Neuromuscular block with vecuronium reduces the rapidly extracted auditory evoked potentials index during steady state anesthesia

PURPOSE: During clinical monitoring, vecuronium appeared to reduce the rapidly extracted auditory evoked potentials index (A-line ARX index or AAI) to some extent. A prospective and randomized study was designed to analyze this phenomenon. METHODS: Forty adult patients undergoing elective surgery were studied. After tracheal intubation, anesthesia was maintained with an end-tidal isoflurane concentration (F(ET)ISO) of 1.0% for 20 min, then a 10-mL dose of either vecuronium 0.05 mg*kg(-1), 0.1 mg*kg(-1), 0.2 mg*kg(-1) or saline was administered in a randomized, double-blind design. The AAI and bispectral index (BI(hx)) were monitored throughout the study and analyzed off-line. RESULTS: BI(hx) was unaltered after the administration of saline or vecuronium. The mean of the averaged (per patient) AAI values recorded from two minutes to ten minutes after the administration of saline or vecuronium 0.05 mg*kg(-1) did not differ significantly from the corresponding mean recorded from 15 min to 20 min after F(ET)ISO maintained 1.0% (P = 0.678, 0.169), however after the administration of vecuronium 0.1 mg*kg(-1) or 0.2 mg*kg(-1), AAI was reduced from 18.3, 18.0 to 14.8, 13.4 (P = 0.016, 0.017). CONCLUSIONS: Neuromuscular block with vecuronium reduces AAI in patients during steady state anesthesia without surgical stimuli, while BI(hx) is unaltered. The cut-off values of AAI for events should be determined according to the level of neuromuscular blockade when monitoring the depth of anesthesia/sedation.