Minimum alveolar concentration of sevoflurane required to prevent Bell’s phenomenon during examination of the eye under anaesthesia

Abstract

Paediatric ophthalmological examination under anaesthesia (EUA eyes) is usually performed using sevoflurane. The inspired concentration of sevoflurane is determined by the vapouriser dial setting. Apart from dial settings, several factors affect inspiratory and, consequently, end-tidal anaesthetic agent (ETAA) concentrations. These factors include type of anaesthetic system, fresh gas flow rate and duration of anaesthesia. ETAA represents alveolar vapour concentration and is regarded as close to brain Background: Ophthalmological examination under anaesthesia (EUA eyes) in children is usually performed under sevoflurane anaesthesia. Adequate anaesthesia is required to immobilise the eye in a central position. Ocular stimuli at an insufficient depth of anaesthesia can result in Bell's phenomenon, with eyes turning cephalic, delaying the procedure. The aim of this study was to determine the minimum alveolar concentration (MAC) of sevoflurane which inhibits Bell's phenomenon (MACBell) in young children and the main stimuli eliciting this response.Methods: A sequential experimental study was conducted using the up-and-down procedure or method. Children between the ages of 1 month and 10 years, scheduled for EUA eyes were included. Each patient received sevoflurane (in 40% oxygen/air) at a preselected end-tidal sevoflurane concentration that differed by 0.1%, depending on the response evoked in the preceding patient. The stimulus that elicited this response was recorded.Results: Forty-three children were included in this study. The median age was 37.0 months (range 2-120 months). MACBell, determined by the average MAC at the midpoint of the 14 crossover pairs, was 1.74 (SD 0.19) and 1.81% by probit regression analysis (95% confidence interval 1.63-2.14). The main stimuli responsible for eliciting the reflex were forced traction on eye muscles (68% of responses) and lid speculum insertion (28%). Conclusion:MACBell was 1.74 MACs of sevoflurane. A high concentration of sevoflurane is required for EUA eyes to prevent ocular movement. The most powerful stimulus during EUA eyes in children was traction on the eye muscles, followed by the insertion of the lid speculum. Administering other anaesthetic agents prior to the stimuli causing Bell's phenomenon should be considered to reduce sevoflurane requirements.