Platypnoea-orthodeoxia syndrome: the importance of the patient's posture for diagnosis.

Platypnoea-orthodeoxia syndrome (POS) is a rare condition characterised by the association of dyspnoea and arterial oxygen desaturation induced by standing and relieved by recumbency. The precise mechanism remains poorly understood, but it appears to involve both anatomical and functional components producing a significant right-to-left shunting under certain postural conditions. This syndrome is associated with either intracardiac or extracardiac aetiologies, and the diagnosis depends on high clinical suspicion. Intracardiac shunt is the most common cause of POS and can be identified through echocardiography with bubble study. We report the case of a patient with POS secondary to a paradoxical shunt through the patent foramen oval, whose definitive diagnosis was only possible with contrasted transthoracic echocardiography in the sitting and standing positions.