The purpose of this study is to compare the effect of continuous positive airway pressure (CPAP) delivered by the variable generator WhisperFlow System with high flow nasal cannula oxygen therapy in mechanically ventilated patient who are at risk for postextubation failure.

Postextubation respiratory failure is not an uncommon condition after discontinuation of mechanical ventilation. Subsequently reintubation and mechanical ventilation for postextubation respiratory failure may lead to increasing of hospital morbidity and mortality and and prolonging intensive care and hospital stay. Several studies have demonstrated the efficacy of non-invasive ventilation to prevent the postextubation failure in patients with high risk for extubation failure.

The variable generator whisper flow system ((Philips Respironics, Murrysville, PA, USA) is the one of widely used continuous positive airway pressure(CPAP) device in UK. This device employs an oxygen-driven venturi to entrain air and generates high fresh gas flow up to 140 liter per minute. The oxygen/air mixture enters the breathing circuit and exits through a threshold resistor namely CPAP valve. Whisperflow is able to provide the flow into the breathing system via oronasal mask which exceeds the peak inspiratory flow rate of the patient and may improve work of breathing as well as gas exchange. In daily clinical practice, the investigators apply this device with humidifier in patients with weaning difficulty and patients who are at risk for postextubation failure.

High-flow nasal oxygen therapy(HFNO) is a device which heated and humidified oxygen is distributed to the nose at high flow rates. With the high flow rate (up to 60 Liter per minute), it generates low level of positive airway pressure. Physiological benefits of HFNO comprises decreasing of the dead space, work of breathing and the low level of positive airway pressure associated with better lung recruitment. Furthermore, the fraction of inspired oxygen can be adjusted and measured by changing the fraction of oxygen in the driving gas.

In the present randomized, controlled trial, the investigators will compare two devices for oxygen therapy, with low levels of continuous positive airway pressure, nasal high-flow oxygen therapy and CPAP of 5 cmH2O delivered by the variable generator WhisperFlow System with oronasal mask , in critically ill patients who are at risk for postextubation failure. The investigators hypothesize that nasal high-flow is superior to CPAP the in terms of reintubation rate.

Inclusion Criteria:

• Mechanical ventilation > 48 hours and
• high risk for extubation failure namely , congestive heart failure, ineffective cough and excessive tracheobronchial secretions, more than one failure of a weaning trial, more than one co-morbid condition, patients who had hypercapnia, and upper airway obstruction without immediately required intubation

Exclusion Criteria:

• age<18 years
• pregnancy
• tracheostomy
• uncontrolled cardiac ischemia or arrhythmias
• unstable hemodynamic status
• documented swallowing problem
• documented cervical spine injury
• Facial anatomical abnormalities interfering with the mask fit
• agitated or uncooperative state
• lack of informed consent