NIRS in Adults - Effects of Increasing Optode Separation

Abstract

Since its development by Jobsisi Near Infra-red Spectroscopy (NIRS) nas been used to study cerebral oxygenation non-invasively in both neonates^2,3 and more recently in adults^4,5. The penetration of NIRS is said to be a function of the interoptode distance, and Monte Carlo modelling has been used to demonstrate the calculated light path, showing that the majority of the light passes through the intra-cranial tissues above about 4 cm separation, with increasing contamination from surface tissues as the distance is reduced. McCormick⁶ has shown data indicating that there was a significant internal carotid signal with the optodes only 2.8 cm apart, but initial results at this hospital during selective carotid arterial clamping suggested that the relative contribution of the extra-cranial tissues may have been greater than was thought. This study investigates the effect of increasing the optode separation on the amount of internal carotid territory seen by NIRS using hypercarbia to increase internal carotid flow, with Doppler measurement of velocity in Internal and External carotid arteries.