The paradox of platypnoea-orthodeoxia syndrome.

Platypnoea-orthodeoxia is a rare clinical syndrome characterised by dyspnoea and oxygen desaturation in the upright position which improves when supine. It requires two components: a sufficiently sized anatomical vascular defect (typically intra-cardiac or intra-pulmonary) combined with a functional component that promotes positional right-to-left shunting. We describe the rare occurrence of a patient with platypnoea-orthodeoxia syndrome (POS) because of a paradoxical shunt through a patent foramen ovale caused by a large right atrial line-associated thrombus in a male with metastatic oesophageal cancer undergoing chemotherapy. This case is a timely reminder to consider POS amongst differentials for hypoxia as it is often treatable if recognised.