Discussion
COPD is a disease that impairs functional capacity. However,
the degree of exercise intolerance as measured by the 6MWD
test varies widely even among patients with similar airflow limitation. This was evident in large clinical trials such as those of patients enrolled for lung volume reduction surgery, who
by design had a narrow range of airflow limitation but a wide
range of exercise performance. The exact reason for exercise
limitation in COPD is not fully understood but appears to
be somewhat independent of FEV1. In the majority of patients
with COPD, dyspnea is a main limiting factor , likely related
to hyperinflation airflow limitation and impairment in diffusing capacity. In addition, skeletal muscle dysfunction has also been implicated in the genesis of exercise limitation among patients with COPD. The muscle of COPD patients shows structural and metabolic abnormalities, not seen in normal patients of similar age. Whatever the exact reasons for exercise limitation, the evaluation of exercise capacity using the 6MWD in patients with COPD helps assess disease severity. Furthermore, identifying levels of exercise impairment helps with the characterization of these patients and assists with the referral to programs such as pulmonary rehabilitation, lung reduction surgery or lung transplantation.