One common approach to administering anaesthesia during CBP involves the delivery of an inhalational anaesthetic agent, such as isoflurane, into the sweep gas supply to the oxygenator. Whilst classic terminology describes agents such as isoflurane as inhalational, in CPB this may cause confusion due to the delivery of the agent through the oxygenator as opposed to the lungs. Therefore, the alternative name of volatile anaesthetic agents is utilised in this protocol. Isoflurane concentrations ranging between 1% and 2.5% have been used by several anaesthetists at the Royal Infirmary of Edinburgh (RIE) for many years. This pragmatic approach has been found to produce clinically adequate depth of anaesthesia when either of the two oxygenators in routine use at the RIE are employed. Indeed, it has been shown using an electro-encephalographic (EEG) technique of assessing depth of anaesthesia, known as the Bispectral Index (BIS), that administering 2.5% isoflurane produces a more than adequate depth of anaesthesia. However, as discussed below, the interpretation of this finding is limited by the profound effect of hypothermia, which is commonly induced on CPB, has on BIS.