Patients in ICU who need help with their breathing are put onto a machine called a ventilator and are also given a medicine, called a sedative, which helps them to sleep and makes them more comfortable. Midazolam is a sedative that is routinely used for these purposes.

For most patients the aim of sedation is to make them sleepy but still able to respond to nursing staff (light sedation)

Dexmedetomidine is a new sedative for use in intensive care and in this clinical study, dexmedetomidine is compared to midazolam. It is thought that dexmedetomidine might be slightly better at allowing patients to be sleepy but still respond to people around them. It also does not appear to affect patient's breathing. the purpose of this study is to test whether dexmedetomidine really does have these advantages compared to midazolam.

in this study we hope to show that: dexmedetomidine is at least as good as midazolam in helping patients to sleep better and making them more comfortable, and that they are able to co-operate better with the staff treating them, and that patients treated with dexmedetomidine require a shorter time on the ventilator than those treated with midazolam.

This is a phase III, multi-centre, prospective, randomised, double-blind, double-dummy, active comparator study. The study consists of three periods: screening, double-dummy treatment and follow-up period.

All patients admitted to ICU will be pre-screened according to inclusion and exclusion criteria prior to informed consent using available clinical data.

Informed consent, screening and randomisation procedures should be completed within 72 hours from the time of admission to ICU and within 48 hours from starting continuous sedation. Eligible study subjects requiring light to moderate sedation (Richmond Agitation-Sedation Scale [RASS] = 0 to -3) will be randomised to either continue on midazolam or switch to dexmedetomidine. Patients should not have received any other continuously or regularly administered sedative agent than midazolam infusion during the last 12 hours except for opioid analgesics. Study treatments will be titrated to achieve an individually targeted sedation range determined on a daily basis. Rescue treatment (i.e. propofol boli) may be given if needed to achieve the target depth of sedation. Continued need for sedation will be assessed at a daily sedation stop, conducted at the same time each day. First sedation stop may be 12-36 hours from randomisation, depending on the time of day the study subject is randomised. The duration of study treatment is limited to a maximum of 14 days from randomisation. Following withdrawal of sedation, study subjects will be monitored for 48 hours and contacted by telephone 31 and 45 days after randomisation.

Inclusion Criteria:

• Age 18 years and over
• Clinical need for sedation of an initially intubated (or tracheotomised) and ventilated (with inspiratory assistance) patient
• Prescribed light to moderate sedation (target RASS = 0 to -3) using midazolam infusion
• Patients should be randomised within 72 hours from ICU admission and within 48 hours of commencing continuous sedation in the ICU
• Patients should have an expected requirement for sedation of at least 24 hours from time of randomisation
• Written informed consent must be obtained according to local regulations before starting any study procedures other than pre-screening.

Exclusion Criteria:

• Acute severe intracranial or spinal neurological disorder due to vascular causes, infection, intracranial expansion or injury
• Uncompensated acute circulatory failure at time of randomisation (severe hypotension with MAP < 55 mmHg despite volume and pressors)
• Severe bradycardia (HR < 50 beats/min)
• AV-conduction block II-III (unless pacemaker installed)
• Severe hepatic impairment (bilirubin > 101 µmol/L)
• Need for muscle relaxation at the time of randomisation (may only be used for intubation and initial stabilization)
• Loss of hearing or vision, or any other condition which would significantly interfere with the collection of study data
• Burn injuries requiring regular anaesthesia or surgery
• Use of centrally acting α2 agonists or antagonists at the time of randomisation, notably clonidine (see section 5.7 for prior and concomitant treatments)
• Known allergy to any of the study drugs or any excipients of the study drugs
• Patients who have or are expected to have treatment withdrawn or withheld due to poor prognosis
• Patients receiving sedation for therapeutic indications rather than to tolerate the ventilator (e.g. epilepsy)
• Patients unlikely to require continuous sedation during mechanical ventilation (e.g. Guillain-Barré syndrome)
• Patients who are unlikely to be weaned from mechanical ventilation; e.g. diseases/injuries primarily affecting the neuromuscular function of the respiratory apparatus such as clearly irreversible disease requiring prolonged ventilatory support (e.g. high spinal cord injury or advanced amyotrophic lateral sclerosis)
• Distal paraplegia
• Positive pregnancy test or currently lactating
• Received any investigational drug within the preceding 30 days
• Concurrent participation in any other interventional study (any study in which patients are allocated to different treatment groups and/or non-routine diagnostic or monitoring procedures are performed)
• Previous participation in this study
• Any other condition which, in the investigator's opinion, would make it detrimental for the subject to participate in the study