Rationale: The extreme increase of obesity in the last years had led to this study. There is no consensus about how to anaesthetise morbidly obese patients. The amounts of narcotics given vary widely and rather depend on the anaesthetist than on the pharmacokinetics and dynamics in the morbidly obese patient. Reason for this is that it is not clear in what extend the pharmacokinetics and dynamics are affected in the morbidly obese patient.

Objective: The study is performed in order to develop a population pharmacokinetic and pharmacodynamic model of Propofol when used for induction and maintenance of anaesthesia in the morbidly obese patient (BMI > 40). A covariate analysis will be performed in order to account for variability in pharmacokinetic and/or pharmacodynamic parameters. This model will take into account patient and procedure bound covariates. The results will be used to develop individualised dosing schemes of Propofol when used for induction and maintenance of anaesthesia in morbidly obese patients.

Study design: A randomised, therapeutic and non-invasive study.

Study population: Morbidly obese patients with a Body Mass Index > 40 undergoing laparoscopic banding or gastric bypass surgery, 18-60 year old.

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).

Main study parameters/endpoints: Primary endpoints: pharmacokinetic parameters; clearance, intercompartmental clearance, volume of central compartment and volume of peripheral compartment.

Secondary endpoints: pharmacodynamic parameters; time to induction of anaesthesia (stop counting, eyelash reflex, quality of anaesthesia, corresponding dose required for induction of anaesthesia for both induction doses), EC50 using BIS, required doses of Propofol during maintenance of anaesthesia, wake-up time.

Nature and extent of the burden and risks associated with participation benefit and group relatedness: A maximum amount of 50 milliliters of blood will be sampled from an indwelling arterial line. The patient will be asked to count slowly during induction of anaesthesia. Both induction doses of 200 and 350 milligrams are currently used standard induction doses for morbidly obese patients.