Platypnoea-orthodeoxia after left total knee replacement.

Platypnoea-orthodeoxia syndrome (POS) is a rare disorder characterised by both dyspnoea (platypnoea) and arterial desaturation (orthodeoxia) in the upright position, with improvement in the supine position. We report an unusual case in which an 82-year-old woman developed severe hypoxaemia with POS after left total knee replacement. A significant difference in alveolar-arterial blood gas oxygen tension was demonstrated, and hypoxaemia failed to respond to 100% oxygen supply. A patent foramen ovale with a right-to-left shunt was evident on transoesophageal echocardiogram employing colour Doppler and agitated normal saline studies. Interestingly, the patient's symptoms resolved within 6 months with ongoing chest physiotherapy, without surgical or medical intervention.