Microbiote's Evolution After Use of Nitrous Oxide on Pediatric Patient Needing Airway Samples

Inclusion Criteria: * Children aged from 3 months to 8 years old * Patients requiring ECBC in routine practice: acute respiratory distress with suspected bacterial respiratory infection (superinfected bronchiolitis or asthma attack, suspected pneumonia) and severe asthma assessment * Child unable to expectorate effectively * Free, informed and written consent of the minor patient's legal guardians; * Enrolled in the social security system. Exclusion Criteria: 1. Contraindication of MEOPA : * Patients requiring 100% oxygen ventilation * Intracranial hypertension * Altered state of consciousness preventing cooperation * Any condition where air is trapped inside the body and expansion could be dangerous (cranial and/or maxillofacial trauma, pneumothorax, emphysema, gas embolism, following recent scuba diving, decompression sickness following gas encephalography, during middle ear/internal ear or sinus surgery, abdominal gas distension, when air is injected into the epidural space during epidural anesthesia, if the patient has received ophthalmic gas (SF6, C3F8, C2F6) used in eye surgery for as long as the gas bubble persists and up to 3 months afterwards) * Known, unsubstituted vitamin B12 or folic acid deficiency * Recent, unexplained neurological abnormalities 2. Contraindications to induced expectoration : * Clinical instability not conducive to induced expectoration (at practitioner's discretion) * Severe respiratory insufficiency, * Severe spasticity. * Hemoptysis * Decompensated heart failure * Contraindication of hypertonic saline : * Hypersensitivity of the bronchial system to hypertonic sodium chloride solutions. * Hemoptysis * Contraindication to the use of Salbutamol : * Hypersensitivity to any of the constituents of salbutamol-based pressurized or inhaled products. * Intolerance to salbutamol-based pressurized or inhaled products (coughing or bronchospasm occurring immediately after inhalation of the product). * Contraindication of the maneuver performed by the physiotherapist: bullous emphysema, confirmed hiatal or diaphragmatic hernia and rib fragility. 3. Patient with minor parent(s) 4. Uncooperative child 5. Antibiotic therapy administered within the previous 24 hours.