An assessment of contributions made by extracranial tissues during cerebral oximetry

This study was designed to determine the extent of contribution made by extracranial tissues to estimation of regional cerebrovascular saturation (ScO2) during cerebral oximetry. Thirty four patients undergoing carotid endarterectomy under regional anesthesia were studied. Bilateral ScO2 monitoring with two INVOS 3100 A cerebral oximeters was used. Effect of occlusion of external carotid artery (ECA) for five minutes on ScO2 readings followed by occlusion of internal and common carotid arteries was studied. ScO2 readings at 1 minute intervals were stored on computer disks for offline analysis. Numerical data were subjected to a two way repeated measures analysis of variance to study the effect of side (ipsilateral or contralateral) and phase (pre clamp, ECA clamp, ICA clamp and post clamp) of operation. A value of p<0.05 was considered significant, There was no significant change in ScO2 on the contralateral side. On the ipsilateral hemisphere ScO2 decreased from 67.4 +/- 8.5 to 65.6 +/- 8.3 with ECA occlusion and to 61.4 +/- 9.6 after ICA occlusion returning to 64.8 +/- 9.8 after all clamps were released. Decrease after ECA occlusion was not significant (p = 0.12) while that after ICA occlusion was significant when compared to pre clamp value (p<0.001). After release of all clamps ipsilateral ScO2 returned toward baseline but remained significantly lower (p<0.05) than pre clamp values. When readings from two hemispheres were compared, a significant difference (p<0.001) was noted during ICA occlusion only. We conclude that the mathematical algorithm used for calculation of ScO2 by INVOS 3100 A cerebral oximeter measures predominantly the intracranial cerebrovascular saturation.