Evaluating the effects of chronic therapy in patients with irreversible air-flow obstruction

The efficacy of treatment in patients with irreversible chronic obstructive pulmonary disease (COPD) is frequently evaluated after prolonged therapy. While objective measurements such as spirometry and exercise testing have been shown to be reproducible on consecutive days, their variability over longer intervals is unknown. To evaluate this, spirometry and exercise testing were measured in 13 clinically stable COPD patients with chronic obstructive pulmonary disease (12 male, 63 (+/- 4) yr of age) on two occasions one month apart. Minute ventilation, maximal O2 consumption (VO2max), CO2 production, and heart rate during cycle ergometry (10-watt increments/1-min stages) to symptomatic limitation on the two days were compared. For the group as a whole, the spirometric measurements and response to incremental exercise on the second test day were similar (all p greater than 0.1) to those observed one month earlier. The coefficient of variation of the ventilatory and gas exchange measurements during exercise ranged from 6 to 10% which was similar to the variability observed in spirometry. The correlation between changes in air flow and VO2max in individual patients was poor (r = 0.37, p greater than 0.25). We conclude that the modest improvements in objective measurements of air flow or exercise performance in patients with chronic obstructive pulmonary disease may reflect intrasubject variability rather than therapeutic effectiveness. This variability should be considered when evaluating therapy.