January 4, 2010
Dr. Paul Fleser
Blood clotting is an important mechanism to help the body repair injured blood vessels. Complex mechanisms exist in the bloodstream to form clots where they are needed. If the lining of the blood vessels becomes damaged, platelets are recruited to the injured area to form an initial plug. These activated platelets release chemicals that start the clotting cascade, activating a series of clotting factors.
The medical term for a blood clot is a thrombus. When a thrombus is formed as part of a normal repair process of the body, there is little consequence. Unfortunately, there are times when a thrombus (blood clot) will form when it is not needed, and this can have potentially significant consequences.
Blood clots form when there is damage to the lining of a blood vessel, either an artery or a vein. The damage may be obvious, such as a laceration, or may occur on the microscopic level. As well, blood will begin to clot if it stops moving and becomes stagnant.
Venous thrombosis or blood clots in a vein occur when a person becomes immobilized and muscles are not contracting to push blood back to the heart. This stagnant blood begins to form small clots along the lining walls of the vein that gradually grow to partially or completely occlude the vein. An analogy to this process is a slow moving river. Over time, weeds and algae start to accumulate along the banks of the river where the water flows more slowly. Gradually, as the weeds start to grow, they begin to invade the center of the river because they can withstand the pressure of the water.
Arterial thrombi (blood clots in an artery) occur by a different mechanism. For those with atherosclerotic disease, plaque deposits form along the lining of the artery and grow to cause narrowing of the vessel. This is the disease process that may cause heart attack, stroke, and peripheral artery disease. If a plaque ruptures, a blood clot can form at the site of the ruptured plaque that can completely or partially occlude the blood flow at that point.
What are the risk factors for blood clots?
The risk factors for arterial clots are those that are common to all diseases that cause narrowing of blood vessels. They include:
Venous clots are formed due to one of two main reasons. Most commonly, when the body stops moving, the risk of blood clots increases since the lack of muscle movement allows blood to become stagnant in veins. This typically may occur when a person is hospitalized or bedridden after illness or surgery. It may also occur with long trips either in a car or a plane where hours may pass without a person getting up to walk or stretch. Orthopedic injuries and casting also put the patient at risk. Pregnancy is a risk factor for forming blood clots in the legs and pelvis, since the growing uterus may slow blood flow back to the heart to a sufficient extent that blood clots may form. There may be a genetic or inborn error in the clotting mechanism, making a person at greater risk for forming clots.
Blood clots may cause life-threatening medical conditions, and for that reason are foremost in the mind of health care practitioners when it comes to diagnosis and prevention.
Deep venous thrombosis and pulmonary embolism
Deep venous thrombosis may lead to a pulmonary embolism, a condition in which the clot breaks off in a leg vein, travels in the veins back to the heart, and is pumped out of the heart through the pulmonary artery to the lungs with blood to be oxygenated in the lungs. In the lungs, the blood clot becomes lodged in the small blood vessels of the lung.
An embolus is the medical term for a blood clot that has moved with the bloodstream to a different location. With pulmonary embolus (pulmonary embolism), two issues occur.
Even if venous blood clots do not embolize, they may cause significant local issues with swelling and pain. Since blood cannot return to the heart if a vein is blocked by a clot, the limbs may chronically swell and have decreased function in a condition called chronic thrombophlebitis.
An arterial thrombus stops the blood supply to the tissues beyond the blockage, depriving cells of oxygen and nutrients. This quickly leads to tissue death. Arterial thrombus is the mechanism that causes:
What are the symptoms of blood clots?
Venous clots do not allow blood to return to the heart and symptoms occur because of this damming effect. Most often occurring in the legs or the arms, symptoms include:
Arterial clots do not allow blood get to the affected area. Pain is the initial symptom of this ischemia, or oxygen deprivation due to loss of blood supply. Other symptoms depend upon the location of the clot, and often the effect will be a loss of function. Heart attack and stroke are self-explanatory.
