Air embolism happens when gas bubbles enter the bloodstream and block blood flow, much like an air pocket trapped in a water pipe. The most common causes are medical procedures involving veins or arteries, scuba diving accidents, and chest trauma. While a small amount of air in a vein is usually harmless and gets absorbed, as little as 300 to 500 milliliters injected rapidly into a vein can cause cardiac arrest, and even a tiny bubble in an artery can trigger a stroke or heart attack.
How Air Gets Into the Bloodstream
For air to enter your blood vessels, two things need to happen at the same time: there has to be an opening in a blood vessel, and there has to be a pressure difference that pulls or pushes air inward. Veins, especially those near the heart, naturally have low pressure. If a vein is open to the outside air (through a needle, catheter, or wound), the slight vacuum effect of breathing in can actually suck air into the vessel. The higher up on the body the opening is, and the more upright you are, the greater the pressure difference and the easier it is for air to enter.
Once inside, air bubbles travel through the veins toward the heart. A large collection of air can get trapped in the right side of the heart, forming what’s called an “air lock.” The heart essentially tries to pump a compressible pocket of air instead of blood, which dramatically reduces blood flow to the lungs and the rest of the body. Smaller bubbles may pass through to the lungs, where they block tiny blood vessels and interfere with oxygen exchange.
Medical Procedures Are the Leading Cause
The majority of air embolisms today are iatrogenic, meaning they result from medical interventions rather than accidents or injuries. Central venous catheters (the large IV lines placed in the neck, chest, or groin) are the single most common source. Air can enter during insertion, while the line is in use, or even during removal if the site isn’t properly sealed. The reported incidence rate for air embolism with central lines ranges from 0.03% to 2%, which sounds small but represents a meaningful number given how frequently these lines are placed in hospitals.
A wide range of other procedures carry risk as well:
- Endoscopy and colonoscopy, where air is deliberately pumped into the digestive tract and can enter through damaged tissue
- Hemodialysis, which cycles blood out of the body and back through tubing that can introduce air
- Neurosurgery, particularly when performed with the patient in a seated position, which maximizes the pressure gradient pulling air into open veins in the skull
- Cardiac and abdominal surgery, where large veins are exposed during the operation
- Thoracentesis (draining fluid from around the lungs) and tissue biopsies
- Angiography and other procedures involving catheters in arteries
These non-surgical procedures have actually overtaken surgery and trauma as the more significant causes of air embolism in recent years, largely because they’re performed so frequently in routine hospital and outpatient care.
Scuba Diving and Lung Overexpansion
Scuba diving is the best-known non-medical cause. The mechanism is straightforward: as a diver descends, the air in their lungs gets compressed by increasing water pressure. If the diver ascends toward the surface without exhaling, that compressed air expands. At a certain point, the expanding gas exceeds what the lung tissue can stretch to accommodate, and the tiny air sacs in the lungs rupture.
When those air sacs tear, gas bubbles can escape into the blood vessels surrounding them. From there, the bubbles travel through the pulmonary veins to the left side of the heart and get pumped directly into the arteries. This is called arterial gas embolism, and it’s one of the most dangerous forms because the bubbles head straight to the brain and heart. It most often happens when a novice diver panics and kicks rapidly toward the surface without breathing out, but it can occur with an ascent of just a few feet if the lungs are full and the airway is closed.
Trauma to the Chest
Both penetrating injuries (like stab wounds or gunshot wounds) and blunt force trauma to the chest can cause air embolism. The mechanism is similar to what happens in diving: damage to lung tissue creates a connection between the airways and the blood vessels, allowing air to cross into the bloodstream. In some cases, a direct injury to a large vein in the chest or neck can also expose the vessel to outside air. Because the chest contains both the lungs and major veins returning blood to the heart, it’s the most vulnerable area for trauma-related air embolism.
Why Arterial Air Embolism Is More Dangerous
The body handles venous and arterial air very differently. In the venous system (the blood returning to the heart), small amounts of air are typically filtered out by the lungs and absorbed without causing symptoms. It takes a large, rapid dose, on the order of 300 to 500 milliliters delivered at about 100 milliliters per second, to overwhelm the right side of the heart and cause cardiovascular collapse.
Arterial air embolism operates on a completely different scale. Even a tiny bubble in an artery can be fatal because arteries carry blood directly to organs. A bubble that reaches the brain can block blood flow and cause stroke-like symptoms: sudden weakness on one side, seizures, loss of consciousness, or coma. A bubble reaching the coronary arteries can cause a heart attack. There is no safe threshold for air in the arterial system.
The Patent Foramen Ovale Problem
About 25% of adults have a small opening between the right and left sides of the heart called a patent foramen ovale, or PFO. This is a remnant from fetal development that never fully closed. In most people it causes no problems, but in the context of air embolism, it creates a dangerous shortcut. Air bubbles that would normally be trapped in the right side of the heart or filtered by the lungs can instead cross through the PFO directly into the arterial circulation. This is called a paradoxical embolism, and it means that even a modest amount of venous air, which would otherwise be harmless, can reach the brain or heart and cause serious damage. Most people with a PFO don’t know they have one until a complication like this reveals it.
What Air Embolism Feels Like
Symptoms depend on where the air ends up and how much enters the bloodstream. A small venous air embolism may cause no noticeable symptoms at all. Larger amounts typically produce sudden shortness of breath, chest pain, lightheadedness, and a rapid heart rate. If enough air accumulates in the heart, blood pressure drops sharply and the skin may turn bluish from lack of oxygen.
When air reaches the brain through the arteries, neurological symptoms appear quickly: confusion, difficulty speaking, vision changes, weakness in the limbs, or seizures. In a hospital setting, one of the classic signs that alerts clinicians is a sudden, unexplained drop in blood pressure combined with low oxygen levels during or immediately after a procedure. In severe cases, a distinctive churning sound called a “mill-wheel murmur” can be heard through a stethoscope over the chest, caused by the heart beating against a pocket of trapped air and blood.
How Air Embolism Is Treated
Treatment focuses on two goals: stopping more air from entering and moving the air that’s already there out of critical positions. If a catheter or surgical site is the source, it gets clamped or sealed immediately. The patient is then repositioned: for venous air embolism, lying on the left side with the head tilted downward encourages the trapped air bubble to float out of the heart’s main pumping chamber and into the upper right chamber, breaking the air lock that stalls blood flow. For arterial air embolism, lying flat is preferred because a head-down position can worsen brain swelling.
The definitive treatment for serious cases is a hyperbaric oxygen chamber, which works by compressing the air bubbles to a smaller size and speeding up their absorption into surrounding tissue. High-concentration oxygen delivered by mask also helps by replacing the nitrogen in the bubbles with oxygen, which the body absorbs much faster. Recovery depends heavily on how much air entered, where it traveled, and how quickly treatment began. Brain injuries from arterial air embolism can leave lasting neurological deficits if treatment is delayed.