 Patients with chronic obstructive pulmonary disease COPD may develop low oxygen levels, because of damage to their lungs. Long term oxygen therapy LTOT is given for at least 15 hours per day, and has established indications and benefits in COPD. However, the indications for and benefits from ambulatory oxygen supplementation oxygen just when walking or exercising are less well understood, in part due to heterogeneity of previous study designs, and lack of long term follow up. This is a pilot study of supplementary ambulatory oxygen in COPD, which allows us to ascertain mechanisms of disease by measuring their degree of systemic inflammation pre and post oxygen supplementation, and measuring changes in gene expression in muscles by means of microarray profiling. Secondly, our study will utilise follow up of clinical parameters including home activity monitoring to ascertain mediumlong term benefits of oxygen supplementation in a real life setting. Our hypothesis is that exertional hypoxia results in muscle dysfunction and this could be prevented by oxygenation. 