Maximal consumption of oxygen (VO2max) during exercise is used in patients with chronic obstructive pulmonary disease (COPD) to stratify perioperative risk. However, the impact of supplemental oxygen to prevent hypoxemia during exercise on maximal oxygen consumption and other ventilatory parameters during maximal exercise in the resting normoxic Chronic Obstructive Pulmonary Disease population is poorly defined. The investigators performed a randomized controlled trial in patients with COPD who underwent cardiopulmonary exercise tests on room air and supplemental oxygen. The investigators compared maximal oxygen consumption and other ventilatory parameters in each individual subject under the two conditions.

Inclusion Criteria:

• Clinical diagnosis of Chronic Obstructive Pulmonary Disease (COPD).
• Pulmonary function characterized by forced expiratory volume in 1 second/forced vital capacity ratio<0.7, and forced expiratory volume in one second <80% predicted.

Exclusion Criteria:

• Subjects with a recent COPD exacerbation requiring emergency department visit or hospitalization within the last month.
• Subjects scheduled for a major pulmonary intervention in the next 3 months
• Subjects with severe peripheral vascular disease or other physical conditions that would preclude exercise testing