Effect of nitrous oxide on the bispectral index and the 95% spectral edge frequency of the electroencephalogram during surgery

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Abstract
We studied the effect of nitrous oxide on the bispectral index and 95% spectral edge frequency of the electroencephalogram in 20 patients undergoing lumbar surgery under general anaesthesia combined with epidural administration of 5 mg morphine. Anaesthesia was induced with propofol and sufentanil, and maintained with sevoflurane in air/oxygen adjusted to keep the bispectral index between 40 and 60. One and a half hours after the start of surgery, nitrous oxide was administered in a randomised sequence of concentrations (20, 40 and 60% end-expired). Under steady-state conditions, mean (SD) bispectral index and spectral edge frequency decreased as end-tidal concentration of nitrous oxide increased, from 47.7 (4.3) and 15.6 (1.3) at 0% nitrous oxide to 39.8 (6.3) and 14.3 (1.8) at 60% nitrous oxide. A negative correlation was found between nitrous oxide concentration and bispectral index (r = -0.48; p < 0.01) and spectral edge frequency (r = -0.39; p < 0.05). We conclude that this dose-dependent decrease in bispectral index and spectral edge frequency induced by nitrous oxide may reflect the level of analgesia associated with the anaesthetic regimen.

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