An air embolism, or gas embolism, is a medical emergency where a gas bubble becomes trapped in the circulatory system, obstructing blood flow. This blockage can lead to serious complications, including stroke, heart attack, or respiratory failure. The consequences can be severe even with small volumes of gas. The severity of an air embolism depends highly on the location in the body where it occurs and the rate at which the air enters the circulation.
The Pathophysiology of Air Embolism
Air embolisms are categorized based on where the gas enters the body: Venous Air Embolism (VAE) and Arterial Air Embolism (AAE). VAE occurs when air enters the venous circulation and travels toward the right side of the heart, eventually reaching the lungs. The bubbles accumulate in the right ventricle and pulmonary artery, creating a physical obstruction known as a “gas lock.”
This gas lock prevents the right side of the heart from effectively pumping blood into the lungs, leading to acute right-sided heart strain, pulmonary hypertension, and a rapid drop in blood pressure. The air also causes a ventilation-perfusion mismatch in the lungs, resulting in low oxygen levels and high carbon dioxide levels in the blood. The bubbles themselves can also cause inflammatory changes in the lung’s blood vessels.
Arterial Air Embolism, by contrast, is generally more dangerous, as the air enters the arterial system and travels to vital organs. This often happens when air enters the left side of the heart and is pumped out to the body’s systemic circulation. The bubbles travel through arteries until they become lodged in smaller vessels, causing immediate local tissue ischemia.
When AAE travels to the brain, it is termed a cerebral air embolism; when it affects the heart muscle, it causes myocardial ischemia. A VAE can sometimes convert to an AAE, known as paradoxical air embolism. This occurs if the air passes through a small hole between the heart’s chambers, such as a patent foramen ovale, allowing the venous air to bypass the lungs’ filtering capacity and enter the arterial circulation.
Critical Volume and Factors Influencing Toxicity
No single, fixed volume of air is universally lethal, as the toxicity of an air embolism is highly variable. The critical volume differs dramatically between the venous and arterial systems because the lungs act as a filter for small venous bubbles. For Venous Air Embolism (VAE), the introduction of 3 to 5 milliliters of air per kilogram of body weight is considered the lethal dose. For a typical adult, this volume can range from 20 milliliters to 350 milliliters of air introduced into the venous system.
The speed at which the air enters the bloodstream, known as the rate of entry, is a significant determinant of severity. A rapid, massive influx of air, such as 300 to 500 milliliters at a rate of 100 milliliters per second, can quickly overwhelm the heart’s ability to function and is often fatal. Rapid infusion of a large air bolus is far more dangerous than the slow, steady entry of the same volume, as the slower rate allows the air to be broken down and absorbed by the lungs.
The location of entry is also a dominant factor, especially for Arterial Air Embolism (AAE). The lethal volume for AAE is dramatically smaller; as little as 0.5 to 2 milliliters of air introduced directly into the coronary arteries can lead to cardiac arrest. Similarly, injecting just 2 to 3 milliliters of air into the cerebral circulation, which supplies the brain, can be fatal.
Patient position during the air entry event also affects the outcome. Having a patient in a seated position during a procedure, such as certain types of neurosurgery, can increase the risk, as the pressure difference allows air to be sucked into open veins. Conversely, placing a patient in a head-down (Trendelenburg) position can lessen the severity of a VAE by trapping the air in the apex of the right ventricle, away from the pulmonary artery entrance.
Common Sources of Air Entry
Air embolisms most commonly arise from medical procedures, though they can also occur due to trauma or pressure changes. Procedures involving the central venous system carry a notable risk, such as the insertion or removal of central venous catheters placed in large veins near the heart. A pressure difference between the vein and the atmosphere can cause air to be drawn into the circulation, especially if the patient takes a deep breath.
Other medical sources include hemodialysis treatments, cardiac surgery, lung biopsies, and various surgical procedures. Neurosurgery, particularly when performed with the patient in a sitting position, has a higher incidence of VAE because of the pressure gradient in the veins above the heart.
Outside of clinical settings, pulmonary barotrauma is a significant cause of Arterial Gas Embolism (AGE), a form of AAE. This frequently occurs in scuba divers who ascend too quickly or hold their breath during ascent. The rapid decrease in surrounding water pressure causes the air in the lungs to expand, rupturing small air sacs and forcing air directly into the pulmonary veins. Trauma is another cause, particularly penetrating chest injuries that create an open pathway for air to enter a large vein or artery.
Recognizing the Signs
Recognizing the signs of an air embolism is essential, as the condition requires immediate medical attention. Symptoms manifest suddenly and depend heavily on whether the embolism is venous or arterial.
For Venous Air Embolism (VAE), which travels to the lungs, a patient may experience:
- Sudden shortness of breath
- A rapid heart rate or chest pain
- Wheezing and coughing
- A rapid drop in blood pressure
In severe cases, a characteristic “mill-wheel” murmur may be heard over the heart, which is the sound of air and blood mixing in the right ventricle. A sudden, unexplained decrease in exhaled carbon dioxide is a sensitive sign monitored during high-risk medical procedures.
Arterial Air Embolism (AAE) presents with immediate symptoms due to the blockage of blood flow to vital organs. If the air reaches the brain, symptoms mimic a stroke, including sudden loss of consciousness, confusion, seizures, or focal paralysis. When the heart is affected, the patient may experience chest pain or irregular heart rhythms. Any sudden neurological change or signs of acute cardiovascular distress following a high-risk procedure should raise suspicion of an air embolism.