Significance of EEG changes at carotid endarterectomy

Visually apparent EEG changes associated with clamping the internal carotid artery appeared in 55 of 176 consecutive patients (31%) undergoing carotid endarterectomy without shunt. Attenuation of higher frequency activity was the most common change. Changes were moderate in 33 patients (19%) and major in 22 (12.5%). Major changes usually commenced earlier than less severe alterations. EEG changes resolved within 10 minutes of clamp release in 36 of 55 patients (65%) after an average clamp time of 36.25 minutes. Changes occurred more commonly when pre-operative EEGs were abnormal contralateral to clamping and when the contralateral carotid artery was more than 90% stenosed. Of the inhalational anesthetics employed with nitrous oxide and oxygen, isoflurane was associated with the lowest incidence of clamp-associated EEG change. Post-operative strokes occurred in 2 of 22 patients (9%) with major clamp-associated EEG changes, none of 33 patients with moderate changes and none of 121 without changes. However, the mechanism of this relationship remains in doubt.