Pulmonary Embolus

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A pulmonary embolus is a blood clot that becomes lodged in the pulmonary artery or one of its branches, causing at least partial obstruction of the artery. Because the pulmonary artery has the critical job of carrying "used" blood to the lungs to be replenished with oxygen, an obstruction of this artery is a very serious matter.

The blood clots (also called thromboembolus) that produce a pulmonary embolus usually are caused by deep venous thrombosis (DVT) in the deep veins of the groin or thighs.

The severity of a pulmonary embolus is generally determined by its size. If a pulmonary embolus is large enough, it can cause a high degree of obstruction, leading to severe cardiovascular distress. A massive pulmonary embolus is often associated with a dangerous drop in blood pressure, and a severe drop in the oxygen content of the blood.

A smaller pulmonary embolus can still cause significant symptoms. Also, smaller clots can sometimes move into one of the smaller branches of the pulmonary artery and completely occlude it, leading to a pulmonary infarction -- the death of a portion of lung tissue.

Up to 30% of people who have untreated pulmonary embolus die. Even with medical treatment, the mortality rate with pulmonary embolus remains above 5%.

What Are The Symptoms and Consequences of Pulmonary Embolus?

The most common symptoms of pulmonary embolus are shortness of breath, "pleuritic" chest pain (that is, chest pain that is worse when you take a breath), and wheezing.

The shortness of breath occurs very quickly, usually within a few seconds after a pulmonary embolus occurs. 

If the embolus produces pulmonary infarction, a cough with bloody sputum (hemoptysis) is often seen.

A massive pulmonary embolus, large enough to cause a severe blockage of blood flow to the lungs, can produce sudden cardiovascular collapse.

Cardiac arrhythmias -- especially atrial fibrillation -- are often seen with pulmonary embolus.

If pulmonary embolus recurs, pulmonary hypertension can develop.

About half the people who experience acute pulmonary embolus also have signs or symptoms of DVT in one or both legs.

Who Is At Risk for Pulmonary Embolus?

In most cases, pulmonary embolus is caused by DVT. So people who have DVT are have the greatest risk of having a pulmonary embolus. In fact, it is estimated that about 50% of people with untreated DVT will experience a pulmonary embolus.

Not surprisingly, most people who have pulmonary embolus turn out to have medical conditions that predispose them to DVT, such as prolonged periods of bedrest, recent surgery, recent stroke or paralysis, cancer, chronic heart disease, or a history of prior DVT. Obesity and smoking are also significant risk factors for DVT - especially in women.

How Is Pulmonary Embolus Diagnosed?

Unfortunately, diagnosing a pulmonary embolus is not always easy to do.

For one thing, the symptoms of pulmonary embolus are rather non-specific - that is, a lot of conditions aside from pulmonary embolus also produce shortness of breath and chest pain. For another thing, the “best” test for diagnosing pulmonary embolus is a pulmonary angiogram - an invasive test in which dye is injected through a catheter into the pulmonary artery, so that any blood clots can be visualized on x-ray. Because pulmonary angiography is an invasive test that carries a certain amount of risk, doctors are appropriately reluctant to use this test except when absolutely necessary.

To reduce the need to perform a pulmonary angiogram, doctors usually use a three-step approach in diagnosing pulmonary embolus.

In Step One, the doctor estimates whether the patient's chances of actually having a pulmonary embolus are high or low. This is done by taking a careful medical history (paying special attention to risk factors for DVT), performing a physical examination, measuring the content of oxygen in the blood, and possibly doing a compressive ultrasound testto look for DVT.

In Step Two, more specific testing is done. If the probability of pulmonary embolus during Step One is thought to be low, the doctor may order a D-dimer test. The D-dimer test is a blood test that measures whether there has been an abnormal level of clotting activity in the bloodstream - as there usually is in people who have DVT or pulmonary embolus. If the clinical probability of pulmonary embolus (as estimated in Step One) is low and the D-dimer test is negative, pulmonary embolus can be ruled out, and diagnostic efforts can move on to other potential causes of the symptoms.

