 Methods: Patients will be recruited by professional research staff and will be randomised into one of two groups 90 in each group. They will receive a standardized technique for anaesthesia, cardiopulmonary bypass and postoperative ICU treatment. The only difference between the 2 groups will be as to which anaesthetic agent they receive during the surgical period, desflurane or propofol. Measurements will involve i brain function testing before and 3 months after surgery a set of 10 verbal or manual tests, ii incidence of delirium in the immediate postoperative period a survey form, iii incidence of total postoperative morbidity and iv cost of hospital stay. Data collection will be by anaesthesia and research staff and a neuropsychologist will employed for performing the brain function testing. 