Primary hypothesis: 1. The volume of colloids that is given to volume optimise an anesthetized patient using Pleth Variability Index shows a good correlation to the volume used if volume optimisation is undertaken by the guidance of oesophageal doppler. 2. Data from the two methods correlate and discriminates similarly volume responders from non responders. 3. A volume kinetic model that indicates dehydration can predict the need for rehydration in order to achieve a well hydrated patient at start of surgery.