All patients would receive standard medical treatment with inhaled bronchodilator, systemic steroid, antibiotics according to our local bacteriology. Acute NIV was initiated by trained respiratory nurses according to standardised protocols. The nurses would be at the bedside during the initial acclimatization. BIPAP vision was used to provide bi-level pressure support ventilation. Interfacing with different types of nasal or full-mask would be individualised. NIV was used for as many as possible in the first day, at least 20 hrs. Throughout the NIV treatment, the following parameters will be monitored when NIV is started: arterial blood gases, respiratory rate, heart rate and blood pressure, mental state, pulse oximetry. If a patient failed to respond to NIV, he or she would be intubated if appropriate. Criteria for failure include lack of clinical improvement with increasing dyspnea and deterioration of blood gases, hemodynamic instability, uncontrolled ventricular arrythmia, development of hypercapnic coma and cardiopulmonary arrest.