Blood Clots: Pulmonary Embolism

  • Basics

    Pulmonary embolism is a sudden blockage of one of the blood vessels in the lungs. It is usually caused by a blood clot from the leg that travels through the body and the heart before reaching the lungs. Figure 01 Large clots can seriously affect the lung's ability to transfer oxygen to the blood, resulting in shortness of breath, chest pain, and a rapid heart rate. These symptoms indicate an emergency situation, and cause sudden death in about one-third of the cases.

    Pulmonary emboli are a serious risk to older people, and are more likely to occur in those who have been immobilized for long periods; for example, in those recovering from leg or hip surgery.

    Pulmonary embolism can be fatal if not detected and treated quickly. Fortunately, treatment is life-saving in almost all cases. People who have sudden life-threatening symptoms of pulmonary embolism have a death rate of 30% if not treated promptly. With rapid diagnosis and treatment, mortality rates drop to only 3%.

    Click to enlarge: Pulmonary embolism

    Figure 01. Pulmonary embolism

  • Causes

    The vast majority of emboli are caused by blood clots that arise from veins in the legs. About 80% of emboli arise from leg veins, although they can also arise in other blood vessels. Under certain circumstances, blood tends to form clots in the deep veins, the larger veins located inside the leg (not the visible veins). This is a condition known as deep venous thrombosis. (Thrombosis is the formation of a blood clot, or thrombus. Once it has lodged in a blood vessel, it is referred to as an embolus.) Deep venous thrombosis tends to occur:

    • if circulation is slow, such as occurs with lack of activity
    • if there is an injury to the blood vessel, such as when a leg has broken, or following leg or hip surgery
    • if a blood vessel is inflamed or damaged
    • if blood tends to clot easily, as occurs with some hereditary conditions and in pregnant women and women taking birth control pills. Cancer also increases the risk of blood clots.

    Emboli can result from substances other than blood. Emboli can also arise from a piece of fat or bone marrow that has escaped from the inside of a fractured long bone. In very rare cases, a piece of a malignant tumor can break off, invade the bloodstream, and cause pulmonary embolism. Air emboli from catheters or IV drug use can also cause pulmonary embolism, as can emboli from parasite eggs (such as those in schistosomiasis), or from amniotic fluid released during labor.

    An embolus can cause part of the lung to die. An embolus can be large enough to cut off the blood supply to surrounding lung tissue. If the blood is stopped for long enough, the tissue in the area dies.The area of dead tissue is known as an infarction.

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