Mechanical ventilation is the first artificial support used in intensive care. After a period of invasive mechanical ventilation, patients should be separated from the ventilator (weaning period of mechanical ventilation). If weaning and extubation (removal of the tracheal tube) are successful in approximately 80 to 90% of resuscitation patients, 10 to 20% will develop acute respiratory failure (ARF) in the days following extubation. Obesity concerns 20 to 30% of resuscitation admissions in France. The pathophysiological changes in the obese patient explain the over-risk of desaturation and ARF in the post-extubation period. In order to decrease the incidence of extubation failure (need for reintubation within 48-72h post-extubation) of the most fragile patients, it is recommended in intensive care unit to prophylactically use various ventilatory support strategies and / or oxygenation, among which noninvasive ventilation (NIV) and oxygen therapy, which can be administered in two ways: High-Flow Humidified Nasal Oxygen Therapy (HFNO) or standard oxygen therapy. These strategies have never been compared in the obese post-extubation critically ill patient. Our hypothesis is that NIV is superior to oxygen to prevent the development of ARF in obese extubated patients in intensive care unit.

Mechanical ventilation is the first artificial support used in intensive care unit (ICU). After a period of invasive mechanical ventilation, patients should be separated from the ventilator (weaning period of mechanical ventilation). If weaning and extubation (removal of the tracheal intubation tube) is successful in approximately 80 to 90% of resuscitation patients, 10 to 20% will develop acute respiratory failure (ARF) in the days following extubation . This incidence is higher in the patients most fragile in the respiratory field (obese patients, chronic obstructive pulmonary disease (COPD), elderly, heart failure, postoperative cardio-thoracic and / or abdominal surgery ...) . The management of post-extubation ARF consists of etiological treatment associated with ventilatory support which usually requires the use of new endotracheal intubation to deliver "invasive" artificial ventilation. This invasive ventilation is responsible for excess morbidity and excess mortality . Therefore, it is fundamental for the physician to prevent by all means the occurrence of ARF post-extubation. One of the most important causes of extubation failure is the imbalance between the burden imposed on the respiratory system and its ability to overcome it, leading to gas exchange abnormalities, namely hypoxia and hypercapnia. Ensuring adequate oxygenation is therefore essential after extubation. In ICU, oxygen therapy is commonly used to improve and provide adequate oxygenation .

Obesity, a major public health problem in the industrialized countries, concerns 10 to 20% of resuscitation admissions in France. It is associated with excess morbidity and longer mechanical ventilation time compared to the general population. Effect on mortality is controversial , some studies suggesting a protective or neutral effect of obesity , named "obesity paradox" . At the ventilatory level, several pathophysiological changes are combined and contribute to an increase in the incidence of respiratory complications . Pulmonary volumes are amputated, the Body Mass Index (BMI) being negatively correlated with the functional residual capacity and vital capacity. The compliance of the respiratory system is reduced by the weight of the chest wall. Decreased compliance and increased airway resistance lead to increased respiratory muscle work. All of these pathophysiological changes in the obese patient explain the over-risk of desaturation and ARF in the post-extubation period. In addition to the intrinsic risk factors associated with obesity, the post-extubation period is marked by numerous risk factors for the development of ARF and extubation failure.

However, the exploration of different modern modalities of adequate post-extubation oxygen therapy in obese resuscitation patients to prevent the occurrence of extubation failure or post-extubation ARF has never been studied.

For over 20 years, Non-Invasive Ventilation (NIV) has been an essential modality in the prevention ("preventive NIV") and management ("curative NIV") of respiratory failure in ICU. The advantages of this ventilation modality are: respect for airway defense mechanisms (speech and cough maintenance), elimination of laryngeal and tracheal trauma induced by an intubation tube, improved comfort, reduction of the risk of bronchopulmonary nosocomial infection .

An alternative to NIV is the simple administration of oxygen. Two devices delivering low oxygen flows are commonly used for oxygen therapy after extubation: the Venturi mask and the nasal cannulas. The nasal cannulas are preferred by patients and less likely to be removed, but they can cause discomfort due to dryness of nasal mucosa when oxygen flows greater than 4 L/min are used. The Venturi mask delivers predetermined oxygen concentrations ranging from 24 to 60%, but the mask is generally less comfortable than the nasal cannula and more likely to be displaced or removed . In clinical practice, the Venturi mask is more frequently used after extubation because critically ill patients breathe preferentially through an open mouth rather than the nose.

