The oxygenator acts as the key interface between the patient's blood and volatile anaesthetic agent, and their availability is essential for CPB to go ahead. The RIE stock and routinely use two makes of oxygenator in case of manufacture supply problems - in which event the perfusionist would be familiar with using both. Both oxygenators are membrane in nature, consisting of microporous polypropylene hollow fibres. One is the Medtronic Affinity Fusion ('Fusion') and the other is Sorin Inspire ('Inspire'), with the choice of which is used being at the discretion of the perfusionist. Both oxygenators possess a similar design, with the patient's blood flowing over the fibers and a mixture of air and oxygen passing through them, allowing for gas exchange to occur, and crucially allowing the passage of volatile anaesthetic agent into the patient's blood. As the manufacture and design of various oxygenators are different, each possesses a unique set of transfer characteristics, which has been shown in bench tests to affect the uptake and elimination of isoflurane. The combined experience of the anaesthetists at the RIE indicates there is no clinical difference between the two different oxygenators in routine use or the resulting anaesthesia, however these bench tests suggest there may be some difference in transfer characteristics which may be established by small differences measured in the arterial blood level of isoflurane.