In an arm or leg, in addition to pain, the limb may appear white, and weakness, loss of sensation, or paralysis may occur. If the blood supply is lost to an area of the bowel, in addition to intense pain, there may be bloody diarrhea.
The initial step in making the diagnosis of a blood clot is obtaining a patient history. Since people rarely are able to tell that they have a blood clot, the health care practitioner will ask questions about what might be occurring. If a blood clot or thrombus is a consideration, the history may expand to explore risk factors or situations that might put the patient at risk for forming a clot.
Venous blood clots often develop slowly with a gradual onset of swelling, pain, and discoloration. Symptoms of a venous thrombus will progress over hours. Arterial thrombi occur as an acute event. Tissues need oxygen immediately, and the loss of blood supply creates a situation in which symptoms begin immediately.
Physical examination can assist in providing additional information that may increase the suspicion for a blood clot. Venous thrombi may cause swelling of an extremity. It may be red, warm, and tender; sometimes the appearance is difficult to distinguish from cellulitis or an infection of the extremity. If there is concern about a pulmonary embolus, the examiner may listen to the lungs, looking for abnormal sounds caused by an area of inflamed lung tissue.
Arterial thrombus symptoms are much more dramatic. If a leg or arm is involved, the tissue may be white because of the lack of blood supply. As well, it may be cool to touch and there may be loss of sensation and movement. The patient may be writhing in pain. Arterial thrombus is also the cause of heart attack (myocardial infarction) and stroke (cerebrovascular accident) and their associated symptoms.
Testing for venous blood clots
Venous blood clots may be detected in a variety of ways, though ultrasound is most commonly used. Occasionally, the patient's size and shape limit the ability for ultrasound to provide a definitive answer.
Venography is an alternative test to look for a clot. In this test a radiologist injects contrast dye into a small vein in the hand or foot and using fluoroscopy (video X-ray) and watches the dye fill the veins in the extremity as it travels back to the heart. The area of clot or obstruction can thus be visualized.
Should a blood clot embolize to the lung, this may be a medical emergency. There are a variety of tests to look for pulmonary emboli. Computerized tomography (CT scan) is often the test of choice when suspicion of pulmonary embolus is high. Contrast material is injected intravenously, and the radiologist can determine whether a clot is present in the pulmonary vessels.
Testing for arterial blood clots
Arterial thrombosis is an emergency, since tissue cannot survive long without blood supply before there is irreversible damage. When this occurs in an arm or leg, often a surgeon is consulted on an emergency basis. Arteriography may be considered, a test in which contrast material is injected into the artery in question to look for blockage on imaging studies. Sometimes, if there is a large artery that is occluded, this test is done in the operating room with the presumption that a surgical procedure will be needed to open the vessel and restore blood flow.
Depending upon their location, blood clots may be aggressively treated or may need nothing more than symptomatic care.
Venous thrombosis in the leg may occur in the superficial or deep systems of veins. Clots in the superficial system are often treated symptomatically with warm compresses and acetaminophen or ibuprofen since there is no risk for clots in the superficial veins to embolize to the lung. They are connected to the deep system by perforator veins that have valves that act like a sieve to strain and prevent any clots form getting to the lung.
Deep venous thrombosis usually requires anticoagulation to prevent the clot from growing and causing a pulmonary embolus. Blood clots below the knee are at lower risk for embolization to the lung, and an alternative to anti-coagulation treatment is serial ultrasound examinations to monitor the clot to see if it is growing or being resorbed by the body.
Pulmonary emboli are treated similarly to deep venous thrombosis, but often the patient is admitted to hospital for observation. This is especially true if lung function is compromised and the patient is short of breath or is experiencing hypoxia, or low oxygen levels in the blood.
Arterial blood clots are often managed more aggressively. Surgery may be attempted to remove the clot, or medication may be administered directly into the clot to try to dissolve it.