 A goal-oriented sedation complies the ability to sedate the patient as deeply as necessary, and allow a modern ventilation regimen with early spontaneous breathing and a pain-free cooperative patient. The ideal sedative agent - with a short duration of action, predictable wake-up times, low drug toxicity, haemodynamic stability and less side effects, and a rational pharmacoeconomic impact nowadays - has still to be found. Inhalative anaesthetics show these properties, but until the introduction of AnaConDa Anesthetic Conserving Device, ACD in 2005, the use of volatile anaesthetics on the intensive care unit ICU required specific evaporating devices or scavenging systems. The ACD, a modified heat- moisture filter, is connected to the breathing circuit of conventional ICU ventilators and a syringe pump delivers the volatile anaesthetic to the ACD where it is vaporized through a rod. Most of the exhaled gas is absorbed in a charcoal filters membrane and reflected to the patient in the following inspiration. Randomised, controlled and comparative studies to the use of volatile anaesthetics in ICU via this innovative device are still missing. Isoflurane has been studied in small patient populations and in comparison to midazolam, while Sevoflurane - a newer volatile agent with short action, brief elimination time, and low hepatic biodegradation - has only been studied intraoperatively and in short-term sedation. This is the first prospective, randomised, clinical study on the feasibility of sevoflurane via the ACD for sedation in ICU patients until 72 hours in comparison to a standard intravenous sedation with propofol. The investigation will work on potential benefits and limitations of the use of volatile agents on the ICU, the quality of sedation Richmond Agitation Sedation Scale, BIS, infusion rate stability of sevoflurane and respiratory parameters, short-term recovery time from discontinuation of infusion until following verbal commands and extubation, haemodynamics, renal and hepatic function and adverse side effects. 