Obstruction of blood flow in a vessel by a blockage that has formed in that vessel or has travelled from elsewhere in the body
Both thrombosis and embolism can be serious and potentially fatal conditions. In thrombosis, a blood clot, known as a thrombus, forms in a blood vessel and obstructs the flow of blood. Any blood vessel in the body may become blocked by a thrombus, but thrombosis is more serious in arteries and in deep veins in the legs (see Deep vein thrombosis). In embolism, a plug of material called an embolus travels through the bloodstream until it becomes lodged in an artery. Although some emboli consist of substances such as tissue or fat, most are pieces of blood clot that have become detached from a larger clot elsewhere in the body.
If an artery is blocked by either a thrombus or an embolus, and blood cannot reach the tissues beyond the blockage by an alternative route, those tissues are deprived of oxygen. Unlike thrombosis, which often develops gradually, the effects of embolism usually develop immediately and may be severe if the blood vessel becomes completely obstructed. Blockage of the arteries that supply the brain (see Stroke), the lungs (see Pulmonary embolism), or the heart (see Myocardial infarction) often proves fatal.
Blood flowing through an artery is normally under pressure so that clots are unlikely to form. If the flow of blood slows down, a clot is more likely to develop. Decreased blood flow through an artery may be caused by narrowing of the vessel due to a gradual build-up of fatty deposits in its walls, known as atherosclerosis. The long-term risk of developing atherosclerosis, and therefore thrombosis, is increased by factors such as smoking and a diet high in fat.
Thrombosis is more likely to develop if there is an increase in the natural tendency of the blood to clot, a condition known as hypercoagulability. In rare cases, the condition is inherited. Hypercoagulability may also occur as a result of taking a combined oral contraceptive (see Contraception) or hormone replacement therapy, being pregnant, or having surgery.
Blood clots sometimes form in the heart when the heartbeat is weak and irregular because the upper chambers (atria) are not totally emptied of blood at each beat (see Atrial fibrillation). These blood clots may then be released from the atria into the coronary or peripheral arteries.
The symptoms caused by thrombosis or embolism vary depending on which blood vessel is blocked. If the blockage affects blood supply to the legs, symptoms may develop within a few hours, and they may be more severe if there is already a reduction in blood supply to this region (see Lower limb ischaemia). Symptoms include:
Pain in the legs, even at rest.
Pale, cold feet.
If the arteries supplying the intestines are affected, symptoms may include:
Severe abdominal pain.
Left untreated, the reduction in blood supply may eventually result in tissue death, which may be life-threatening. The affected tissues will change colour over several days, eventually becoming black (see Gangrene). If you develop these symptoms, you should seek emergency medical attention.
If your doctor suspects that you have thrombosis or embolism, he or she will have you admitted to hospital immediately for treatment. The blood flow through your blood vessels may be measured using pulse volume recording and imaging, such as Doppler ultrasound scanning. Angiography (see Contrast X-rays), sometimes combined with MRI, may be used to obtain detailed images of the blood vessels and look for obstruction.
Depending on the site and size of the clot, you may be given drugs to dissolve it and prevent further clots from forming (see Drugs that prevent blood clotting). Emergency surgery may be necessary to remove the clot or to bypass it using a graft made from synthetic material. Alternatively, the affected artery may be widened by angioplasty, a procedure in which a balloon mounted on the tip of a catheter is passed into the artery and then inflated to widen the obstructed area.
After the thrombosis or embolism has been treated, you may need to continue medication for several months to prevent further blood clots from forming. For example, you may be advised to take a low dose of aspirin daily. If you smoke, you should stop immediately. You should also try to eat a low-fat diet (see A healthy diet) and exercise regularly. If you are taking a combined oral contraceptive or hormone replacement therapy, you may be advised to stop.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.