More recently, a new oxygen therapy device has been marketed. This device (Optiflow®, Fisher & Paykel, New Zealand) of High Flow Nasal Humidified Oxygen Therapy (HFNO) is able to deliver oxygen humidified by nasal cannulas. This system has several theoretical advantages: high flow rates reduce the dilution of inhaled oxygen and allow precise distribution of FiO2 throughout the inspiratory phase by adapting the peak flow rate to the patient. The high oxygen flow can also have a washing effect on the dead space of the nasopharynx. In addition, a flow-dependent effect helps to generate a continuous positive end-expiratory pressure (PEEP) , related to an air entrainment mechanism, which has been documented in healthy volunteers and COPD patients. Finally, the use of a high level of humidity could prevent alterations of the ciliated epithelium of the respiratory tract, maintain the activity of the muco-ciliary system, and facilitate the elimination of secretions . In a study in non-obese patients, HFNO was shown to improve oxygenation compared to the Venturi mask, while reducing respiratory rate, PaCO2 and discomfort in patients receiving oxygen therapy after extubation. These positive effects of HFNO were associated with less interface displacement and less oxygen desaturations than the Venturi mask. A secondary result was that the need for reintubation at 48 h was lower with HFNO than with the Venturi mask (4% vs 21%).

In summary, in order to decrease the incidence of extubation failure (need for reintubation within 48-72h post-extubation) of the most fragile patients (including obese patients), it is recommended in intensive care unit to prophylactically use various ventilatory support strategies and / or oxygenation, among which:

• NIV which allows to deliver an established level of oxygen delivered via ventilatory assistance using two levels of pressure (inspiratory aid + positive expiratory pressure), but intermittently, with duration of sessions dependent on the tolerance of the patient.
• Oxygen therapy, which can be administered in two ways: HFNO or standard oxygen therapy.

3.2 Knowledge gap and research hypothesis In an observational study of 124 patients, EL Sohl et al. compared NIV to standard oxygen to prevent extubation failure, and showed a 16% absolute risk reduction of ARF using NIV compared to standard oxygen following extubation. In 155 post cardiac surgery obese patients, Corley et al. compared HFNO and standard oxygen to prevent extubation failure, without showing any difference.

However, none of these studies compared simultaneously the most recent devices available: NIV, HFNO and standard oxygen, nor their association. HFNO is now often used, and the PEP issued by HFNO is much lower than that issued by the NIV. The opening of the cells and the probable maintenance of the residual functional capacity is less when using HFNO than NIV. Thus, the two oxygenation methods (NIV and HFNO) appear complementary in case of ARF following extubation in obese patients. The benefit of NIV +/- HFNO compared to oxygen (standard oxygen or HFNO) to improve the quality of post-extubation oxygenation of overall ICU obese patients has never been studied.

In this multicenter, randomized, controlled, interventional study in mechanically ventilated obese critically ill patients, we will test the hypothesis that NIV (associated to HFNO or standard oxygen) is superior to oxygen (HFNO or standard oxygen) to prevent the development of ARF in obese extubated patients in intensive care unit.

3.3 Originality and innovative aspects of the study NIV has proven effective in small observational studies in preventing ARF following extubation of obese patients, in ICU or postoperative setting. The control group was standard oxygen therapy, which was the standard of care a few years ago. Nowadays, HFNO is more and more used, and has proven to be non-inferior to NIV in ARF patients following cardiothoracic surgery and in high risk patients after extubation in the ICU. To date no prospective randomized study has compared NIV (alternated with HFNO or standard oxygen) with oxygen therapy (HFNO or standard oxygen) to prevent extubation failure in obese patients. This study would be the first to compare the association of the most recent advances in term of oxygenation and lung recruitment in critically ill patients: NIV, HFNO and standard oxygen.

Inclusion Criteria:

• Adult (age ≥ 18 years)
• Subjects must be covered by public health insurance
• Obese patients defined by a body mass index ≥ 30 kg/m²
• Extubation after a length of mechanical ventilation >= 6 hours
• Written informed consent from the patient or proxy (if present) before inclusion or once possible when patient has been included in a context of emergency.

Exclusion Criteria:

• Refusal of study participation or to pursue the study by the patient
• Hypercapnia with a formal indication for NIV (PaCO2 ≥ 50 mmHg, formal indication for NIV)
• Isolated cardiogenic pulmonary edema (formal indication for NIV). Patients with pulmonary edema associated with another ARF etiology can be included.
• Pregnancy or breastfeeding
• Anatomical factors precluding the use of NIV and/or HFNO
• Absence of coverage by the French statutory healthcare insurance system
• Patients with tracheostomy
• Patients with CPAP for obstructive apnea syndrome
• Patients with decision of no-reintubate (limitation)

12 months after the main publication

Data are provided to qualified investigators free of charge. Required documents to request data include a summary of the research plan, request form, and IRB review. Dataset will be shared after careful examination by the study board of investigators.