Respiratory failure is the danger for the patients admitted in ICU, mechanical ventilation could save a lot of patients' life, however, it prolongs patients' ICU stay and hospital stay. High flow nasal cannula (HFNC) has advantages of PEEP effects, reducing dead space, increasing EELV and improving comfort, and it has been used to treat respiratory failure patients. In recent study, HFNC could improve mortality of respiratory failure patients with P/F<200mmHg in the subgroup. The investigators want to start a randomized controlled study to evaluate the effects of HFNC on the patients with respiratory failure.

just as the brief summary

Inclusion Criteria:

1. Age>18 years

2. patients met all four of the following criteria:

   • a respiratory rate of more than 25 breaths per minute,
   • a ratio of the partial pressure of arterial oxygen (Pao2) to the Fio2 of 300 mm Hg or less while the patient was breathing oxygen at a flow rate of 10 liters per minute or more for at least 15 minutes,
   • a partial pressure of arterial carbon dioxide (Paco2) not higher than 45 mmHg, and
   • an absence of clinical history of underlying chronic respiratory failure

Exclusion Criteria:

1. Paco2 of more than 45 mm Hg
2. exacerbation of asthma or chronic respiratory failure
3. cardiogenic pulmonary edema
4. severe neutropenia
5. hemodynamic instability, use of vasopressors,
6. Glasgow Coma Scale score of 12 points or less (on a scale from 3 to 15, with lower scores indicating reduced levels of consciousness)
7. contraindications to noninvasive ventilation
8. urgent need for endotracheal intubation, a do-not-intubate order, and a decision not to participate.