However, if the probability of a pulmonary embolus during Step One is judged to be high (or if the D-dimer test has been performed and is positive), then usually either a V/Q scan or a CT scan of the chest is done.

A V/Q scan - also called a ventilation/perfusion scan, or simply a lung scan - uses a radioactive dye (injected into a vein) to assess the flow of blood in the lung tissue. If the pulmonary artery is partially blocked by an embolus, the corresponding part of the lung tissue will receive less than the normal amount of the radioactive dye.

The CT scan is a non-invasive, computerized x-ray technique that allows the doctor to visualize the pulmonary arteries, so that an obstruction caused by an embolus can be seen. In general, the CT scan technique is preferred over the V/Q scan, but not all hospitals have the capacity to perform a CT scan for pulmonary embolus.

Step Three is usually unnecessary today, since modern noninvasive tests usually allow the diagnosis to be confirmed or ruled out. But if the diagnosis after Step Two remains unclear, it may be necessary to move to Step Three -- pulmonary angiography. 

By using this approach, the correct diagnosis can be made in the large majority of people who have symptoms suggesting pulmonary embolus, usually without needing Step Three. 

How Is Pulmonary Embolus Treated?

The main treatment for pulmonary embolus is to use anticoagulant drugs ("blood thinners") to prevent further blood clotting.

Once a diagnosis of pulmonary embolus is confirmed, therapy is begun immediately with either intravenous heparin or a derivative of heparin that can be given subcutaneously (such as Arixtra, or fondaparinux). Often, this therapy is begun even earlier if Step One suggests a very high probability of pulmonary embolus. The heparin family of drugs provide an immediate anticoagulant effect and help to prevent further blood clots from forming.

Once this acute therapy is instituted, chronic anticoagulation with an oral medication is also begun. Traditionally Coumadin has been the drug of choice, but in recent years the newer anticoagulation drugs - apixaban (Eliquis), rivaroxaban (Xarelto), edoxaban (Savaysa) and dabigatran (Pradaxa) - have come into widespread use for long-term therapy in patients with pulmonary embolus.

In people whose pulmonary embolus is massive enough to cause cardiovascular instability, anticoagulation therapy is often not enough. These patients often require powerful "clot busting" drugs - fibrinolytic agents such as streptokinase. These drugs are aimed at trying to dissolve the clot that is obstructing the pulmonary artery. Fibrinolytic therapy carries substantially more risk than therapy with standard anticoagulants, in particular, a risk of serious bleeding complications. However, when a pulmonary embolus is severe enough to be life-threatening, the risk of ”clot busters" is usually outweighed by the potential benefits. 

In extremely dire circumstances, surgery can be employed to directly remove the pulmonary embolus. This surgery (called embolectomy surgery) is quite risky and is not aways effective, so it is reserved for patients who are judged to have a very low chance of surviving without it.

Sometimes patients will have a second or third pulmonary embolus, even with adequate anticoagulation therapy. In these cases, a filter may be placed in the inferior vena cava (the large abdominal vein that connects the leg veins to the heart) in order to "intercept" any further clots that may break loose from the leg veins.

Summary

Pulmonary embolus is a common problem, most often seen in people who have a medical condition (or habits) that predispose them to DVT. If a pulmonary embolus remains untreated -- especially if it is a large pulmonary embolus -- there is a substantial risk of death.

If you have symptoms suggestive of a pulmonary embolus -- sudden, unexplained shortness of breath, which may or may not be accompanied by chest pain -- it is important that you get checked by a physician immediately. Whether or not you actually turn out to have a pulmonary embolus, symptoms like these are seldom caused by benign conditions that will simply get better on their own.

Sources:

Kearon C, Akl EA, Ornelas J, et al. Antithrombotic Therapy for VTE Disease: CHEST Guideline and Expert Panel Report. Chest 2016; 149:315.

Holbrook A, Schulman S, Witt DM, et al. Evidence-based management of anticoagulant therapy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest 2012; 141:e152S.

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