[Evaluation exercise tolerance in COPD patients: the 6-minute walking test].

Dyspnea brought on by exercise is one of the basic symptoms of patients with chronic obstructive pulmonary disease (COPD). It appears in the initial phases of the disease, affects activities of daily living,1 and determines, to a large extent, the perception of the degree of illness and extent of deterioration in quality of life. Tolerance to exercise in these patients is an indicator of severity, regardless of forced expiratory volume in the first second.2-4 In fact, the evaluation of both factors—severity of obstructive ventilation (forced expiratory volume in 1 second) and the limitation of tolerance to exercise—are vital in controlling the progression of COPD. Among the simple tests used to evaluate tolerance to exercise, the 6-minute walking (6MW) distance has proved to be a predictor of survival independent of other variables5 and, at the same time, indicates frequency of hospitalizations from exacerbations.6 This is important considering that the progression of the disease is influenced by the frequency of these episodes which signify an associated mortality of from 3% to 10%, or more if the patient is admitted to intensive care.7 The 6MW distance has also been shown to predict postoperative outcome in candidates for volume reduction surgery by identifying patients with a potentially unfavorable outcome.8 Among patients that present postoperative functional improvement, an improvement in tolerance to exercise has a longer duration than changes in forced expiratory volume in 1 second.9 This discrepancy can be explained by the fact that exercise tests reflect not only lung changes but also interactions with other systems that respond to exercise such as the cardiovascular and muscular systems. Thus, the high predictive value of exercise tolerance lies in its multifaceted nature, being affected by: a) airflow limitation from mechanical dysfunction; b) impaired gas exchange; c) insufficient increase in heart rate during exercise, and d) peripheral muscle dysfunction. This multifaceted nature together with lung function limitations measured at rest makes exercise tolerance valuable in characterizing COPD patients.10 However, exercise protocols that can be clinically applied outside lung function laboratories are a practical consideration.