The investigators designed a cross-over, randomized trial to assess the physiological effects of helmet pressure support ventilation (PSV) and continuous positive airway pressure (CPAP) as compared to high-flow nasal cannula during the early phase of acute hypoxemic respiratory failure

Setting: 21-bed general ICU, emergency room of the Emergency department (ED), Fondazione Policlinico Universitario "Agostino Gemelli" IRCCS, Rome, Italy..

Patients: adult hypoxemic non-hypercapnic patients admitted to the emergency department or the ICU with de novo acute respiratory failure will be assessed for the enrolment.

Each eligible patient, in the absence of exclusion criteria, will receive 15 minutes of heated and humidified 60% oxygen at a rate of 50 l/min via a non-rebreathing face mask. An ABG will be then collected and PaO2/FiO2 ratio computed: given the high flows used, actual FiO2 will be approximated to the set one.

Nonhypercapnic patients with a PaO2/FiO2≤200 mmHg will be enrolled. In the absence of exclusion criteria and if all other inclusion in criteria are met, patients showing PaO2/FiO2≤300 and >200 mmHg will be treated according to the clinical practice eventually reassessed for the presence of oxygenation criterion subsequently.

Enrolled patients will receive all the interventions (helmet CPAP, PSV and high-flow nasal cannula) in a randomized, cross-over fashion, for 40 minutes each.

At the end of the study, the patient will receive the treatment that will be shown as more appropriate for the patient, according to the attending physician, who will be informed about the results of the study on the individual patient.

Inclusion Criteria:

Patients: adult hypoxemic non-hypercapnic patients admitted to the emergency department or the ICU with de novo acute respiratory failure will be assessed for the enrolment.

Patients will be considered eligible whether all the following inclusion criteria are met:

1. Respiratory rate>25 bpm.
2. PaO2/FiO2 ≤200 in the supine position, measured after 15 minutes of high flow treatment with face mask (60 l/min, temperature of the humidification chamber set at 37°C, FiO2 set to achieve a SpO2 >92% and <98%). Given the use of the high flows, nominal FiO2 will be considered a reliable estimate of the actual one.
3. PaCO2 <45mmHg
4. Absence of history of chronic respiratory failure or moderate to severe cardiac insufficiency.
5. Written informed consent

Exclusion Criteria:

• Exacerbation of asthma or chronic obstructive pulmonary disease;
• Cardiogenic pulmonary oedema;
• Haemodynamic instability (Systolic blood pressure <90 mmHg or mean arterial pressure <65 mmHg) and/or lactic acidosis (lactate >5 mmol/L) and/or clinically diagnosed Shock
• Metabolic Acidosis (pH <7.30 with normal- or hypo-carbia);
• Glasgow coma scale <13;
• Recent head surgery or anatomy that prevent the application of helmet or nasal cannula to patient's face.

Data will be made available upon a reasonable request