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A pulmonary embolism is a blood clot that forms in the lung as a result of an embolus, an entity that travels from somewhere else in the body getting stuck once it enters a blood vessel two narrow for it to get through. The embolus comes from a blood clot that begins, usually in a deep vein, most commonly in the leg or in the pelvis. The venous clot embolizes, meaning that pieces of it break off and travel wherever the blood takes them. Just to be clear on the order of events leading to pulmonary embolism: A blood clot forms in a deep vein. The clot embolizes, sending an embolus (or multiple emboli) through a pathway of increasingly wider veins until the embolus reaches the right side of the heart. From there, the embolus travels through the pulmonary trunk, then into either the right or left pulmonary artery, then through more branches of increasingly narrow vessels, until it can no longer get through. If large enough to get stuck in a fairly large vessel, the embolus becomes an embolism, as platelets (clotting cells) and various clotting proteins deposit onto it. Since it happens in a lung, it is called a pulmonary embolism. This distinguishes it from other emboli that can form in other organs.
Because deep venous thrombosis and central venous thrombosis (clot in a pelvic vein) are typical sources of pulmonary emboli, doctors use the term venous thromboembolism (VTE) in reference to the category of conditions that includes both venous thrombosis and its complication, pulmonary embolism. While deep veins are by far the most common source of emboli that get to the lungs, in very rare cases a blood clot in a superficial vein also can embolize but do keep in mind that this is very rare.
Pulmonary embolism is an issue for pregnancy because the risk for developing VTE is particularly elevated during pregnancy and through the first few post-partum weeks. The elevated risk relates to two of three prongs that comprise what doctors call Virchow’s triad. Named for the 19th century Prussian physician, Rudolf Virchow, the Virchow triad consists of stasis, hypercoagulability, and damage the lining of the blood vessel or the lining of whatever is holding the blood. The more prongs of the triad that are present and the worse they are, the greater the risk of forming blood clots. In the case of pregnancy, stasis together with hypercoagulability are the two phenomena that affect the blood in ways that make it more likely to clot. Stasis is when blood slows down or stops moving, which happens in deep veins because of the growing womb pushing on the large veins of the trunk. Hypercoagulability means that blood likes to clot more than it usually does. This happens during pregnancy because of hormones, such as estrogen, causing changes that cause concentrations of certain clotting proteins to rise in the blood.
A woman suffering from a pulmonary embolism typically suffers from a constellation of symptoms that include difficulty breathing (dyspnea), chest pain, and often heart palpitations, anxiety, sweating, dizziness, lightheadedness, or fainting, and sometimes coughing up blood. These symptoms begin suddenly, which is an important clue to distinguish pulmonary embolism from chronic pulmonary disease. If there is a woman who is pregnant or just gave birth, who has not been suffering from pulmonary disease, and she suddenly develops chest pain and dyspnea, she should be considered to have a pulmonary embolism until proven otherwise.
We noted that pregnancy is a risk factor for pulmonary embolism. The reason for this is that pregnancy is a risk factor for venous thrombosis, blood clots in a deep vein. Other pulmonary embolism risk factors are the same risk factors for venous thrombosis. So really we are talking about risk factors for VTE. These include smoking, various types of cancer, and a variety of genetic conditions. The combination of smoking with pregnancy elevates the risk higher than either smoking or pregnancy alone elevates the risk. The same is true when smoking is combined with oral contraceptives, which also constitute a risk factor, although you would not be using oral contraceptives while pregnant. Finally, there’s another risk factor important to mention, namely sitting down, or otherwise not movement for a long time. Especially when you are pregnant, or when you fly, you should rise up at least once per hour and move your legs.
Along with blood tests to reveal certain hints of pulmonary embolism, doctors diagnose pulmonary embolism by means of special imaging studies. The main imaging modalities include CT pulmonary angiography and ventilation/perfusion (V/Q) scanning. Some people like to use ultrasound instead, because it does not expose the pregnant woman to ionizing radiation, but ultrasound is not as revealing as these other methods. If ultrasound identifies a pulmonary embolism, doctors can proceed with treatment. If the ultrasound is negative, however, this does not rule out pulmonary embolism. In such cases, doctors will need use one of the other imaging modalities, because pulmonary embolism that is not treated immediately can kill the patient, along with her fetus.
Treatment for pulmonary embolism involves medications that can break up the blood clot quickly to restore blood flow to the affected area of the lung. Doctors will then put the woman onto heparin therapy that often will be low molecular weight heparin (LMWH), which is administered by injections below the skin (subcutaneous). Then, after you deliver your baby, if doctors decide that you are still at risk for developing clots in deep veins, they will switch you to a category of drugs called direct acting oral anticoagulants (DOACs), which you take as pills.