| Air embolism | |
|---|---|
| Classification and external resources | |
| ICD-9 | 673.0, 999.1 |
| eMedicine | emerg/787 |
| MeSH | D004618 |
An air embolism, or more generally gas embolism, is a pathological condition caused by gas bubbles in a vascular system. The most common context is a human body, in which case it refers to gas bubbles in the bloodstream (embolism in a medical context refers to any large moving mass or defect in the blood stream). However air embolisms may also occur in the xylem of vascular plants, especially when suffering from water stress.
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Small amounts of air often get into the blood circulation accidentally during surgery and other medical procedures (for example a bubble entering an intravenous fluid line), but most of these air emboli enter the veins and are stopped at the lungs, and thus a venous air embolism that shows any symptoms is very rare.
For venous air embolisms, 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).[1] [2]However, experiments in animals show that the amount of gas necessary for this to happen is quite variable, and also depends on several other factors, such as body position. Very large and symptomatic amounts of venous air emboli may also occur in rapid decompression in severe diving or decompression accidents, where they may interfere with circulation in the lungs and result in respiratory distress and hypoxia.[3]
Gas embolism into an artery, termed arterial gas embolism (AGE), is a more serious matter than in a vein, because a gas bubble in an artery may directly stop 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, is affected.[3] The amount of arterial gas embolism that causes symptoms depends on location, but in the brain may be a bubble with a volume only a fraction of a milliliter.
Air embolism can occur whenever a blood vessel is open and a pressure gradient exists favoring entry of gas. Because the circulatory 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. [4] 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[3].
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. 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.
There have been rare cases of air embolism being caused by air entering the bloodstream from the uterus or tears in female genitalia.[5][6] The risk appears to be greater during pregnancy.[5] Cases have been reported that resulted from attempts to perform an abortion by syringing.[6] These appear to have been due to damage to the placenta allowing air to enter the bloodstream.[6] there has been a reported case of autoerotic equipment causing air embolisms.[6] Outside of pregnancy there is at least one case where an air embolism resulted from the insertion of an object into the vagina during masturbation.[6]
Gas embolism is a diving disorder suffered by compressed air divers and can happen in two distinct ways:
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.
If an arterial 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 agitating 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.
Recompression is the most effective treatment of an air embolism.[2] 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[1]. 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-mediated ischemic-reperfusion injury.
It is also important to promptly place the patient in Trendelenburg position (head down) and on their left side (left lateral decubitus position). This positioning helps to trap air in the apex of the ventricle and prevent it from reaching the lung circulation.[1][7]
Oxygen first aid treatment is useful for suspected gas embolism casualties or divers who have made fast ascents or missed decompression stops.[8] 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. However pure oxygen from an oxygen tank through a Non-rebreather mask is always the most optimal way to deliver needed oxygen to a decompression sickness patient[3].
Air embolisms generally occur in the xylem of vascular plants because a fall in hydraulic conductivity results in cavitation. Possible causes of falling hydraulic conductivity include water stress and the freeze-thaw cycle.
A number of physiological adaptations serve to prevent embolisms. These include variations in the length and diameter of vessel elements in different parts of the plant; and the ability to reduce transpiration by closing off leaf stomata.
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