Summary
Background. This study was designed to determine whether the use of bispectral index monitoring (BIS) can be used as an indicator of the most suitable moment for endotracheal intubation or laryngeal mask airway (LMA) insertion. Methods. Sixty-one adult ASA I-III patients were premedicated with midazolam one hour before induction, and allocated to two groups to receive either thiopentone or etomidate. Vecuronium, alcuronium or succynylcholine were used for muscle relaxation. Intubation or LMA insertions were performed by an anaesthesiologist blinded for BIS readings, which were noted by an independent observer. The following times were noted: from induction to intubation (t _ANEST/INT ), from induction to the lowest BIS reading (t _{ANEST/BIS-MIN} ), from induction to the BIS<50 (t _ANEST/BIS<50 ) and the length of BIS<50 (t _BIS<50 ). Results. The mean TBIS<50 was 143±102 sec, although the BIS never decreased below 50 in 5 cases. Forty-two patients (69%) were intubated at BIS<50, 16 (26%) at BIS>50. There was no correlation between the time of intubation (or LMA insertion) and the lowest BIS readings. T _BIS<50 correlated positively (p<0.05) with a total thiopentone dose. t _ANEST/INT and t _{ANEST/BIS-MIN} correlated positively (p<0.05) with BIS readings at intubation. High BIS readings correlated positively with low oxygen saturation during intubation. Discussion and conclusions. The study suggests that clinical observation is inadequate for choosing the optimal moment for intubation or LMA insertion, which was usually performed too early and was occasionally associated with hypoxic events. Optimal depth of anaesthesia was related more to the time course of anaesthesia than to the anaesthetic agent used for induction. BIS can be regarded as a useful tool for optimizing intubating conditions and reducing the incidence of awareness episodes.