Correlation between bispectral index, end-tidal anaesthetic gas concentration and difference in inspired–end-tidal oxygen concentration as measures of anaesthetic depth in paediatric patients posted for short surgical procedures

BACKGROUND AND AIMS: Measurement of end-tidal anaesthetic gas concentrations (ETAG) is currently a pragmatic indicator for monitoring anaesthetic depth. We aimed to assess the performance of ETAG for sevoflurane (ETAG-sevo) with bispectral index (BIS) and difference between inspired and end-tidal oxygen concentration (Fi−Et)O(2)% in measuring anaesthetic depth in toddlers and preschool children. Primary outcome was to correlate BIS with ETAG-sevo. Secondary outcome was to correlate (Fi−Et)O(2)% with ETAG-sevo and to derive cut-off value of (Fi−Et)O(2)%which corresponds with light planes of anaesthesia [minimum alveolar concentration (MAC <0.6)]. METHODS: Thirty patients between 1 and 5 years of age undergoing short procedures were included. ETAG, MAC, BIS and (Fi−Et)O(2)% were measured at intubation, maintenance phase, last 15 min of surgery, end of surgery, extubation, recovery. Pearson's correlation coefficient was used to measure correlation. Receiver operating characteristic (ROC) curves were used to derive cut-off value of (Fi−Et)O(2)% which corresponded with MAC <0.6. RESULTS: BIS correlated poorly with ETAG at all time intervals. Significant correlation was seen between (Fi−Et)O(2)% and ETAG at intubation (P = 0.042), last 15 min of surgery (P = 0.019) and end of surgery (P = 0.001). Cut-off value >7 was obtained for (Fi−Et)O(2)% corresponding to MAC <0.6 at extubation with area under ROC curve0.955 (95% confidence interval 0.811–0.997), with sensitivity 0.8571 and specificity 1.00. CONCLUSION: BIS was an unreliable measure of anaesthetic depth. (Fi−Et)O(2)% values >7 corresponded with light planes of anaesthesia.