Chronic obstructive lung disease is a disabling disease that affects people usually after several years of smoke tobacco exposure and affects millions of patients worldwide. The disease is marked by multiples episode of worsening, termed exacerbations necessitating frequent hospitalizations. During these exacerbations, patients present breathless, and in the most severe cases, are admitted to an Intensive Care Unit (ICU) for respiratory assistance. Currently, respiratory assistance is provided by a ventilator via a oronasal mask (referred to non-invasive ventilation, NIV), that helps patients to cope with their breathless. The mask is not always well tolerated and the ventilator sessions are delivered intermittently. In the past decade, a new technique that provides air-oxygen with high flow has been developed. This technique, called High Flow via Nasal Cannula (HFNC) can deliver from 21 to 100% heated and humidified air-oxygen at a high flow of gas via simple nasal cannula. Recent studies have shown that the technique is very efficient to treat patients presenting with acute respiratory failure who don't have any underlying chronic pulmonary disease. Whether the technique would be also efficient in patients with COLD presenting with severe exacerbations has not yet been demonstrated. Since HFNC does not require any mask, it is thought that the comfort of the patient would be much better in comparison to NIV and could potentially help to treat many patients with the disease. The objective of the present study is to study the physiological effect of HFNC as compared to NIV in patients with severe exacerbations of COPD and to show that it is non-inferior to NIV.