<unk>  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<unk>  embolus does not often happen when a blood vessel is injured. In the veins above the heart, such as in the head and neck, the venous pressure may be less than atmospheric and an injury may let<unk>  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<unk>  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. Venous or pulmonary<unk>  occurs when<unk>  enters the systemic veins and is transported to the right side of the heart and from there into the pulmonary arteries, where it may lodge, blocking or reducing blood flow. Gas in the venous circulation can cause cardiac problems by obstructing the pulmonary circulation or forming an<unk> -lock which raises central venous pressure and reduces pulmonary and systemic arterial pressures. Experiments on animals show that the amount of gas necessary for this to happen is quite variable. Human case reports suggest that injecting more than 100 mL of<unk>  into the venous system at rates greater than 100 mL/s can be fatal. Very large and symptomatic amounts of venous<unk> ay 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. Gas<unk>  in a systemic artery, termed arterial gas<unk>  (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 for a cerebral arterial gas<unk>  (CAGE) or heart attack if the heart is affected. The amount of arterial gas<unk>  that causes symptoms depends on location — 2 mL of<unk>  in the cerebral circulation can be fatal, while 0.5 mL of<unk>  into a coronary artery can cause cardiac arrest. Prevention and screening If a patent foramen ovale (PFO) is suspected, an examination by echocardiography may be performed to diagnose the defect. In this test, very fine 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 underwater diving, where bubbles may grow during decompression. A PFO test may be recommended for divers intending to expose themselves to relatively high decompression stress in deep technical diving.
Air embolism
Pulmonary aspiration
Aspiration pneumonia
Pulmonary hemorrhage
Airway obstruction
Right heart strain
Thoracic insufficiency syndrome
Lung infarction