Almost a million people in the US each year are diagnosed with venous thromboembolism (VTE), manifested by both a deep vein thrombosis (DVT) and a pulmonary embolism (PE), which has an upper limit mortality rate of 10%. To understand venous thromboembolism, it becomes necessary to understand the pulmonary and systemic circulation. Through the superior and inferior vena cava, blood from the entire body goes into the right atrium of the heart before entering the right ventricle. This deoxygenated blood is pumped to the lungs by the pulmonary artery before being returned to the left atrium by the pulmonary veins. The oxygenated blood enters the left ventricle and is pumped to the entire body via the aorta, only to have the whole cycle repeat again. The vessels that take blood away from the heart are known as arteries while the vessels that bring blood back to the heart are known as veins. Arteries have more smooth muscle than veins do in order to help them effectively perfuse the entire body with blood and deal with high pressures. Veins, on the other hand, are thinner so that they can deal with high volumes, and they have one-way valves to prevent backward blood flow. DVTs affect veins and PEs affect arteries, and the pulmonary and systemic circulation moves the clot that gives rise to the patient’s VTE.

Pulmonary and Systemic Circulation

A DVT is when a blood clot forms in the patient’s veins, most often in the legs but also possible in the arms and other regions of the body. When there is blood vessel damage, platelets form a plug, adhering to the vessel’s connective tissue. Clotting factors from the platelet then convert fibrinogen into fibrin, which forms the clot itself and promotes healing. In the cases of myocardial infarctions or ischemic strokes, the clots generally form due to excess cholesterol; however, for DVTs, blood clots form because blood pools in the veins. Veins rely on muscle contractions like those of calf muscles to pump blood back to the heart. Therefore, after long periods of muscle inactivity like with sitting, the blood can be moving too slowly in the deep veins and clot. A PE can occur when the DVT breaks from the veins and moves into the heart and then the arteries in the lung where it causes a blockage. Without its blood supply, pulmonary infarction and then death occurs, making a pulmonary embolism life-threatening. A venous thromboembolism is the occurrence of both a DVT and PE, and, according to World Thrombosis Day, it is the “leading cause of death and disability worldwide.”

The biggest risk factor for VTE is long periods of immobility like in an airplane or a hospital, which makes constant movement paramount. Old age (60+), smoking, alcohol, and obesity increase the chance of developing a clot and thus increase the risk of a VTE. Interestingly enough, cancer also drastically increases the risk of a VTE usually either because of vessel damage caused by tumors or the tumors themselves triggering clotting. While VTE can be entirely asymptomatic, there are some characteristics symptoms for a DVT and PE. With a DVT in the leg, there is sometimes pain, heat, redness, and swelling, which also happen to be the four characteristic signs of inflammation. With a PE, there is sometimes chest pain, dyspnea (shortness of breath), and tachycardia (fast heart rate). The tachycardia results as the lungs do not do gas exchange properly, so the body needs to pump the low oxygenated blood more frequently to meet organ oxygen demands. The two largest complications of a VTE are post-thrombotic syndrome (PTS) and chronic thromboembolic pulmonary hypertension (CTEPH). PTS is when patients do not recover from their DVT, which results in long term leg pain, chronic swelling, and sometimes the formation of leg ulcers (sores). CTEPH is high lung blood pressure because of lingering blood clots or scar tissue from the PE in the pulmonary arteries. Because of this arterial narrowing, the pressure of blood going through the pulmonary arteries increases, and this can cause chronic chest pain and dyspnea (not acute like with a PE).

DVT and PE

To diagnose VTE means diagnosing the DVT and PE separately. The gold standard for diagnosing deep vein thrombosis is doing a duplex ultrasonography of the leg. Basically, sound waves are used to image the moving blood to determine aspects of its flow to determine if there is a thrombosis, or blood clot. A positive D-dimer blood test can also indicate a DVT as D-dimer is a part of a protein made in detectable quantities only when a blood clot is dissolving. Furthermore, contrast venography can be done where IV contrast is given before getting a leg CT scan as the contrast helps visualize the deep veins and any potential DVT in them. The gold standard for diagnosing a PE is a computed tomographic pulmonary angiography. Once again, IV contrast is given before getting a chest CT scan in order to visualize the pulmonary arteries and see if there is an embolism. However, a ventilation-perfusion scan can also be done where IV radioactive albumin is given to determine pulmonary perfusion (blood flow) and radioactive technetium aerosol is given to determine pulmonary ventilation (gas exchange). With a patient with PE, there will be V/Q mismatch, or wedge-shaped defects on the perfusion, but everything will be clear on ventilation. This is because the pulmonary artery is occluded by the blood clot, but ventilation should still be relatively normal because the airway is clear. Treatment for VTE generally involves anticoagulants to thin the blood to reduce the chance of forming more clots and sometimes thrombolytics, or clot breakers, in order to dissolve any future clots. In extreme cases like in the case of an extremely large clot, surgery like a thrombectomy can be done to remove the blood clot from the deep veins of the leg or the pulmonary arteries. Generally, the blood clot is either split into smaller pieces and removed (mechanical thrombectomy) or simply suctioned out (catheter aspiration thrombectomy).

A venous thromboembolism can be life-threatening, especially considering how a pulmonary embolism can cause the death of the lung tissue, which in turn reduces the oxygenation of other important organs. With proper attention to the risk factors and potential symptoms, diagnosis and treatment of deep vein thrombi and pulmonary embolisms can be expedited to allow for the best prognosis possible. Considering the high incidence and mortality of this disease, awareness regarding VTE is absolutely paramount.

 

References

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“Venous Thromboembolism (Blood Clots) Data & Statistics.” CDC, www.cdc.gov/ncbddd/dvt/data.html. Accessed 19 June 2019.