Thrombosis, or abnormal blood clotting, is often a very dangerous condition that produces two general types of medical problems. First, thrombosis inside an artery can block the flow of blood, producing damage to the organs that are supplied by the blocked artery. Myocardial infarctions (heart attacks) usually involve thrombosis within a coronary artery, and strokes are often caused by thrombosis within one of the arteries that supply the brain. (These are called thrombotic strokes.)
Second, thrombosis that occurs inside a vein or inside the heart can embolize. That is, the blood clot can break off and travel through the vascular system, doing damage wherever it finally lodges. A pulmonary embolus is damage to lung tissue caused by a blood clot that embolizes to the lungs (typically, from a vein in the leg). Strokes are often caused by an embolus that travels to the brain, usually from a thrombus within the heart, most often in association with atrial fibrillation. (These are called embolic strokes.)
Drugs That Prevent or Treat Blood ClotsBecause thrombosis can be so dangerous, people who are at elevated risk for this condition often need treatment to prevent thrombosis from occurring or to attempt to dissolve blood clots that have already formed. There are three general categories of drugs these people might receive - the anticoagulant drugs, the fibrinolytic drugs and the anti-platelet drugs. While each of these drugs has its own profile of side effects, one side effect common to all is excessive bleeding. So, all of these drugs must be used with appropriate precautions.
Anticoagulant DrugsThe anticoagulant drugs inhibit one or more of the clotting factors that are responsible for blood clotting. These drugs include:
Heparin. Heparin is an intravenous drug that has an immediate (within seconds) inhibitory effect on the clotting factors. Doctors can adjust its dosage frequently, as needed, following the partial thromboplastin time (PTT) blood test. The PTT reflects how much the clotting factors have been inhibited. (That is, it reflects the "thinness" of the blood.) Heparin is used exclusively in hospitalized patients.
Low Molecular Weight Heparin: enoxaparin (Lovenox), dalteparin (Fragmin). These drugs are purified derivatives of heparin. Their major advantages are that they can be given as skin injections (which almost anyone can learn to do in a few minutes) and do not need to be closely monitored with blood tests. So, unlike heparin, they can be administered with relative safety on an outpatient basis.
Newer Intravenous or Subcutaneously-Administered Anticoagulant Drugs. Several "heparin-like" anticoagulant drugs have been developed, including argatroban, bivalirudin (Angiomax), fondaparinux (Arixtra) and lepirudin (Refludan). The optimal time and place to use all of these drugs are being slowly worked out.
Warfarin (Coumadin). Until recently, Coumadin was the only orally administered anticoagulant drug available. The biggest problem with Coumadin has been in adjusting its dosage. When first taken, Coumadin's dosage must be stabilized over a period of weeks with frequent blood tests (the INR blood test). Even after stabilization the INR still needs to be monitored periodically and the dosage often re-adjusted. So, getting to and maintaining the "right" dose of Coumadin has always been difficult and inconvenient.
Dabigatran (Pradaxa) and Other "New" Oral Anticoagulant Drugs. Because the dose of warfarin can be relatively difficult to regulate, drug companies have worked for years to come up with "warfarin-substitutes." Dabigatran was the first of these new anticoagulants to be approved by the U.S. Food and Drug Administration. (Rivaroxaban and apixaban are two similar drugs that are just now reaching the market.) The chief advantage of all these drugs is that they can be given in fixed daily dosages and do not require blood tests or dosage adjustments.
Fibrinolytic DrugsStreptokinase, urokinase, alteplase, reteplase, tenecteplase. These powerful drugs are given acutely and intravenously to dissolve blood clots that are in the process of forming. For the most part their use is limited to patients who are within the first few hours of an acute heart attack or stroke, to attempt to re-open a blocked artery and prevent permanent tissue damage. They can be tricky to use and carry a substantial risk of bleeding complications. However, in the right circumstances the use of these drugs can prevent death or disability. Of these drugs, streptokinase is the most frequently used fibrinolytic drug world-wide because it is relatively cheap. In the United States, tenecteplase is currently the drug of choice because it appears to cause fewer disastrous bleeding consequences and is easier to administer than the other drugs.
Anti-Platelet DrugsThree groups of drugs are used to reduce the "stickiness" of platelets, thus preventing platelets from clumping together to form platelet plugs. The anti-platelet drugs are most effective in preventing abnormal blood clots from forming in arteries (as opposed to veins).
Aspirin and diypyramidole (Aggrenox). These drugs have a modest effect on platelet "stickiness" but have fewer bleeding-related side effects than the other anti-platelet drugs. They are often used in an attempt to reduce the risk of heart attack or stroke in people whose risk is elevated.
Ticlopidine (Ticlid), clopidrogel (Plavix) and prasugrel (Effient). These drugs are more powerful (and therefore riskier) than the first group. They are commonly used when the risk of arterial clotting is especially high. Their most common application is in people who have received coronary artery stents. Here, decisions about when and how long to use them have become controversial.angioplasty and stent placement) and in patients with acute coronary artery syndrome. These drugs are very expensive and (in general) must be given intravenously.
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