Ketamine Sedation in Mechanically Ventilated Patients

* Mechanical ventilation is widely used in critically ill patients. Sedation is used in most instances to alleviate symptoms of pain and distress. It usually consists of an association of opiates and benzodiazepines. * In some instances, pain and agitation persist despite this combined sedation regimen. In such cases, the clinicians have the choice between increasing dosage of these 2 substances which may increase their adverse effects (mainly hypotension) and/or adding a neuromuscular blocking agent which is not devoid of adverse effects (mainly the onset of neuromyopathy of critical illness). * This study will assess the safety, efficacy and cost-effectiveness of adding ketamine, a well known anesthetic agent, to a combination of midazolam and sufentanil when this combination is not sufficient to reach acceptable sedation target.