Cerebral monitoring by means of oximetry and somatosensory evoked potentials during carotid endarterectomy

Object. Cerebral ischemia that occurs during carotid endarterectomy is commonly monitored by means of somatosensory evoked potentials (SSEPs) and electroencephalography (EEG). The authors conducted this study to determine whether cerebral ischemia could also be reliably detected by cerebral oximetry. Methods. Twenty-nine patients who underwent carotid endarterectomy were monitored by means of SSEPs, EEG, and cerebral oximetry with a model NIR0500 (20 patients) or INVOS3100A (nine patients) oximeter. Changes in amplitude of SSEPs were graded as follows: 0, no change; 1, decrease of less than 50%; 2, decrease of greater than 50%; and 3, 100% decrease. As measured with the NIR0500 oximeter, closing the common ca ro-tid artery decreased mean oxyhemoglobin levels twice as much (p < 0.005) in the group with SSEPs of 1 to 3 (- 13.11 +/- 5.59 mu M [mean +/- standard deviation], 12 patients) as in the group with SSEPs of 0 (-6.22 +/- 5.59 mu M, eight patients). The rise in deoxyhemoglobin was also greater (p < 0.05). Two of nine patients monitored with the INVOS3100A oximeter had SSEPs of 1 and 3, and their regional saturation of oxygen (rSO(2)) values fell by -11.50 and -11.51, respectively. In the remaining seven patients with SSEPs of 0, the rSO(2) ranged between -2.00 and -6.10 with no overlap with the group with SSEPs of 1 to 3. The increase in oxyhemoglobin monitored using the NIR0500 oximeter and rSO(2) monitored using the INVOS3100A machine after opening the external carotid artery was less than that seen after opening the internal carotid artery. Both types of oximeters could detect cerebral ischemia but whereas false negatives occurred with the NIR0500, none was observed with the INVOS3100A. Extracranial contamination was also four times less frequent with the INVOS3100A than with the NIR0500 monitor. Conclusions. The results indicate that at least as measured with the INVOS3100A instrument, a decrease in rSO(2) of - 10 or more or a decrease below an rSO(2) of 50 is indicative of cerebral ischemia of sufficient severity to decrease the amplitude of SSEPs.