Light levels of anaesthesia after relaxation for tracheal intubation - comparison of succinylcholine and cis-atracurium.

Use of a single bolus of a hypnotic together with non-depolarizing muscle relaxants for anaesthesia induction may cause inappropriate light levels of anaesthesia (ILLA). The purpose of this study was to compare the incidence of ILLA during anaesthesia induction using either cis-atracurium (CIS) or succinylcholine (SUC). Patients (n = 65) received fentanyl and propofol. Relaxants were randomly chosen and were either CIS 0.15 mg/kg, or SUC 1 mg/kg. After achieving relaxation, ILLA were assessed double-blinded by the isolated forearm technique and electroencephalogram -derived values. Time from induction to complete relaxation was 335 ± 55 s with CIS and 141 ± 26 s with SUC. Nine patients in the CIS group (26%), but no patient in the SUC group responded to commands before endotracheal intubation (P < 0.01). During the entire induction up to 1 min after intubation in the CIS group, 24 of 35 patients (68%) showed 31 episodes of ILLA, as defined as responsiveness to commands and spontaneous movements. With SUC, 8 of 30 patients (27%) showed 11 episodes of ILLA (P < 0.01). In patients with ILLA, state entropy (SE) and spectral edge frequency (SEF) were 68 ± 10 (mean ± standard deviation) and 17 ± 4 Hz, respectively, and in patients without ILLA, SE and SEF were 40 ± 14 and 12 ± 3 Hz, respectively (P < 0.01). The onset time of a muscle relaxant has substantial impact on the incidence of ILLA during induction of anaesthesia. Entropy and SEF may indicate the presence of ILLA.