  The technique of near infrared spectroscopy (NIRS) allows to measure the
oxygenation of the brain tissue. The particular problems involved in detecting
regional brain oxygenation (rSO2) are discussed. The dominant chromophore
(light absorber) in tissue is water. Only in the NIR light region of 650-1000
nm, the overall absorption is sufficiently low, and the NIR light can be
detected across a thick layer of tissues, among them the skin, the scull and
the brain. In this region, there are many absorbing light chromophores, but
only three are important as far as the oxygenation is concerned. They are the
hemoglobin (HbO2), the deoxy-hemoglobin (Hb) and cytochrome oxidase (CtOx). The
devices that were used in our experiments were : Somanetics INVOS Brain
Oximeter (IBO) and Toomim's HEG spectrophotometer. The performances of both
devices were compared including their merits and drawbacks. The IBO use for
research has two drawbacks: the sampling rate is too small and the readings are
limited to only two significant digits. The HEG device does not have these
drawbacks, but is not developed sufficiently at this time to measure rSO2. We
have measured the HEG readings and compared them with the rSO2 readings of the
IBO. Results of an experiment are presented whose aim is to explore the
relationship between respiration and cerebral oxygenation. Measurements of end
tidal CO2 (EtCO2) were taken simultaneously with cerebral oxygen saturation
(rSO2) using the INVOS Cerebral Oximeter of Somanetics. Due to the device
limitations we could explore only subjects who could perform with a breathing
rate of around 2/min or less. The results of all subjects clearly show a
periodic change of cerebral oxygenation with the same period as the breathing
exercises.
