How and Why Exercise Is Impaired in COPD

Published data indicate that exercise in COPD is more often limited by leg effort than breathlessness. This casts some doubt on the classical belief that inability to ventilate limits exercise performance. In fact, symptoms limiting exercise appear to be essentially the same in COPD and in health or congestive heart failure, where exercise is limited by inadequate energy supply to locomotor muscles. In COPD, impaired O 2 delivery to locomotor muscles is suggested by: (1) the O 2 cost (v̇ O2 ) of breathing may be ∼50% of whole body v̇ O2 ; (2) decreasing the work of breathing improves performance and v̇ O2 of locomotor muscles, and (3) locomotor muscle v̇ O2 is greater when it is the only muscle exercising than during whole body exercise. Excessive expiratory pressures when expiratory flow is limited may lead to decreased venous return and contribute importantly to exercise limitation.