Intervention (if applicable): Patients will be randomised into two groups, one group will be given 200 milligrams of Propofol and the other group will be given 350 milligrams of Propofol. During the induction of anaesthesia with Propofol over 60 seconds, the patient is asked to count in order to measure time to induction of anaesthesia. During and following anaesthesia a maximum of 50 ml of blood will be taken from an indwelling arterial line. Depth of sedation will be measured using non-invasive Bispectral Index (target 40-60) and other standard measures (heart frequency and blood pressure).