Pulmonary embolism (PE) refers to the occlusion of the pulmonary arteries by an intravascular object. The condition is relatively common such that a few hundred thousand cases occur in the United States each year, resulting in tens of thousands of annual deaths. In fact, almost every type of physician who takes care of patients must know about PE. It can develop in a variety of clinical settings.
In general, the term "embolize" means to travel down the bloodstream and lodge into something. The words "embolism" and "embolus" refer, respectively, to the action of embolizing and the object that embolizes. Therefore, a PE is a condition in which something travels into the lung circulation and gets stuck in at least one branch of the pulmonary arteries.
Causes
The most common cause of PE is deep vein thrombosis (DVT), the formation of a blood clot within the veins of the leg. A variety of factors can lead to DVT, such as surgery, trauma, prolonged leg immobilization, certain blood clotting disorders, malignant tumors, pregnancy, oral contraceptives, and estrogen replacement therapy. The prime concern with DVT is that the clot can break off and embolize to the lung.
Besides blood clots, PE can occur due to other types of emboli. They include air bubbles inadvertently introduced via invasive medical procedures (air embolism), globules of fat following fracture of long bones (fat embolism), amniotic fluid from a pregnant uterus (amniotic fluid embolism), and debris from an infectious process (septic embolism). These nonthrombotic causes of PE occur in a small portion of cases.
Pathophysiology
When a branch of the pulmonary artery is obstructed, the right side of the heart delivering blood to the lungs may experience strain and failure as it pumps against higher than usual pressure. Meanwhile, blood cannot pick up oxygen from the portion of lung whose incoming blood supply is blocked. This results in lower than normal levels of blood oxygen (hypoxemia). In approximately 10% of cases, the affected portion of lung may die (pulmonary infarction), but this is often prevented by a collateral blood supply.
The degree of physiologic events triggered by PE depend on the number and size of emboli as well as any preexisting lung conditions. Small emboli may have little to no impact in the lungs whereas large emboli can produce effects that warrant intervention. In the most severe case, PE involves an embolus so massive that it blocks the left and right pulmonary arteries, cutting off all circulation to both lungs. This type of embolus, called a saddle embolus because it straddles two arteries, is uncommon but fatal.
Symptoms and Signs
Patients with PE due to large emboli typically complain of difficulty breathing (dyspnea) and/or chest pain with inhalation (pleuritic chest pain). Other symptoms, such as coughing and loss of consciousness, are uncommon. A physical examination of the patient may reveal rapid breathing (tachypnea), an elevated heart rate (tachycardia), and a drop in blood pressure (hypotension).
Diagnosis
Because the symptoms and signs of PE are not specific to only that condition, testing is necessary to diagnose PE. The gold standard is pulmonary angiography, but other imaging studies are appropriate. Helical computed tomography (CT), also called spiral CT, can visualize the intravascular obstruction. Magnetic resonance imaging (MRI) can do the same as CT but is reserved for patients who cannot undergo CT. Ventilation-perfusion (V/Q) scanning evaluates the degree of air flow and blood flow in the lungs to see if there is a definite mismatch between the two. In addition to these tests, a physician may attempt to rule out DVT with CT, MRI, or ultrasonography of the legs.
Treatment
All patients with PE, regardless of the cause, are treated with oxygen to correct hypoxemia and vasopressors to manage hypotension. If PE is due to a blood clot, then anticoagulation is also administered. The patient is given the blood-thinner heparin or other similar drug, such as enoxaparin (Lovenox), to inhibit formation of new clots, allowing the body to break down the lung clot. Simultaneously, the patient is given warfarin (Coumadin), which decreases the production of certain clotting factors that form clots. Once the blood level of warfarin is adequate, heparin can be discontinued. The duration of warfarin treatment depends on the patient's risk for subsequent DVT and PE.
Prevention
For patients with PE secondary to DVT, studies have shown that the mortality rate of PE is reduced to 5% with anticoagulant therapy. Anticoagulation may be used to prevent DVT and PE, particularly for hospitalized patients. Other means to prevent these conditions include compression stockings and intermittent pneumatic compression devices. Certain patients may benefit from a filter surgically placed in the inferior vena cava (IVC), one of the big veins entering the heart. This IVC filter, also called a Greenfield filter, prevents any clots originating in the lower half of the body from embolizing to the lungs.
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