Recovery of Gas Exchange Variables and Heart Rate
To patients with COPD, exercise performance is limited by reduced ventilatory capacity. Although many studies have documented the correlation between reduced work capacity and spirometric indices of airflow obstruction, recovery from exercise has received little attention in COPD. The metabolic cost of exercise involves not only the performance of external work, but the demands of the oxygen transport system itself, including the work performed by the muscles of respiration, and persistent hypermetabolism during recovery. In normal subjects after maximal exercise, Vo2 uring recovery declines in a biexponential manner and reaches near baseline values in approximately five to ten minutes. We hypothesized that patients with COPD have more prolonged hyperpnea after exercise than normal subjects due to airflow limitation and reduced rate of elimination of excess carbon dioxide formed during exercise. The clinical implications of prolonged hyperpnea after exercise in patients with COPD may include greater metabolic demand for a given task and reduced resistance to fatigue, particularly for repetitive tasks. In addition, delayed elimination of increased body stores of carbon dioxide would impair the ability to adjust to alterations of acid-base status in these patients.