Simultaneous NIR-EPR spectroscopy of rat brain oxygenation.

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.