Nearinfra red spectroscopy NIRS

• Near-infrared (700-1000nm) light propagated across the head is absorbed by haemoglobin (oxy- and de-oxy), myoglobin and oxidised cytochrome aa3 (the terminal part of the respiratory chain involved in oxidative phosphorylation).

• The sum of (oxy- + deoxy-) haemoglobin is considered an index of cerebral blood volume (CBV) change, and the difference as an index of change in haemoglobin saturation assuming no variation occurs in CBV. CBV and flow can be quantified by changing the FIO2 and measuring the response.

• Cerebral blood flow is measured by a modification of the Fick principle. Oxyhaemoglobin is the intravascular non-diffusible tracer, its accumulation being proportional to the arterial inflow of tracer. Good correlations have been found with the xenon-133 technique.

• Cytochrome aa3 cannot be quantified by NIRS but its redox status may be followed to provide some indication of mitochondrial function.

• Movement artefact must be avoided and some devices require reduction of ambient lighting.

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