When a patent foramen ovale becomes pathological

Platypnoea–orthodeoxia syndrome (POS), first described in 1949, remains one of the most fascinating examples of positional physiology in cardiovascular medicine. Patients experience dyspnoea and arterial desaturation that worsen when upright and improve when supine. At first glance, the mechanism seems simple: an interatrial communication allows right-to-left shunting. In practice, however, POS is rarely straightforward.