Mechanical ventilation is one of the most important means of resuscitating patients with critical illnesses such as respiratory failure and played a great role during the SARS epidemic. Clinically, mechanical ventilation is divided into two categories, invasive and non-invasive, according to the way the patient is connected to the ventilator. Non-invasive ventilation refers to ventilation by connecting the patient to the ventilator in a non-invasive way, such as through a nasal mask (nasal mask) or an oral-nasal mask (mask); invasive ventilation refers to ventilation by connecting the patient to the ventilator through an artificial airway, such as a tracheal intubation or incision. In recent years, non-invasive positive pressure ventilation (NPPV) technology has made significant progress and is being used more and more widely, and it is clinically popular because it can avoid tracheal intubation. A large number of randomized controlled clinical studies at home and abroad have shown that patients are conscious and can cooperate with the use of NPPV; they have some ability to cough up and expel sputum; and the arterial blood gas and acid-base disturbances are not yet serious enough to affect the stability of important organs, such as cardiovascular organs, which are all necessary conditions for the use of NPPV. Therefore, there is actually a therapeutic window for NPPV during the occurrence and development of acute respiratory failure, and the use of NPPV within this therapeutic window is easy to succeed. It is also important to note that NPPV should not be administered too early, as its use when the patient does not require ventilatory support is not only unhelpful to improve the prognosis, but can also result in a waste of resources. The following conditions should be considered as contraindications to NPPV: 1. Respiratory depression or arrest. 2. Unstable cardiovascular system (hypotension not easily corrected, severe arrhythmias). 3. Those prone to aspiration (severe upper gastrointestinal bleeding, obvious abnormal swallowing reflex). 4.Patients with nasal and facial deformities or upper airway obstruction. 5.Recent facial, upper respiratory or gastrointestinal surgery. 6, The patient has poor general condition, confusion (Glasgow Coma Score < 8-10) and thick sputum or excessive airway secretions, and cannot perform effective sputum production. In addition, correction of the indication for the application of NPPV according to the patient's initial treatment response may improve the success rate of NPPV use. If the patient's breathing slows down after 1~2h on the machine, arterial blood gas analysis (pH, PaCO2 and PaO2) improves, and coughing and sputum evacuation ability increases, it indicates that NPPV is effective and can be used under close observation. What to do if complications occur with NPPV Mask leakage Air leakage is related to improper mask position, loose fixation straps, high peak airway pressure, and mask not fitting the patient's face shape. Adjusting the position of the mask, increasing the tension of the fixation band and lowering the ventilation pressure can reduce or eliminate the air leak. If the mask used does not fit the patient's face, it should be replaced with another type of mask in a timely manner. Facial compression injury Compression injury is related to the pressure of the mask on the face and the mask material. Air cushion pressure exceeding capillary pressure can cause local bruising or even necrosis. Therefore, the pressure inside the air cushion should be controlled as much as possible or a mask-type mask should be used, and the ventilator should be discontinued for as long as necessary. In addition, the alternate use of face mask and nasal mask is also an effective method to prevent and control facial pressure injury. Gastric distention Gastric distention is mainly associated with high airway pressure that exceeds the patient's lower esophageal sphincter tone, which can fall below this value in patients with respiratory failure, resulting in gas entering the stomach. In addition, involuntary swallowing causing gastric distention can occur when the patient is confused or has poor compliance with the ventilator. A gastric tube should be routinely left in place to drain gas in a timely manner when the patient requires higher pressure support or is confused, and gastrointestinal decompression should be performed promptly if automatic drainage is ineffective. During use, if there is aggravation, unconsciousness, vomiting and gastric acid aspiration by mistake, noninvasive ventilation should be terminated immediately and tracheal intubation and invasive mechanical ventilation treatment should be performed.