pulmonary arteriovenous malformation

Normally blood from a pulmonary artery must pass through a fine mesh of capillaries in the lung, where it picks up oxygen and is filtered, before returning through the pulmonary veins. A pulmonary arteriovenous malformation is an abnormal direct connection that bypasses this mesh, letting artery flow shortcut straight into a vein, like a road skipping the toll plaza where every car should be checked.

Because the blood detours past the gas-exchanging capillaries, some passes through without being oxygenated, which can lower blood oxygen and cause breathlessness. Equally important, the missing capillary filter means small clots or bacteria that the lung would normally trap can slip through into the general circulation, raising the risk of stroke and brain abscess. Some malformations also bleed, causing coughing up blood.

Many pulmonary arteriovenous malformations are linked to an inherited condition called hereditary hemorrhagic telangiectasia, in which abnormal vessels form throughout the body. When they are significant, they are commonly treated by embolization, threading a catheter to the feeding artery and plugging it from inside, which restores the protective filtering and improves oxygenation.