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BJA Advance Access first published online on May 23, 2006
This version published online on June 1, 2006
British Journal of Anaesthesia, doi:10.1093/bja/ael131
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© The Board of Management and Trustees of the British Journal of Anaesthesia 2006. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Accepted April 18, 2006

Clinical Investigation

A comparison of the SNAP IITM and BIS XPTM indices during sevoflurane and nitrous oxide anaesthesia at 1 and 1.5 MAC and at awakening

C. A. Wong 1, R. J. Fragen 1, P. Fitzgerald 1, and R. J. McCarthy 1 *

1 Department of Anesthesiology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA

* To whom correspondence should be addressed.
R. J. McCarthy, E-mail: r-mccarthy{at}northwestern.edu


  Abstract

Background. Monitoring level of consciousness during anaesthesia, with the ability to predict the intentional or unintentional return to consciousness, is desirable. The purpose of this study was to compare two processed electroencephalographic depth of anaesthesia monitors (SNAP IITM and BIS XPTM) during sevoflurane and sevoflurane/nitrous oxide anaesthesia.

Methods. In total, 42 subjects received an interscalene block, followed by general anaesthesia with sevoflurane or sevoflurane/nitrous oxide. The indices were recorded at baseline, at 1.5 and 1.0 minimum alveolar concentration (MAC) equivalents, and during emergence.

Results. The SNAP and BIS indices decreased from baseline at 1.5 and 1.0 MAC equivalents, but there was no difference within groups between subjects who received nitrous oxide and those who did not. The SNAP index returned to baseline by 1 min before awakening and was higher than baseline at eye opening, but the BIS index remained below baseline at awakening. There was a bias of -1 (95% CI: -3 to 1) between the SNAP and BIS at baseline; this increased to 21 (95% CI: 19-23) during maintenance of anaesthesia and was 6 (95% CI: 4-8) at awakening.

Conclusions. The SNAP index tracks loss of consciousness and emergence from sevoflurane and sevoflurane/nitrous oxide anaesthesia. There is significant bias between the SNAP and BIS indices and therefore, the indices are not interchangeable. The SNAP index returns to baseline before awakening, whereas the BIS index remains below baseline at awakening, suggesting that the SNAP index may be more sensitive to unintentional awareness.

Keywords: monitoring, bispectral index; monitoring, electroencephalography; monitoring, intraoperative; monitoring, SNAP II index.
This is a new version of this article as there was an error in the equation in the legend to Fig. 3 in the previous version
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