The lungs are special because they have two circulations - the bronchial or systemic circulation and the pulmonary circulation. The pulmonary circulation is important for gas exchange. As the pulmonary arteries that come out of the right heart become smaller and smaller, pulmonary capillaries are formed, which links very closely to air sacs (called alveoli) which take in oxygen and release carbon dioxide from the body. When the oxygen in the air that is breathed in  and ends up in the alveoli cannot participate in gas exchange, the body becomes started of of oxygen needed for important functions in the different organ systems, including the brain.


 Pulmonary arterio-venous malformations (PAVMs) are abnormal connections of the low-flow pulmonary circulation in   the lungs (https://www.youtube.com/watch?v=NDk8fmIl9V8).


 PAVMs are associated with a diagnosis of HHT in 90% of cases. Sometimes a bad infection in the lungs that remains   untreated for a while can also lead to a focal PAVM close to the lesion. Sometimes trauma can lead to a focal PAVM. It   is extremely rare to see multiple PAVMs outside of HHT.


 PAVMs can be silent and not cause symptoms until there develops a complications. Some patients have finger clubbing (a bump in the nails of the fingers and toes) that doctors and nurses look for. Some patients with PAVMs complain of headaches related to the shunt caused by abnormal blood flow through these lesions.


 Complications of PAVMs include low oxygen levels, bleeding (pulmonary hemorrhage), strokes and brain abscess.   Therefore it is important to screen patients at high risk for this.


 For further detailed information on normal pulmonary circulation and respiratory physiology, please see the following   video.


 If you have any questions about HHT and PAVMs, talk to your doctor about it.