<qa>  What is the primary endpoint of the study?Exercise duration on constant load exercise testing was chosen as the primary endpoint of this study because it has been shown to be a reproducible and highly responsive measure for physiologic benefit in COPD. It is unknown whether theophylline will provide any additional increase in exercise duration, when added to a combined long-acting bronchodilator regimen. If an improvement is found with theophylline, a follow-up study of larger size and longer duration will be required to investigate whether this benefit will translate into improvements in clinical outcomes such as quality of life or frequency of exacerbations. Alternatively, if no improvement can be detected, the current COPD treatment recommendation should be re-evaluated. 