The objective of this thesis was to determine the impact of right to left shunting (RLS) through patent foramen ovale (PFO) on oxygen saturation in obstructive sleep apnoea (OSA) and chronic obstructive pulmonary disease (COPD). The first aim was to determine the prevalence of PFO in severe OSA and whether PFO closure improves nocturnal oxygen saturation. The data showed that PFO with large shunts were more prevalent, however overall prevalence was not statistically different when...