A pulmonary arteriovenous malformation (AVM) is an abnormal connection between an artery and a vein in the lungs that allows blood to bypass the tiny air sacs where oxygen exchange normally happens. Instead of flowing through the lung’s capillary network, blood takes a shortcut directly from the arterial side to the venous side, returning to the body without picking up enough oxygen or being filtered. Pulmonary AVMs affect an estimated 38 per 100,000 people, with a strong predominance in women.
How a Pulmonary AVM Disrupts Normal Lung Function
In a healthy lung, blood flows from the heart’s right side into progressively smaller vessels until it reaches capillaries wrapped around tiny air sacs called alveoli. There, carbon dioxide leaves the blood and oxygen enters it. The freshly oxygenated blood then returns to the heart’s left side and gets pumped to the rest of the body. The capillary network also acts as a filter, trapping small blood clots, bacteria, and debris before they can reach organs like the brain.
A pulmonary AVM creates a direct, capillary-free tunnel between the pulmonary artery and vein. Blood flowing through this tunnel never contacts ventilated air sacs, so it arrives back in the general circulation still carrying too much carbon dioxide and too little oxygen. This is called a right-to-left shunt. The larger the AVM, or the more AVMs present, the greater the proportion of blood that skips gas exchange, and the lower a person’s oxygen levels drop. The body often compensates by increasing the drive to breathe, which is why some people with pulmonary AVMs feel chronically short of breath even though their lungs are structurally normal in every other way.
Connection to Hereditary Hemorrhagic Telangiectasia
Between 80% and 95% of people diagnosed with a pulmonary AVM have a genetic condition called hereditary hemorrhagic telangiectasia, or HHT. This inherited disorder causes blood vessels throughout the body to form abnormally, leading to fragile, dilated vessels in the nose, skin, lungs, liver, and brain. About half of all people with HHT develop pulmonary AVMs at some point in their lives.
HHT follows an autosomal dominant pattern, meaning a single copy of a faulty gene from either parent is enough to cause the condition. Roughly 44% of HHT cases trace to mutations in a gene called ENG, and 52% to a gene called ACVRL1. Pulmonary and brain AVMs are more common in people carrying ENG mutations specifically. Because of this strong genetic link, all adults with confirmed HHT are screened for pulmonary AVMs at specialized centers, and 82% of North American HHT centers now also screen children routinely.
Symptoms and Physical Signs
Many pulmonary AVMs cause no symptoms at all and are discovered incidentally on a chest scan done for another reason. When symptoms do appear, they typically reflect low oxygen levels: shortness of breath during exertion, fatigue, and sometimes a bluish tint to the lips or fingers (cyanosis). Digital clubbing, where the fingertips widen and the nails curve downward, can develop over time in people with large or multiple AVMs.
One distinctive symptom is called platypnea: breathlessness that worsens when standing up and improves when lying down. This happens because most pulmonary AVMs sit in the lower portions of the lungs, where gravity directs more blood flow when you’re upright, pushing a larger volume of blood through the shunt. About one-third of people with pulmonary AVMs show a measurable drop in oxygen saturation of 2% or more simply from moving from lying down to standing, a finding called orthodeoxia.
Serious Complications
The biggest risks from pulmonary AVMs aren’t in the lungs themselves. They’re in the brain. Because the capillary filter is bypassed, tiny blood clots and bacteria that would normally be trapped can travel freely into the arterial circulation and lodge in brain vessels. This process, called paradoxical embolism, can cause two major problems.
The first is stroke. Small clots passing through the AVM can block cerebral arteries, and studies report stroke rates ranging from about 12% to 17% of PAVM patients in large series, though some smaller studies have found rates as high as 45%. The second is brain abscess, where bacteria slip through the shunt and seed an infection in brain tissue. Brain abscess rates in published studies range from roughly 4% to 19% of PAVM patients. Both complications can occur even when the AVM itself seems small and causes no respiratory symptoms, which is why treatment guidelines have shifted toward intervening on all treatable PAVMs regardless of size.
How Pulmonary AVMs Are Diagnosed
The first-line screening test is a contrast echocardiogram, commonly called a bubble study. A small amount of agitated saline is injected into a vein in the arm while an ultrasound monitors the heart chambers. Normally, the bubbles get trapped in the lung capillaries and never appear on the left side of the heart. If bubbles show up in the left chambers after three to eight heartbeats, it indicates blood is crossing through an abnormal connection in the lungs.
The bubble study also provides a rough sense of severity. Fewer than 30 bubbles crossing over (grade 1) is not associated with increased risk of brain complications. Thirty to 100 bubbles (grade 2) or more than 100 bubbles (grade 3) independently predict a higher chance of stroke or brain abscess. If the bubble study is positive, a thin-slice CT scan of the chest is the gold standard for confirming the diagnosis, pinpointing the AVM’s location, measuring the feeding artery, and planning treatment.
Treatment With Embolization
The standard treatment for pulmonary AVMs is transcatheter embolization, a minimally invasive procedure performed by an interventional radiologist. A thin catheter is threaded through a vein, typically in the groin or wrist, and guided into the feeding artery of the AVM. Once in position, the radiologist deploys small metallic coils, a self-expanding mesh plug, or a combination of both to block blood flow through the malformation.
Historically, AVMs with feeding arteries smaller than 3 millimeters were considered low risk and left untreated. That threshold has been abandoned. Strokes and brain abscesses have been documented in patients with AVMs below that size, and current guidelines recommend embolizing all treatable PAVMs if technically feasible. The immediate technical success rate for the procedure is very high, effectively reaching 100% for complete blockage at the time of treatment. Most patients go home within two to three days.
The main limitation is that AVMs can reopen over time. Recanalization rates for coils alone range from about 4% to 28% depending on the study. Vascular plugs have a recanalization rate of roughly 5% to 33%. Using coils and plugs together appears to produce the best long-term results. In one study, 25 out of 26 AVMs treated with the combined approach remained fully blocked at 12 months. Because of the possibility of reopening, follow-up visits are typically scheduled at one month and six months after treatment, with periodic imaging thereafter.
Pregnancy and Pulmonary AVMs
Pregnancy poses particular risks for women with pulmonary AVMs. The normal cardiovascular changes of pregnancy, especially the significant increase in blood volume and cardiac output during the second and third trimesters, can cause an existing AVM to expand rapidly. Hormonal changes also affect vessel walls. Together, these factors raise the risk of AVM rupture, which can lead to bleeding into the chest cavity and, in severe cases, life-threatening blood loss.
For women who know they have pulmonary AVMs, treatment before becoming pregnant is strongly preferred. If an AVM is discovered during pregnancy, embolization can be performed in the second or third trimester by an experienced team, when radiation exposure to the fetus is considered minimal. Any pregnant woman with worsening or unexplained shortness of breath, particularly one with a personal or family history of HHT, should be evaluated for a possible pulmonary AVM.