Guided isoflurane injection in a totally closed circuit

Twenty-six patients undergoing middle ear surgery were anaesthetised using a completely closed circuit into which liquid isoflurane was injected using a syringe pump. The injection rate was guided by a table of calculated rates of isoflurane uptake, utilising the square root of time principle, at succeeding time intervals in different weight groups. A constant alveolar concentration of 1.3 MAC was aimed for. The injection rate was changed to that for the higher or lower weight group appropriate to the time interval whenever clinical signs indicated inappropriate depth of anaesthesia. A mean of 10.3 ml liquid isoflurane was used to maintain anaesthesia for a mean of 82.3 minutes. Blood pressure was maintained at 75% of the pre-operative value. Mean arterial blood gas data and changes in plasma catecholamine levels were within the normal range. The mean recovery time was 10.5 minutes. Syringe pump delivery of liquid isoflurane avoided fluctuations that result from bolus injections of unit dose. The implemented injection rate table, working as a therapeutic window, guided redosing, reduced calculation time and served as a record. The technique is safe, economic and adaptable to variations in uptake and response to inhalational anaesthetics.