A number of studies have demonstrated that whilst at home, patients on LTOT experience episodes of intermittent hypoxia (SpO2 <90%) and spend significant amounts of time with SpO2 <90%. These episodes of desaturation occur most frequently during activities of daily living and may be harmful as they could lead to pulmonary hypertension, arrhythmias and ischaemic heart disease. Simply increasing the LTOT flow rate by 1 or 2 litres in one option but this exposed the patients to hyperoxaemia and the risk of hypercapnia. An another method optimising LTOT is to utilise an auto-titrating oxygen system. These system allow close control of SpO2 by automatically adjusting the flow of oxygen to match a target SpO2. At the Royal Brompton Hospital and Imperial College London, the investigators have developed our own auto-titrating oxygen system - called intelligent oxygen therapy (iO2t).