air embolism: see embolus.

An air embolism, or more generally gas embolism, is a medical condition caused by gas bubbles in the bloodstream (embolism in a medical context refers to any large moving mass or defect in the blood stream). Small amounts of air often get into the blood circulation accidentally during surgery and other medical procedures, but most of these in veins are stopped at the lungs, and a venous air embolism that shows symptoms are very rare. Death may occur if a large bubble of gas becomes lodged in the heart, stopping blood from flowing from the right ventricle to the lungs (this is similar to vapor lock in engine fuel systems). However, the amount of gas necessary for this to happen is quite variable, and also depends on a number of other factors, such as body position.

Gas embolism into an artery, termed arterial gas embolism, or AGE, is a more serious matter than in a vein, since a gas bubble in an artery may directly cause stoppage of blood flow to an area fed by the artery. The symptoms of AGE depend on the area of blood flow, and may be those of stroke or heart attack if the brain or heart, respectively, are affected.

Pathogenesis

Air embolism can occur whenever a blood vessel is open and a pressure gradient exists favoring entry of gas. Because the pressure in most arteries and veins is greater than atmospheric pressure, an air embolus does not always happen when a blood vessel is injured. In the veins above the heart, such as in the head and neck, the pressure is less than atmospheric and an injury may let air in. This is one reason why surgeons must be particularly careful when operating on the brain, and why the head of the bed is tilted down when inserting or removing a central venous catheter from the jugular or subclavian veins.

When air enters the veins, it travels to the right side of the heart, and then to the lungs. This can cause the vessels of the lung to constrict, raising the pressure in the right side of the heart. If the pressure rises high enough in a patient who is one of the 20% to 30% of the population with a patent foramen ovale, the gas bubble can then travel to the left side of the heart, and on to the brain or coronary arteries. Such bubbles are responsible for the most serious of gas embolic symptoms.

Trauma to the lung can also cause an air embolism. This may happen after a patient is placed on a ventilator and air is forced into an injured vein or artery, causing sudden death. Breath-holding while ascending from scuba diving may also force lung air into pulmonary arteries or veins in a similar manner, due to the pressure difference.

Air can be injected directly into the veins either accidentally or as a deliberate act. Examples include misuse of a syringe, and industrial injury resulting from use of compressed air. However, the amount of air that would be administered by a single small syringe is, in most cases, not enough to suddenly stop the heart, nor cause instant death. Single air bubbles in a vein do not stop the heart, due to being too small. However, such bubbles may occasionally reach the arterial system through a patent foramen ovale, as noted above, and cause random ischemic damage, depending on their route of arterial travel.

Gas embolism in diving

Gas embolism is a diving disorder suffered by SCUBA divers and can happen in two distinct ways:

• Pulmonary barotrauma: Air bubbles enter the bloodstream as a result of gross trauma to the lining of the lung following a rapid ascent while holding the breath; the air held within the lung expands to the point where the lungs burst (pulmonary barotrauma). This is easy to do as the lungs give little warning through pain until they do burst. The diver will arrive at the surface in pain and distress and may froth or spit blood. A pulmonary barotrauma is very obvious and presents quite differently from the decompression sickness below.
• Decompression sickness (DCS): Air bubbles precipitate out into the bloodstream if the gas dissolved in the blood at pressure is not allowed sufficient time to out-gas on ascent. The symptoms may be subtle and not immediately noticeable.

Bubbles in the bloodstream from any source are dangerous as they can form clots and precipitate stroke or thrombosis. Pulmonary barotrauma, although more dramatic, is less likely to affect oxygen supply to the brain because bubbles tend to be introduced into the venous system and are trapped and managed at the lung. Gas embolism arising from decompression sickness are potentially more dangerous as they can form in the arterial system, the bubbles are smaller and they can travel to and lodge in the brain where they can cause stroke. The first aid treatment for both is to administer oxygen, treat for shock and get to hospital; at the hospital both may use a hyperbaric chamber but otherwise treatment is different.

Treatment

Recompression is the most effective treatment of an air embolism. Normally this is carried out in a recompression chamber. This is because as pressure increases, the solubility of a gas increases. Additionally, owing to Boyle's law, the size of the gas bubble or bubbles decreases in proportion to the increase in atmospheric pressure. In the hyperbaric chamber the patient breathes 100% oxygen. Under hyperbaric conditions, oxygen diffuses into the bubbles, displacing the nitrogen from the bubble and into solution in the blood. Oxygen bubbles are more easily tolerated. Air is composed of 21% oxygen and 78% nitrogen with trace amounts of other gases. Additionally, diffusion of oxygen into the blood and tissues under hyperbaric conditions supports areas of the body which are deprived of blood flow when arteries are blocked by gas bubbles. This helps to reduce ischemic injury. Finally, the effects of hyperbaric oxygen antagonize leukocyte-medicated ischemic-reperfusion injury.

Oxygen first aid treatment is useful for suspected gas embolism casualties or divers who have made fast ascents or missed decompression stops. Most fully closed-circuit rebreathers can deliver sustained high concentrations of oxygen-rich breathing gas and could be used as an alternative to pure open-circuit oxygen resuscitators.

Pop culture

In season 4, episode 15 of House, a bus driver had an air embolism from dental surgery.

During the hospital scene in the 2006 re-make of The Omen, Katherine Thorn (played by actress Julia Stiles) has a lethal amount of oxygen injected into her IV (by her son's babysitter) giving her an air embolism followed by almost instantaneous cardiac arrest, killing her.

In the film Apt Pupil, the Nazi war criminal commits suicide in the hospital by intentionally blowing air into his IV line.

In the film The Ring Two when the psychiatric doctor tells Aiden (who is possessed by Samara, the girl who died in the well) that he cannot go home, he "shows" the doctor something by some means of mental projection, that makes her inject herself with a full syringe of air.

In an episode of The Simpsons Homer is injected with a syringe full of air, after which he passes out.

In the movie Vertical Limit, one of the climbers commits homicide by injecting a fellow climber with a syringe full of air.

In an episode of Nip/Tuck, when Sean McNamara decides to go diving with his son Matt, a fellow diver suffers an arterial gas embolism.

In an episode of ER (TV Series) , September 25th, 2008, Dr. Greg Pratt dies of an air embolus sustained from an explosion.

References

External links

• Arterial Gas Embolism