FiO2 in Chronic Obstructive Pulmonary Disease (COPD) Patient Normobaric O2 Versus NIV

in a prospective observational study the investigators will register FiO2 in 15 voluntaries COPD patients as following: * Pharyngeal FiO2 (G5® Philips™ side stream FiO2) with O2 supply through nasal cannula at basal flow (flow needed by patient). In order to compare FiO2s the investigators will measure FiO2 with 150%, 75%, 50% and 25 % of basal Flow * And FiO2 during nocturnal NIV (pressure support ventilation; Vivo 50®, Bréas™), using facial or nasal masque (Confort Gel® Respironics™). O2 flow will be basal flow.