Helmet NIV vs. CPAP vs. High-flow Nasal Oxygen in Hypoxemic Respiratory Failure

Inclusion Criteria: * Acute symptoms of respiratory failure PaO2/FiO2 ratio ≤ 200; PaCO2≤45mmHg; Absence of history of chronic respiratory failure or moderate to severe cardiac insufficiency (NYHA\>2 or left ventricular ejection fraction\<50%); Informed consent Patients that have already received NIV, CPAP continuously for more than 24 hours before the screening visit will be excluded. Other Exclusion Criteria: * Pregnancy; * Exacerbation of asthma or chronic obstructive pulmonary disease; * Hypercapnia (PaCO2\>45 mmHg) with or without respiratory acidosis; * More than 2 organ failures, including the lung. * Documented pneumothorax; * Clinical diagnosis of Cardiogenic pulmonary edema; * Haemodynamic instability (Systolic blood pressure\<90 mmHg or mean arterial pressure\<65mmHg) and/or lactic acidosis (lactate\>5 mmol/L) and/or clinically diagnosed Shock requiring administration of vasoactive agents (norepinephrine\>0.1 mcg/Kg/min); * Metabolic Acidosis (pH \<7.30 with normal- or hypo-carbia); * Chronic kidney failure requiring dialysis before ICU admission; * Chronic hypoxemic respiratory failure requiring long-term oxygen therapy; * Altered neurological status that requires immediate intubation and/or making the patient uncooperative; * Urgent need for endotracheal intubation, according to the decision of the attending physician; * Do not intubate order; * Decision of withdrawal of life-sustaining therapy; * Thoracic or abdominal surgery in the previous 7 days; * Any condition that makes the patient very likely to require endotracheal intubation due to a reason different from respiratory failure; * Recent head surgery or anatomy that prevent the application of helmet or HFNC to patient's face.