The optimal noninvasive management of acute hypoxemic respiratory failure is debated. Helmet noninvasive ventilation may be more effective than facemask noninvasive ventilation for these patients. Putatitve benefits of helmet use are the possibility to apply significantly higher positive end-expiratory pressure without air leaks and with good patient's comfort.

In this randomized crossover study, the investigators will assess the physiological effects of helmet compared to facemask noninvasive ventilation, with the latter applied with different ventilator settings (similar to or different from helmet settings).

Inclusion Criteria:

• Acute hypoxemic respiratory failure and PaO2/FiO2<200 mmHg
• PaCO2<45 mmHg
• Respiratory failure not caused by exacerbation of chronic pulmonary disease, cardiac failure or fluid overload

Exclusion Criteria:

• Pregnancy
• Contraindication to helmet or facemask noninvasive ventilation
• Contraindication to esophageal manometry
• Contraindication to electrical-impedance tomography monitoring
• Recent surgery involving the abdomen or the thorax
• Pneumothorax or documented barotrauma

Data will be made available upon reasonable request