This is a single-center, open-label, randomized controlled trial to evaluate the effect of high-flow nasal cannula with a flow rate of 60 L/min versus 40 L/min after planned extubationon on a composite outcome of reintubation and use of NIV.

This is a single-center, open-label, randomized controlled trial to evaluate the effect of high-flow nasal cannula with a flow rate of 60 L/min versus 40 L/min after planned extubationon on a composite outcome of reintubation and use of NIV in patients who are intubated for hypoxemic respiratory failure.

[Study Procedures] Set flow rates of HFNC The flow rates of HFNC(high-flow nasal cannula) are set as 40 L/min and 60 L/min, respectively in the two trial groups. FiO2 was down-titrated to the minimal level to keep SO2 ≥ 92%. After that, the flow rate setting is fixed for 24hrs +/- 6 hrs in the two groups. After that, the flow rate setting in both groups would be tapered to 30 L/min and would be kept for 12 hours. Then, HFNC would be changed to conventional oxygen therapy to keep SpO2≥ 92%.

[Outcome Measures] To increase the statistical power of this pilot trial, we used a composite outcome of NIV use and or reintubation in 48 hours for the primary endpoint. Secondary endpoints include mortality of different time points, physiological parameters(respiratory rate, heart rate), ventilation/oxygenation data(PaO2/FiO2 ratio, pH) and patient comfort. Exploratory endpoints include comparison between different flow rate settings.

[Primary endpoint] We hypothesized that higher flow setting of HFNC can reduce work of breathing and can increase washout of dead space, which could result in lower re-intubation rate and lower NIV use rate after planned extubation in hypoxemic patients. The primary endpoint is a composite outcome of NIV use or reintubation in 48 hours between two groups of different flow rates.

[Secondary endpoints] ICU mortality In-hospital mortality 28-day mortality Time to successful liberation from mechanical ventilation AUC(area under curve) of respiratory rate (0-24 hours) AUC of heart rate (0-24 hours) PaO2/FiO2 ratio at 4 and 24 hours Change of arterial CO2 level(mmHg) at 4 and 24 hours Proportion of respiratory acidosis (arterial blood gas: pH<7.35) at 4 and 24 hours Proportion of Intolerance (up titration of flow or/and FiO2; down titration of flow) in 24 hours

[Sample size] The event rate of primary endpoint, defined as a composite outcome of NIV use and or reintubation in 48 hours, was assumed to be 40% in the 40 L/min arms and 20% in the 60 L/min. We estimated that with a sample of 180 patients, the study would have 80% power to detect a 20% absolute reduction in proportion of composite outcomes, at a two-sided type I error rate of 5%.

Inclusion Criteria:

• invasive mechanical ventilation > 48 hours
• acute hypoxemic respiratory failure(PaO2/FiO2 < 300 mmHg) as a main cause of invasive mechanical ventilation.
• planned extubation & already passed a spontaneous breathing trial (SBT)

Exclusion Criteria:

• < 20 years of ages
• refusal to re-intubation
• with terminal cancer
• pregnant women
• with a tracheal stoma or tracheostomy tube in situ
• not feasible for high-flow nasal cannula(decided by the primary care team)
• must required to use non-invasive ventilation immediately after extubation (decided by the primary care team)