Target-controlled administration of inhaled anaesthetics

When using target-controlled administration of a volatile anaesthetic, the purpose is to achieve (and maintain) a clinical effect as rapidly as possible. Therefore, the anaesthetist should be able to change the concentration of the drug in its biophase as quickly as possible. Unfortunately, the biophase of volatile anaesthetics occurs outside the blood compartment and a time delay between dose and response exists. The delay in onset of a ‘target concentration’ depends on the properties of the drug, the administration technique of the inhalational anaesthetic drug and the characteristics of the breathing circuit. Several techniques are described for minimizing this delay. When using an effect measurement of hypnotic depth (e.g. bispectral index), it might be possible to close the loop not only for targeting the end expiratory or brain concentration, but also for targeting the complete anaesthetic–hypnotic effect.