A blood clot is a thickened mass in the blood formed by tiny substances called platelets. Clots form to stop bleeding, such as at the site of a cut. Clots should not form when blood is moving through the body; when clots form inside blood vessels or when blood has a tendency to clot too much, serious health problems can occur.
The formation of a clot in a blood vessel may result in thrombophlebitis. The term refers to swelling of one or more veins caused by a blood clot.
Although some clots occur in the arms or small, surface blood vessels, most occur in the lower legs. When the blood clot occurs in a deep vein, it is called deep vein thrombosis, orDVT.
As many as 350,000 to 600,000 venous blood clots per year occur in the United States. The danger of DVT comes when pieces of the clot, known as emboli or an embolus, break off and travel through the bloodstream to the lungs. About 1 in 3 blood clots to the lungs (pulmonary embolism) are fatal.
As soon as a blood vessel wall is damaged-by a cut or similar trauma-a series of reactions normally takes place to activate platelets to stop the bleeding. Platelets are the tiny particles in the blood released into the bone marrow that gather together and form a barrier to further bleeding. Several proteins in the body are involved in the platelets clotting process. Chief among these proteins are collagen, thrombin, and von Willebrand factor. Collagen and thrombin help platelets stick together. As platelets gather at the site of injury, they change in shape from round to spiny, releasing proteins and other substances that help catch more platelets and clotting proteins. This enlarges the plug that becomes a blood clot. Formation of blood clots also is called "coagulation."
The series of reactions that cause proteins and platelets to create blood clots also are balanced by other reactions that stop the clotting process and dissolve clots after the blood vessel has healed. If this control system fails, minor blood vessel injuries can trigger clotting throughout the body. The tendency to clot too much is called "hypercoagulation." Anytime clots form inside blood vessels, they can lead to serious complications.
A blood clot that blocks an artery to the brain can cause a stroke. If the clot blocks blood flow to the lungs, pulmonary embolism can occur. A blood clot that blocks a coronary artery can cause a heart attack. Certain people are at higher risk for blood clots than others; surgery, some injuries, childbirth and lying or sitting still for extended periods of time put people at higher risk, as do inherited disorders. Once a person has a blood clot, he or she may have to take bloodthinning drugs to prevent clots from recurring. Men and women are at similar risk for blood clots. A recent study in Austria found that men run a higher risk of recurring blood clots than women, though the reason is unknown.
Many causes can lead to blood clots, some genetic and some environmental. An environmental cause of DVT is prolonged inactivity. For instance, having to sit in a car or airplane for a long period of time decreases blood flow in the lower legs. Recent studies have shown that 1% of air travelers develop blood clots, usually on long flights of five hours or more. However, one study in 2004 found that air travelers developed clots on flights as short as three hours, though they often dissolved naturally and did not lead to complications. Other environmental causes of blood clots include use of hormone replacement therapy to ease menopausal symptoms, oral contraceptives for birth control, pregnancy (and a childbirth within the past six weeks), recent surgery or procedures involving use of a central venous access catheter, and cancer. Smoking also is an important and preventable environmental risk for blood clots.
Some people are born with a higher risk for blood clots. Hypercoagulation disorders are genetic conditions. Usually the body doesn't produce enough of the proteins involved in the clotting process, so they cannot do their job to stop the clotting; in other cases, there is an extra protein that causes too much clotting.
Theremay be no symptoms of blood clots until they grow so large that they block the flow of blood through the vein. Then, symptomsmay develop suddenly around the area and include:
Additional symptoms may indicate serious complications of blood clots such as pulmonary embolism, stroke, and heart attack. If vein swelling or pain are accompanied by high fever or shortness of breath, rapid pulse, or chest pain, or other symptoms that may indicate stroke, heart attack, or pulmonary embolism, it is advised to go to an emergency room immediately.
