Platypnea is the sensation of dyspnea while in the upright position that improves or resolved while laying supine. This is the opposite of orthopnea which is commonly present in congestive heart failure.
Platypnea can occur in patients with left atrial myxomas as the upright position is more likely to allow the tumor to obstruct mitral valve inflow than when supine. Also, the “hepatopulmonary syndrome” which consists of pulmonary arteriovenous malformations causing an intrapulmonary shunt can cause platypnea. Most of these malformations are in the base of the lungs and thus while upright, blood flow is redistributed to the lung base causing more shunting. While supine, less shunting occurs and thus the sensation of dyspnea improves.
Platypnea is associated with orthodeoxia which is oxygen desaturation that occurs while upright and improves while supine.