<qa>  What is the randomized double-blinded parallel group clinical trial?This will be a prospective randomized double-blinded parallel group clinical trial evaluating the short-term effects of theophylline therapy on exercise duration and lung function in patients already receiving both tiotropium and a long-acting beta-agonist salmeterol or formoterol. Twenty-four patients with moderate to severe COPD who are already using tiotropium and long-acting beta agonists will be randomized to receive either: a 4 weeks of oral theophylline dosed to achieve therapeutic blood levels, or b 4 weeks of identical placebo therapy. Only patients with stable COPD no exacerbations nor changes in medications over the last 2 months and no contraindications to exercise testing or theophylline use will be enrolled. All patients will undergo baseline pulmonary function testing and incremental and constant load exercise testing prior to randomization. PFTs and constant load exercise testing will be repeated after 4 weeks of therapy. Patients will be instructed to report adverse events and will have their theophylline dose adjusted to ensure therapeutic bloods levels 55-110 umolL, prior to repeat testing. 