Regardless of which of the two routine oxygenators is used during CPB, it has anecdotally been seen that no clinical difference in anaesthetic depth is observed. However, small differences may be detected by measuring the arterial blood concentration of isoflurane. If a difference exists, it suggests that anaesthetists should take the oxygenator transfer characteristics into account when deciding on which isoflurane concentration is to be used. If no difference is established, it will reassure anaesthetists that their interpretation of depth of anaesthesia in the clinical environment is correct.