Obesity may alter the disposition and/or clearance of drugs in the body as well as the response, which should be considered when using anesthetics in these patients. Total intravenous anesthesia (TIVA) with propofol is widely used in children, adolescents and adults undergoing surgery, because of rapid onset of action, ease of titration and rapid offset of action. While extensive research on optimal propofol dosing has been performed in non-obese adults, including in critically ill mechanically ventilated adult patients by the investigators' collaborators, there is no evidence on required dosages in morbidly obese adult or pediatric patients of this highly lipophilic agent. As a consequence, serious problems do arise due to under- and overdosing, increasing the risk of inadequate effects and adverse events, respectively. Crucial additional information is needed on the pharmacokinetics of drugs used in morbidly obese children to improve safety and efficacy.