Chronic respiratory insufficiency and COPD are the third leading cause of death worldwide. Patients decompensate at various stages of their disease and exhibit acute-on-chronic respiratory failure (ACRF), a frequent cause of ICU hospitalization for hypercapnic acute respiratory failure (ARF). Non-invasive ventilation (NIV) is the first line ventilatory treatment for hypercapnic ARF. It is applied intermittently, separated by periods of spontaneous breathing (SB) with standard oxygen (O2).

Standard O2 has drawbacks that limit the benefit of intermittent NIV in hypercapnic ARF: limited gas flow which is well below the patient's inspiratory flow rate, limited capacity and efficiency of oxygenation with non-controlled FiO2 (risk of excessive oxygen and induced hypercapnia), and cold and dry gas leading to discomfort and under-humidification of the airways and tracheobronchial secretions. Benefits in terms of work of breathing and CO2 removal resulting from PEEP and pressure support applied during NIV periods could be rapidly lost during standard O2.

Recently, use of high-flow heated and humidified nasal oxygen therapy (HFHO) has gained enthusiasm among intensivists to manage ARF. HFHO delivers high flows (up to 60L/min, that generate moderate PEEP) of heated and humidified oxygen at a controlled and adjustable FiO2 (21 to 100%) that rapidly improve respiratory distress symptoms, oxygenation, respiratory comfort and outcome of patients with hypoxemic ARF.

These unique features of HFHO could overcome some of the drawbacks of standard O2 during SB periods in hypercapnic ARF. Indeed, PEEP effect, washout of nasopharyngeal dead-space limiting CO2 re-breathing and inspired gas conditioning preserving adequate mucosal function and secretion removal, could potentially contribute to decrease airways resistance, intrinsic PEEP and work of breathing, while improving patient comfort. Investigators aim to determine if the use of HFHO, as compared to standard O2, increases the number of ventilator-free days (VFDs) and alive at day 28 in patients with hypercapnic ARF admitted in an ICU, an intermediate care, or a respiratory care unit, and requiring NIV.

In both groups, treatment will start with a first NIV session of 2 hours, with arterial blood gas measurement between one hour and two hours after initiating the NIV session. The NIV will be extended for those patients with a pH < 7.30. In both groups, patients will be assessed for their tolerance of NIV and their ability to switch to spontaneous breathing every hour +/- 30 min, except during sleep (10 pm-8 am); they will be assessed for their tolerance of spontaneous breathing and for the need of resumption of NIV every 2 hours+/- 30 min and every 4 +/- 1 hours thereafter. To ensure the consistency of indications of NIV and invasive mechanical ventilation (IMV) across centers and reduce potential bias, NIV and IMV will be initiated and stopped in the same way in the two groups, using predefined criteria.

• Inclusion (day 0): informed consent, randomisation (HFHO group/standard O2 group), NIV initiation (for 2 hours), clinical and paraclinical exam including ABG, data collection
• Follow-up (day 1 to day 28) : NIV, clinical exam, ABG, data collection

Inclusion criteria

• Adult patients aged 18 or above, admitted to an ICU, an intermediate care or a respiratory care unit;
• Chronic respiratory disease previously documented or strongly suspected on clinical, radiological and blood gazes data and pulmonary function tests, in connection with an obstructive respiratory disease (COPD, emphysema, overlap-syndrome (COPD + obstructive sleep apnoea) or mixed (bronchiectasis, obesity-hypoventilation syndrome))
• Patients requiring NIV for hypercapnic ARF (whatever the precipitating cause) i.e. with clinical signs of moderate to severe respiratory distress : dyspnea and /or respiratory rate > 25/min and/or use of accessory respiratory muscles and/or paradoxical abdominal motion and/or signs of respiratory encephalopathy (sleepiness, asterixis, confusion); and respiratory acidosis on arterial blood gases, defined by pH<7.35 and PaCO2 > 45 mmHg despite the careful supply of oxygen and appropriate therapy that may include bronchodilators, corticosteroids and antibiotics

Exclusion criteria

• Contraindications to NIV;
• Purely restrictive lung disease (thoracic deformity, neuro-muscular pathology) and pure obstructive sleep apnoea (without spirometric disturbance or daytime gas anomaly)
• Immediate need for intubation (respiratory or cardiac arrest);
• Persistent hemodynamic instability (use of vasopressors for > 1 hour);
• Multiple organ failure (score SOFA>6);
• NIV treatement for >3 consecutive hours (without any interruption) before admission to ICU, intermediate care, or respiratory care unit and before randomization;
• Anticipated difficulties to conduct NIV (facial trauma or deformation, edentulous patient);
• End stage chronic respiratory insufficiency (defined as use of NIV at home or CPAP treatment at home and life expectancy below 6 month);
• Non-treated pneumothorax;
• Impossibility to perform subjective assessment of dyspnea and comfort (cognitive impairment);
• Patient under guardianship or trusteeship;
• Pregnancy/breastfeeding;
• Decision to withhold or to withdraw life-sustaining treatments (including intubation)
• Moribund state
• Current participation in another clinical trial with an endpoint related to NIV.
• No affiliation to social security (beneficial or assignee);
• Refusal to participate to the study (patient or legal representative or family member or close relative if present