspired oxygen fraction (FiO₂) in air from 0.30 at baseline to 0.0, 0.05, 0.10, and 0.15 for 1, 2, 5, and 5 minutes, respectively, followed by reoxygenation with an FiO₂ = 0.30. Although both t-pO₂ and SmcO₂ values showed a decrease during reduced FiO₂ followed by recovery on reoxygenation, it was found that SmcO₂ recovered more rapidly than t-pO₂ during the recovery phase. The recovery of t-pO₂ is not only related to blood oxygenation, but also to delivery, consumption, and diffusion of oxygen into the tissue from the vascular system. Further studies will be required to determine the exact mechanisms for the delay between the recovery of SmcO₂ and t-pO₂.