 Hypoxaemic patients with exacerbations of chronic obstructive pulmonary disease COPD are at some risk of carbon dioxide CO2 retention during oxygen therapy. Main mechanism of CO2 retention is believed to be reversal of preexisting regional hypoxic pulmonary vasoconstriction, resulting in a greater dead space. Risk of CO2 retention during mechanical ventilation remains controversial. Thus recent study suggested limited risk of CO2 retention with controlled oxygen supplementation during mechanical ventilation. Conversely, controlled oxygen supplementation might decrease dyspnea and respiratory workload, increase comfort and improve both urinary output and renal function. 