A physician will diagnose blood clots based on patient history and one of several diagnostic imaging exams. The patient's history will help determine possible risk factors that may lead to suspected blood clots. In addition to family history or known genetic disorders, the patient may mention an environmental factor such as recent air travel or use of high-risk medications.
To help get a picture of suspected clots inside the blood vessels, usually the first test of choice is an ultrasound. Doppler or duplex ultrasound uses sound waves that travel through tissue and reflect back to create images. A computer transforms the sound waves into moving images on the screen that may show the clot, as well as blood flow near the clot and any abnormalities. Ultrasound does not use x rays and is a noninvasive method. Computed tomography (CT) scans also might be used to image the blood vessels. CT scans are similar to x rays, except the images are much like cross-section slices with greater detail that can be computerized and even viewed three-dimensionally. A special dye called a contrast agent may be injected before the exam to help highlight the veins. Magnetic resonance angiography uses magnetic resonance imaging (MRI) to image the blood vessels. It also may involve injection of a contrast dye. Venography is less commonly used, and involves injecting a contrast and using x rays to image the veins.
Medicines can help thin blood, making it less likely to clot. The two most common blood thinners are heparin and warfarin. Heparin works right away, keeping blood clots from growing. It usually is injected. In recent years, more physicians have been prescribing low-molecular weight heparin, purified versions of the drug that can be given with less monitoring. Warfarin (coumadin) often is used for longterm treatment of blood clots and is taken orally. Patients must work closely with their physicians to constantly monitor its effects and adjust dose if necessary. Too little warfarin can lead to clotting, but too much can thin the blood so much that causing lifethreatening bleeding can occur. The same can be true of low-molecular weight heparin when used on a longterm, at-home basis.
Other treatments for blood clots include injecting clot-busting drugs directly into the clot through a catheter, or in rare instances, installation of a filter to block a clot from lodging in the lungs. Sometimes, surgery also is needed to remove a clot blocking a pelvic or abdominal vein or one that is chronic and disabling. A cardiovascular surgeon or interventional radiologist may perform balloon angioplasty or insert a stent to open a narrowed or damaged vessel. In an emergency situation, a drug called tissue plasminogen activator, or tPA, may be given to immediately dissolve a life-threatening blood clot to the brain or heart. In 2004, the U.S. Food and Drug Administration (FDA) approved a new, small, corklike device (called a Merci Retriever) that can be used to remove blood clots from the brains of patients who cannot receive clot-busting drugs. More recently, the FDA approved a suction device that works in much the same way, called the Penumbra device.
Garlic is thought to lower blood clotting potential. Less evidence suggests onions and cayenne pepper may help keep blood thin. New research from Australia adds tomato juice to the list of potential blood thinners. Subjects who drank a glass of tomato juice a day reduced their risk for DVT, stroke, and cardiovascular disease. Research has shown that a natural soy and pine product called pinokinase has been effective in controlling DVT in air travelers. Patients seeking alternative treatments for blood clots should work with certified practitioners and should inform their allopathic provider about their alternative care.
If detected and controlled with medications, blood clots can be safely managed. However, if the clots become dislodged and travel to an artery, they can cause nearly instant death. For instance, more than 600,000 people have a pulmonary embolism each year and more than 10% of them die from the embolism, most within 30 to 60 minutes after symptoms start.
Clots may be avoided by not smoking, and by not using medications that add to the risk. Clotting can be prevented by following physician recommendations concerning medications. Sometimes, physicians will prescribe special support stockings that prevent swelling and reduce chances of DVT. When taking an air flight of six hours or longer, drinking plenty of fluids to avoid dehydration, avoiding tight clothing around the waist, and stretching calves every hour can help prevent DVT. It is advised that those on long flights get up and move about once an hour during the flight. If not possible, moving the legs regularly while seated by flexing the ankles, then pressing the feet against the seat in the row ahead or on the floor can help stretch the calves. A physician may advise those at high risk of DVT wear support stockings during the flight or take low-molecular weight heparin two to four hours before departure.