 In this study, the investigators therefore want to examine which beta-blocker might be the safest for COPD patients, as each work slightly differently. Some beta-blockers may have a more beneficial effect on airways than others, whilst still benefitting the heart. The investigators will study two different beta-blockers; one that potentially narrows airways and one that potentially opens airways. The investigators will be using cardiopulmonary exercise testing an exercise bike that measures both heart and lung function during exercise to look for differences between both beta-blockers primarily in terms of lung function but also with information about the heart. The investigators will recruit people with moderate to severe COPD who are able to complete a cycle exercise test through their respiratory research department. The study will last for 10-12 weeks with 5 main visits to the department for serial exercise tests, breathing tests, simple heart function tests and simple blood tests that will tell the investigators what other effects these beta-blockers are having on the heart and lungs. 