Blood Clots

Overview

A blood clot is a mass of blood that has become partially solid due to the actions of proteins and other elements in the blood that are responsible for clotting. Clotting is a mechanism used by the body to stop bleeding after a person is injured. However, a blood clot may become harmful if it is formed under circumstances that allow it to block an artery or vein and stop blood flow.

If you are diagnosed with pathologic blood clots or considered to be at risk of forming these types of blood clots, then drugs that prevent clotting, referred to as anticoagulants, may be administered to decrease your risk of clotting. Coumadin^® (warfarin) is a common type of anticoagulant drug.

What is a blood clot?

Clotting is a mechanism used by the body to stop bleeding. The first step in clotting is adhesion of platelets, which are fragments of blood cells that circulate in the blood, to the cut edges of a damaged blood vessel. In this way, a platelet plug is formed and external bleeding stops. Next, small molecules called clotting factors cause strands of solid material called fibrin to form, and to stick together and seal the inside of the wound. Eventually, the cut blood vessel heals and the blood clot dissolves after a few days.

When can a blood clot become harmful?

A blood clot becomes harmful when it blocks an artery or vein and stops blood flow. The blood clot is then called a thrombus. Although a thrombus may occur in any blood vessel, it most commonly develops in the veins of the leg and can travel through the major blood vessels of the pelvis to the blood vessels of the lung where it can be fatal. A thrombus in the leg or pelvic vein is called a deep vein thrombosis (DVT).

A thrombus that breaks free and travels through the bloodstream is called an embolus. As it travels, it compromises blood flow and may become lodged in a smaller blood vessel, causing blockage. For example, if an embolus blocks an artery in the lung, it is called a pulmonary embolism. (Pulmonary is a medical term that means “related to the lung.”)

What are the signs and symptoms of a blood clot?

Less common symptoms may include pain in the back, shoulder, or upper abdomen; dizziness; fainting; painful respiration; new onset of wheezing; and any new heart arrhythmia.
An embolus may also travel to the heart, brain, or eye. An embolus in the brain can cause a stroke to occur. An embolus blocking an artery in the heart can cause a heart attack. An embolus may cause sudden blindness in one eye. An embolus that blocks an artery is a life-threatening condition. For this reason, symptoms of a DVT or an embolus require immediate care.

Who is at increased risk of harmful blood clots?

Several factors can increase the risk of blood clots. Patients with cancer are at a higher risk. Some other factors include:

• Decreased mobility, bed rest
  
• Obesity
• Advanced age
• Chemotherapy
• History of deep vein thrombosis
• Fractures
• Arteriosclerosis
• Oral contraceptives (birth control pills)
• Smoking
• Genetic and acquired disorders affecting the clotting system

How are blood clots diagnosed?

Ultrasound - The most common test used to diagnose a DVT is called an ultrasound. This technique uses sound waves to evaluate the flow of blood in your veins. A gel is put on the skin of your leg. A handheld device is placed on the leg and passed back and forth over the affected area. This device sends sound waves from the leg to an ultrasound machine. A computer then creates a visual image based on the different frequency sound waves that are recorded. The picture is displayed on a monitor, allowing your doctor to see the blood flow in your leg.

Venography - If the ultrasound does not provide a clear diagnosis, venography may be performed. A venogram is an x-ray used to examine veins. A dye is injected into a vein and then an x-ray is taken of the leg. The dye makes the vein visible on the x-ray. If the blood flow in the vein is impaired, it will show on the x-ray.

CT scan - Pulmonary embolism and DVT may also be diagnosed with computed tomography or “CAT” scans. A CT scan is similar to a venogram in that is uses x-ray equipment. However, CT provides a 3-dimensional image through the use of special x-ray equipment that is constructed in a ring. When a CT scan is used to diagnose an embolism, a dye is injected into the patient’s bloodstream through the arm. The scanner then rotates around the table, scanning the lungs and the veins of the legs from many different directions. Blood clots are highlighted by the dye.

How are blood clots treated?

If blood clots are diagnosed or you are considered to be at risk of forming blood clots, anticoagulant drugs that prevent clotting will be administered. Warfarin, a commonly used anticoagulant drug, is administered as a pill.

Although treatment with warfarin is usually effective in most people with blood clots, it may be less effective in cancer patients. Also, treatment with warfarin may be complicated because anticlotting is affected by poor diet and liver malfunction, which may be common problems in cancer patients. Low-molecular-weight heparin (LMWH) is another type of anticoagulant drug. It is administered subcutaneously to block the clotting process.

LMWH is similar to heparin, an anticoagulant drug that was first discovered in 1916 and has been in use for many years. Heparin prevents the expansion of a blood clot, but does not affect the size of blood clots that have already formed. A preparation of heparin contains heparin molecules of different sizes. LMWH is a preparation of heparin that has been treated to eliminate the longer heparin molecules, so that only the shorter heparin molecules remain. Two specific types of LMWH, Lovenox^® (enoxaparin) and Fragmin^® (dalteparin), are indicated for the prevention and treatment of DVT.

LMWH is often administered for the first few days of anticoagulant treatment since warfarin takes several days to begin working. For example, if blood clots are found in your legs or lungs, you will typically receive LMWH for a few days to “thin” your blood (make it less likely to form clots). Warfarin will also be prescribed, with a few days of overlap allowed between LMWH and warfarin administration. This overlap is necessary because when warfarin is first administered, it may actually increase the risk of clotting temporarily. Once your risk of clotting has been decreased with LMWH, you will stop receiving this drug and will continue to receive warfarin.

Until recently, heparin was routinely administered as initial anticoagulant treatment for DVT. However, guidelines from the American College of Physicians and the American Academy of Family Physicians now recommend LMWH instead of unfractionated heparin (heparin containing a mixture of large and small heparin molecules) when possible. Some patients with cancer may benefit from extended treatment with LMWH, especially if they have difficulty receiving adequate protection from clotting from oral doses of warfarin.

LMWH is thought to provide protection against clotting more quickly than unfractionated heparin, and to be less likely to cause severe bleeding. LMWH may also be associated less often with an uncommon but very serious complication called heparin-induced thrombocytopenia (HIT), although it is possible for HIT to occur with either LMWH or  unfractionated heparin.

Another type of anticoagulant drug is called Arixtra^® (fondaparinux). Fondaparinux is a pentasaccharide molecule (a chemical made of five sugar groups strung together). Its structure is very similar to the active part of heparin, which is also a pentasaccharide. However, since fondaparinux only consists of a pentasaccharide molecule and does not resemble the rest of the heparin molecule, it (unlike LMWH) is not a type of heparin.

Fondaparinux is indicated for the prevention of DVT in patients undergoing hip or knee replacement surgery, or those undergoing abdominal surgery if they are at risk of harmful blood clots. In addition, fondaparinux is indicated for treating DVT if given together with warfarin, and for treating pulmonary embolism if given with warfarin and given at the hospital. Current guidelines by the American College of Chest Physicians recommend using either fondaparinux or LMWH to prevent DVT in patients undergoing hip or knee surgery.