Anaesthesia for electrocochleography

Until recently Ketamine has been the anaesthetic most commonly used for electrocochleography. It was believed that the use of a conventional anaesthetic machine would produce artifacts in the recordings but experience has shown that this is not the case; in fact quieter tracings are obtained due to the abolition of muscle potentials. Ketamine is preferred for children under 3 years old but the others receive a general inhalation anaesthetic. The intramuscular route is chosen for the administration of Ketamine in order to minimize the occurrence of hallucinations in the recovery period. Consistent results have been obtained with a dosage related to the metabolic rate of the child. One milligram of Ketamine is administered for each 35 cm2 of surface area. General inhalation anaesthesia is given by a semiopen circuit and an oral endotracheal tube is used. Induction is with methohexitone or nitrous oxide and halothane, and maintenance is also with nitrous oxide and halothane. The main drawback is pollution of the atmosphere of the sound-proofed testing room with halothane. This has been overcome by ducting the expired gases outside the room using an Enderby valve.