A pulmonary arteriovenous malformation is an abnormal connection between an artery and a vein within the lungs that allows deoxygenated venous blood to pass directly into the system circulation.
Pulmonary arteriovenous malformations can occur in the setting of hepatopulmonary syndrome. The diagnosis of a pulmonary arteriovenous malformation is made on agitated saline contrast echocardiography — also known as contrast echocardiography or a "bubble study." In this examination, saline microbubbles can be seen entering the left heart chambers; however, it takes three to 10 cardiac cycles before they appear, which helps to distinguish a pulmonary arteriovenous malformation from an intracardiac shunt, which would cause microbubbles to appear in the left heart within one to three cardiac cycles.
Platypnea can occur from pulmonary arteriovenous malformations and is described as the sensation of dyspnea while in the upright position that improves or resolves while laying supine. This is the opposite of orthopnea, which is commonly present in congestive heart failure.
Most pulmonary arteriovenous malformations are in the base of the lungs; 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 — oxygen desaturation that occurs while upright and improves while supine.