STUDY OBJECTIVES: Supplemental oxygen is used in hypoxemic patients with chronic airways obstruction (CAO) because it reduces pulmonary artery pressure and prolongs life. The purpose of this study was to assess at rest the effects of 30% oxygen inhalation on dyspnea, breathing pattern, neuromuscular inspiratory drive based on measurement of mouth occlusion pressure (P(0.1)), and dynamic hyperinflation (DH), as reflected by changes in inspiratory capacity (IC). METHODS: Ten patients with stable CAO receiving long-term oxygen were studied at rest, before and after 5, 15, and 25 min of oxygen administration. Severity of dyspnea was rated using the visual analog scale (VAS). Breathing pattern parameters, P(0.1), IC, and tidal expiratory flow limitation (EFL), were measured sequentially. RESULTS: Eight patients exhibited EFL under baseline condition. During 30% oxygen breathing, the VAS score significantly decreased, associated with a concurrent increase of IC (11%). There was also a significant reduction of minute ventilation and tidal volume (11% and 12%, respectively), which was due to a significant decrease of mean inspiratory flow. Although not significantly, P(0.1) decreased by 13%. Finally, two patients reverted from EFL to no EFL. CONCLUSION: Patients with CAO receiving long-term oxygen may benefit from hyperoxic breathing at rest, since it decreases the ventilation and the degree of DH, with concurrent improvement of dyspnea sensation.