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

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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 NIRO500 (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 NIRO500 oximeter, closing the common caro-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 µM [mean ± standard deviation], 12 patients) as in the group with SSEPs of 0 (−6.22 ± 5.59 µM, 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 (rSO2) values fell by −11.50 and −11.51, respectively. In the remaining seven patients with SSEPs of 0, the rSO2 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 NIRO500 oximeter and rSO2 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 NIRO500, none was observed with the INVOS3100A. Extracranial contamination was also four times less frequent with the INVOS3100A than with the NIRO500 monitor.

Conclusions. The results indicate that at least as measured with the INVOS3100A instrument, a decrease in rSO2 of −10 or more or a decrease below an rSO2 of 50 is indicative of cerebral ischemia of sufficient severity to decrease the amplitude of SSEPs.

Article Information

Address reprint requests to: Edwin M. Nemoto, Ph.D., Department of Neurological Surgery, Suite B-400, Presbyterian University Hospital, 200 Lothrop Street, Pittsburgh, Pennsylvania 15213. email: nemoto@neuronet.pitt.edu.

© AANS, except where prohibited by US copyright law.

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Figures

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    Bar graph showing changes in micromolar increments (µM) in oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) in 12 patients with SSEPs of 1 to 3 and eight patients with SSEPs of 0 after clamping of the CCA. Error bars indicate the standard deviation. *p < 0.004 and †p < 0.04 compared with the corresponding variable in patients with SSEPs of 0.

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    Bar graph showing changes (D) in micromolar increments (µM) for oxyhemoglobin (HbO2) and in the rSO2 index from before (B) and after (A) unclamping of the ECA and the ICA (A–B). The ICA/ECA ratios for HbO2 and rSO2 are ratios of the average changes. *p < 0.013 and †p < 0.002, comparing the changes occurring before and after unclamping of the ICA and ECA.

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