Inclusion Criteria:  Critical ill patients that are mechanically ventilated within the first 48h of admission to the ICU and who would remain in the ICU for over 72h were enrolled in this study. Age 18-80 years. Voluntary informed consent.  Exclusion Criteria:  Surgical treatment is performed within 24 hours or planned during ICU treatment. Craniocerebral injury, post neurosurgery, non drug coma, mental illness or peripheral neuropathy. Heart rates are less than 50 beats / min or severe atrioventricular block without pacemaker support. The mean arterial pressure is less than 55 mmHg even though fluids and vasoactive drugs are administrated. Patients who are known or suspected allergy to benzodiazepines, propofol, dexmedetomidine or opioids. History of alcohol and drug abuse. End stages of lung diseases, such as pulmonary fibrosis, lung damage, etc. Pregnancy.