Use of conventional ECG electrodes for depth of anaesthesia monitoring using the cerebral state index: a clinical study in day surgery.

BACKGROUND The cost-benefit relationship for depth of anaesthesia monitors is complicated by the high cost of specially designed EEG electrodes. The cerebral state index (CSI) monitor will accept regular ECG electrodes with snap connectors. The purpose of this study was to determine if generic ECG electrodes could replace the more expensive proprietary EEG electrodes for the CSI monitor. METHODS Two identical cerebral state monitors were used simultaneously during sevoflurane anaesthesia for knee arthroscopy in 14 ASA I-II patients. One monitor used proprietary (Danmeter) EEG electrodes and the other used ECG electrodes (3M Red Dot Diagnostic ECG Electrodes). Paired CSI values were recorded every other minute. Anaesthetic depth was titrated clinically. Sedation depth was scored according to the Observer's Assessment of Alertness/Sedation (OAAS) scale. RESULTS The agreement between the two measures was found to be high, mean difference--0.23, and the overall repeatability mean bias was 6.6 and 153/163 pairs (94%) were located within the 95% limits of agreement. No major difference was noted in impedance, noise, or artifacts. A large overlap in CSI was noted for each level of the OAAS scale; patients with CSI values as low as 40-50 responded whereas patients not responding to surgical stimulation had CSI values as high as 75. The direct cost of disposables decreased from 4euro to 0.50euro per patient by using ordinary ECG electrodes. CONCLUSIONS Switching from proprietary EEG electrodes to ordinary generic ECG electrodes maintains the same accuracy at about a 10th of the cost when measuring CSI during day surgery with sevoflurane anaesthesia.