The minimum alveolar concentration (MAC) of sevoflurane required to prevent Bell's phenomenon during examination of the eye under anaesthesia
BACKGROUND: In children, sevoflurane is the most commonly used volatile anaesthetic. Its excellent hemodynamic tolerance gives it a wide therapeutic index. This halogenated agent can abolish movement or hemodynamic responses to noxious stimuli in children and in adults. In order to investigate the effect of sevoflurane on Bell’s response, we determined the minimum alveolar concentration (MAC) of sevoflurane inhibiting Bell’s reflex in 50% of the subjects in response to an eye examination (MACBell). Bell’s phenomenon can be seen at any time during anesthesia when the patient’s depth of anesthesia has changed. It is a natural protective reflex in which the globe turns cephalic in response to stimuli such as pressure on the eye by a lid speculum. This occurs both in the awake state and with light planes of anesthesia. It is usually a sign of light anaesthesia although the patient is not awake or aware. In contrast, the reflex is extinguished with deep planes of anesthesia such that the eye remains in neutral gaze, which is the position required by the ophthalmologist when performing an examination of the eyes under anaesthesia. OBJECTIVES: The aim of this study was to determine the effective minimum alveolar concentration of sevoflurane that is necessary to prevent Bell’s phenomenon in children undergoing an eye examination under anesthesia. The question we aimed to answer with this study was the following: What is the minimum alveolar end tidal concentration of sevoflurane that is required to prevent reflex movement of the eyeball during an eye examination under anesthesia (EUA) in children? This study aimed to give anesthesia providers, involved in ophthalmology examinations, an indication of the dose of sevoflurane that is necessary during anesthesia to prevent the eyeball turning upward and thus safely guide the child through the EUA. A secondary aim of the study was to determine what the main stimuli were for eliciting Bell’s reflex during the EUA. METHODS: A prospective, experimental study was designed. Children aged 2 months to 10 years having an EUA between 19/05/2015 and 23/07/2015 were included in the study. UFS Ethics Committee approval of the study protocol and parental consent were obtained. A dose finding study using up-and-down methodology was used to determine the effective dose in 50% of the population (ED50) of sevoflurane that was required to prevent Bell’s phenomenon under anaesthesia. Patients received sevoflurane at preselected concentrations according to the ‘up-and-down’ study design, and after a steady-state period an examination under anaesthesia was performed. Bell’s reflex was graded as either minimal, when the center of the cornea was still visible or as a full response when the center of the cornea was not visible anymore. The stimulus that elicited the response was also recorded. RESULTS: Forty-three children were studied. In a sub analysis of 32 patients the ED50 of sevoflurane was determined. The ED50 of sevoflurane needed to prevent Bell’s phenomenon was found to be a MAC of 1.8. The main stimuli responsible for eliciting the reflex were forced abduction, adduction or traction on the eye muscles. CONCLUSION: MACbell(1.8) was higher than surgicalMAC(1.0). The most potent stimulus that was responsible for eliciting Bell’s reflex was during traction on the eye muscles, adduction and abduction.