At an FIO2 of 0.21, arterial blood gas results below are rep…

At an FIO2 of 0.21, arterial blood gas results below are reported for a patient with COPD and dyspnea:pH                           7.30PaCO2                     60 torrPaO2                        43 torrHCO3                      29 mEq/LBE                           +3 mEq/LThe most appropriate oxygen therapy for this patient is a: