Platypnea refers to shortness of breath (dyspnea) that is relieved when lying down,^[1] and worsens when sitting or standing up. It is the opposite of orthopnea. The word is derived from the Greek platus (= flat) and pnoia (=breath).

1. Left atrial thrombus
2. Left atrial tumors
3. Pulmonary arteriovenous fistula

Platypnea is due to either hepatopulmonary syndrome or an anatomical cardiovascular defect increasing positional right-to-left shunting (bloodflow from the right to the left part of the circulatory system). These defects include rare syndromes in which the venous blood from the liver does not pass through the lungs, or if venous blood from the portal circulation reaches the inferior vena cava without passing through the liver (Abernethy malformation, type 1).

Insufficiency of abdominal muscles causes lower diaphragm position and dyspnea. In clinostatism, abdominal organs push the diaphragm in its normal, upper position, reducing the respiratory effort.

Template:Circulatory and respiratory system symptoms and signs