Physiological Effects of a New Interface on Lung Ventilation and Gas Distribution

Inclusion Criteria: * presence of hypoxemic Acute Respiratory Failure * absence of history of chronic respiratory failure or moderate-to-severe cardiac insufficiency Exclusion Criteria: * reduced level of consciousness, as indicated by a Glasgow Coma Scale \< 12 * severe respiratory distress (i.e. respiratory rate \> 35 breaths/min) * hemodynamic instability, (i.e. systolic arterial pressure \<90 mmHg or mean systolic pressure \<65 mmHg despite fluid repletion) * need for vasoactive agents, i.e. vasopressin or epinephrine at any dosage, or norepinephrine \>0.3 mcg/kg/min or dobutamine\>5 mcg/kg/min * life-threatening arrhythmias or electrocardiographic signs of ischemia * acute respiratory failure secondary to neurological disorders, status asthmaticus, chronic obstructive pulmonary disease (COPD), cardiogenic pulmonary oedema * presence of tracheotomy * uncontrolled vomiting * more than 2 acute organ failures * body mass index \>30 kg/m2 * documented history or suspicion of obstructive sleep apnoea * facial anatomy contraindicating helmet or nasal cannula application * contraindications to placement of EIT (i.e., pneumothorax, pulmonary emphysema, chest burns or thoracic surgery within 1 week) * inclusion in other research protocols.