An assessment of contributions made by extracranial tissues during cerebral oximetry

This study was designed to determine the extent of contribution made by ext racranial tissues to estimation of regional cerebrovascular saturation (ScO 2) during cerebral oximetry. Thirty four patients undergoing carotid endart erectomy under regional anesthesia were studied. Bilateral ScO2 monitoring with two INVOS 3100 A cerebral oximeters was used. Effect of occlusion of e xternal carotid artery (ECA) for five minutes on ScO2 readings followed by occlusion of internal and common carotid arteries was studied. ScO2 reading s at 1 minute intervals were stored on computer disks for offline analysis. Numerical data were subjected to a two way repeated measures analysis of v ariance to study the effect of side (ipsilateral or contralateral) and phas e (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 fr om 67.4 +/- 8.5 to 65.6 +/- 8.3 with ECA occlusion and to 61.4 +/- 9.6 afte r ICA occlusion returning to 64.8 +/- 9.8 after all clamps were released. D ecrease 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 r emained significantly lower (p<0.05) than pre clamp values. When readings f rom two hemispheres were compared, a significant difference (p<0.001) was n oted during ICA occlusion only. We conclude that the mathematical algorithm used for calculation of ScO2 by INVOS 3100 A cerebral oximeter measures pr edominantly the intracranial cerebrovascular saturation.