Dexmedetomidine Versus Standard Clinical Practice During Non Invasive Mechanical Ventilation

Inclusion Criteria: * Patients over 18 years of age. * Competent or with legal representative able to sign inform consent. * Reversible ARF secondary to heart failure, COPD exacerbation, pneumonia, or at risk of pot-extubation failure\* who meet the criteria for starting NIV. * Signs and symptoms of respiratory distress or * Moderate to severe dyspnoea, grater than usual and/or * Respiratory rate greater than 25 in COPD or greater than 30 in hypoxemic ARF and/or * Use of accessory muscles and/or paradoxical breathing and/or * Hypercapnic encephalopathy * And changes in gas exchange * PaCO2\>45 mmHg, pH\<7.35 and/or * PaO2/FiO2 between 300 and 150. \*Patients at risk of post-extubation failure: Patients who meet at least one of the following criteria. * Impaired consciousness. * Age over 65 years * Heart failure with EF \>30% * Severe disease with an Acute Physiology and Chronic Health Evaluation (APSCHE) score \>12. * Protracted weaning before extubation Exclusion Criteria: * Respiratory arrest, direct indication of OTI and IMV. * Severe unstable comorbidity (myocardial ischemia with ejection fraction \<30%, arrythmia, uncontrolled hypotension defined as systolic blood pressure less than 90 mmHg with doses of norepinephrine\>0.5 mcg/kg/min and/or dobutamine\>10 mcg/kg/min). * Inability to protect the airway: bronchial aspiration. * Fixed upper airway obstruction. * Tracheostomy. * Undrained pneumothorax. * Severe agitation or lack of collaboration of the patient despite medication administered. * Facial burns or trauma. * Facial surgery or anatomical changes which prevent mask fitting. * Hypersensitivity to the active substance or to any of the excipients listed in section 6.1. * Allergy to eggs, soya, or peanuts. * HR\< 50 bpm not induced by beta- blockers. * Advanced heart block (grade 2 or 3) unless paced. * Acute cerebrovascular conditions. * Increased intracranial pressure. * Closed angle glaucoma. * Myasthenia gravis. * Concurrent use of CYP3A4 inhibitors (amprenavir, atazanavir, or ritonavir). * Refuse to participate in the trial. * Pregnant or nursing patients.