Acute left heart failure (ALHF) is a sudden attack or deterioration of abnormal left heart function, which may impair myocardial contractility and increase cardiac loading, further result in decreased cardiac output, abrupt elevation of pulmonary and systemic circulation pressure, consequently trigger in acute pulmonary congestion, acute pulmonary edema and cardiac shock .

Hypoxia and severe dyspnea may pose fatal threats for the patients suffered from ALHF should be alleviated as soon as possible, and thus oxygen therapy and ventilation support are regarded as important therapeutic measures for these patients. According to 2017 Chinese College of Emergency Physicians(CCEP) acute heart failure clinical guideline, invasive ventilation should be recommended to those patients whose symptoms still get worsening despite timely treatments.

Intubation may increase the risks of infection and multiple organ failure, so timely extubation contributes to reduce the duration of mechanical ventilation and the rate of complications. But if extubation failure occurs, the patient often requires re-intubation that may induce the outset of complications, extend the length of stay and increase mortality.

Heart failure proves to be high risk factor for extubation failure on the basis of previous studies. It is recommended to apply sequential non-invasive ventilation (NIV) if the patient receiving invasive ventilation for more than 24 hours and having high risk for extubation failure on the basis of 2017 American Thoracic Society(ATS) clinical guideline. It is worthy to note that NIV has many shortcomings, for example, it may induce dry oropharyngeal cavity, skin injury caused by mask oppression, gaseous distention, vomiting, respiratory aspiration, air leak, drying sputum, difficulty in coughing up phlegm and claustrophobia.

As an emerging technology, high flow nasal oxygen (HFNO) has many advantages in airway humidification, tolerance and compliance which also can effectively improve pulmonary oxygenation function of patients with respiratory failure. Which therapeutic measure should be recommended for the patients with ALHF after extubation, NIV or HFNO? It is still unclear according to the latest management guidelines. So one perspective study will be launch to compare the difference between HFNO-group patients with NIV-group patients in re-intubation rate within 48 hours, oxygenation index, length of ICU stay, total hospital stay, mortality and compliance for evidence-based medicine.

1. The anticipated number of patient subjects will reach 120 in the study. The patient's condition should be informed to his/her next of kin and then agree to signs the informed consent.

2. When passing spontaneous breathing trials and removing tracheal tube, the patients should be divided into two groups by random digital table.

3. Group A/HFNO group: the patients should receive the treatment of high flow nasal oxygen immediately after extubation. The initial flow speed was set at 30L/min and increased at 5L/min in titrated type until the patient feels uncomfortable. The temperature was set at 37℃. The fraction of inspiration O2 was set in accordance with specific condition of patient to insure oxygen saturation more than 95%.

   Group B/NIV group: the patients should receive the treatment of non-invasive ventilation immediately after extubation. The initial pressure are set at a lower level (inspiratory pressure: 6-8cmH2O, expiratory pressure: 4cmH2O) and adjusted to a suitable level in 10-20min. The fraction of inspiration O2 was set in accordance with specific condition of patient to insure oxygen saturation more than 95%.

4. The criteria of successful extubation: the patient doesn't require re-intubation within 48 hours after extubation.

   The criteria of extubation failure: the patient needs re-intubation and ventilation support within 48 hours after extubation.

5. (1) Investigators should keep a record of these patient's general condition, for instance, age, sex, diagnosis, Apache II score, treatment, etc.(2) Investigators should routinely detect clinical manifestation, ventilation parameters and physiological indices of the recruited patients. Elementary monitoring indicators consist of vital signs, oxygen saturation, ECG, tidal volume, breathing rate, inspiratory pressure, expiratory pressure and blood gas analysis. Blood gas analysis should be evaluated on an average of 12 hours, within the range of 48 hours after extubation.(3) Clinical prognosis of these patients should be recorded, including the rate of weaning failure, re-intubation rate, time for re-intubation, causes of re-intubation, duration of HFNO/HIV, length of ICU stay, hospitalization time and number of death.（4） The data of adverse effect derived from devices applied in the study should be recorded. HFNO group: dry nasal cavity, dry oropharyngeal cavity, dysphagia.HIV group: dry oropharyngeal cavity, skin injury caused by mask oppression, gaseous distention, vomiting, respiratory aspiration, air leak, drying sputum, difficulty in coughing up phlegm and claustrophobia.

Inclusion Criteria:

• (1) Clinical diagnosis of acute left heart failure
• (2) The patient receives invasive ventilation for more than 24 hours
• (3) Must pass spontaneous breathing trials
• (4) Patient's next of kin agrees to sign the informed consent.

Exclusion Criteria:

• (1) Chronic Obstructive Pulmonary Disease (COPD)
• (2) Disturbance of consciousness
• (3)Bulbar paralysis, dysphagia
• (4) Facial deformity
• (5) Terminal tumor
• (6) Neuromuscular disease