Non-invasive measurement of the superficial cortical oxygen partial pressure

We present a non invasive fluorescein based method to measure and visualise the partial oxygen pressure of the rat cortex in a 2D picture. We studied 10 Wistar rats. A trepanation was done over the hemisphere and the dura was opened. A PMMA cylinder with a calibrated optical membrane was fixed over the surface of the brain. The CCD camera with the light source is placed over the cylinder. This allows the generation of two-dimensional maps of the pO2 pressure. Using the white light picture we defined regions of interest (ROI) in an artery, vein, parenchyma and an overall ROI. For every ROI a mean emission value was calculated. We increased, stepwise, the FiO2 from 30% up to 100%. Thereafter we established ventilation with an FiO2 of 30% and induced a stepwise hypo- and hyperventilation. The ROI's showed significantly different pO2 values. The apO2 showed a good correlation to the pO2 in the ROIs. This new set up seems to give reliable absolute pO2 values of the brain surface. This method seems to be able for the first time to give a non invasive pO2 map of the brain surface reflecting oxygenation and ventilation effects.