Air embolism

Overview
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 is 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, despite being employed by writers of fiction as a clandestine method of murder, amounts of air such as would be administered by a single small syringe are not likely 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 one of the diving disorders SCUBA divers sometimes suffer when they receive pressure damage to their lungs following a rapid ascent where the breath is inappropriately held against a closed glottis, allowing pressure to build up inside the lungs, relative to the blood. It is termed "gas" because the diver may be using a diving breathing gas other than air. The gas bubbles can impede the flow of oxygen-rich blood to the brain and vital organs. They can also cause clots to form in blood vessels.

Gas embolism and decompression sickness (DCS) may be difficult to distinguish, as they may have similar symptoms, especially in the central nervous system. The treatment for both is the same, because they are both the result of gas bubbles in the body. In a diving context, the two are often called decompression illness (DCI).

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.

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.

Prevention
If an arterical gas embolism resulting from patent foramen ovale is suspected, an exam by echocardiography may be performed to diagnose the defect. In this test, very fine (microscopic) bubbles are introduced into a patient's vein by aggitating saline in a syringe to produce the bubbles, then injecting them into an arm vein. A few seconds later, these bubbles may be clearly seen in the ultrasound image, as they travel through the patient's right atrium and ventricle. At this time, bubbles may be observed directly crossing a septal defect, or else a patent foramen ovale may be opened temporarily by asking the patient to perform the Valsalva maneuver while the bubbles are crossing through the right heart-- an action which will open the foramen flap and show bubbles passing into the left heart. Such bubbles are too small to cause harm in the test, but such a diagnosis may alert the patient to possible problems which may occur from larger bubbles, formed during activities like scuba diving.