Impact of Various Sedation Regimens on the Incidence of Delirium

Inclusion Criteria: * Subjects willing to give written informed consent. * Mild, moderate or severe ARDS patients demanding invasive mechanical ventilation. * Subjects whose expected time of mechanical ventilation is longer than 24 hours. * Subjects aged between 18 and 70. Exclusion Criteria: * Subjects with extremely unstable circulation, whose SBP lower than 90mmHg after volume expansion or pressor agent treatment. * Subjects with extremely unstable circulation, whose SBP lower than 90mmHg after volume expansion or pressor agent treatment. * Subjects with heart rates less than 50 beats per minute. * Subjects with second or third degree atrioventricular block. * Subjects with serious cerebral injury, severe neurologic disorder (e.g acute stroke, uncontrolled epilepsy and severe dementia) or coma. * Subjects with acute or severe liver disease (Child-Pugh class C), see attachment 2. * ARDS patients caused by pulmonary fibrosis or COPD. * Subjects on all types hemodialysis. * Subjects with neuromuscular system disease, alcohol withdrawal syndrome or mental disease before entrance of ICU. * Subjects suspected of narcotic analgesics abusing. * Subjects needing neuromuscular blocking agents (except intubation). * Subjects allergic to investigational products or with other contraindication. * Subjects who are breastfeeding or pregnant * Subjects participated in other study within 30 days before entrance of ICU