This study evaluates the work of breathing among patients under various conditions of treatment with high flow nasal cannulas. Sixteen spontaneously breathing patients with respiratory failure (hypercapnic or hypoxemic) will be included. The design of this study is a cross over of four treatment periods with different flow settings.

High flow oxygen therapy (HFOT) is a promising technique increasingly used in the management of acute respiratory failure. In hospitalised hypoxemic patients, recent clinical evidence showed that HFOT can reduce endotracheal intubation and reduce mortality. Physiologically, the HFOT causes a decrease in respiratory rate and minute-ventilation and may be associated with a decrease in PaCO2. It is possible that these effects are associated with decreased work of breathing, which could explain some of the benefits in terms of comfort and efficiency.

The objective of this research is to evaluate the impact of a wash-out of anatomical dead space by high flow nasal cannulas on respiratory parameters and on the work of breathing.

The investigators will evaluate the baseline status with conventional oxygen therapy methods (low flows), and then compare it with three different levels of high flow oxygen therapy in a randomised order. The primary endpoint will be the work of breathing.

Inclusion Criteria:

Spontaneously breathing patients exhibiting a respiratory distress defined by a respiratory rate ≥ 20 breaths/min associated with

• either hypoxemia (SpO2<90% with O2≥3L/min) ;
• or hypercapnia (PaCO2>45 mmHg with a respiratory acidosis (pH<7,38).

Exclusion Criteria:

• Patients below 18yo, pregnant or breastfeeding women;
• Patients enrolled in another study excluding co-enrolment;
• History of ear nose and throat disease (i.e. surgery, epistaxis, trauma), eso-gastric disease (i.e. esophageal varices, digestive haemorrhage), rheumatologic or neurologic disease possibly interfering with the design of the study;
• Acute respiratory or cardiovascular disease contra-indicating the enrolment in the study protocol (i.e. acute coronary syndrome, pulmonary embolism, pneumothorax);
• Need for immediate intubation or for continuous noninvasive ventilation and/or recent (<6h) arterial blood gases showing a respiratory acidosis with a pH<7.30;
• Patient feeling nauseous or under recent fed condition (<1h).