Pulmonary Embolism

Overview

PE refers to obstruction of the pulmonary artery or one of its branches by a thrombus (clot). Acute PE is a common and often fatal disease. Unfortunately, symptoms and signs of PE are highly variable, nonspecific, and common among patients with and without PE. Thus, additional testing is needed to confirm or exclude the diagnosis of PE.

Risk factors that promote the development of deep vein thrombosis (DVT) also increase the risk for PE. (See factors above listed above for DVT).

Pathophysiology

Typically, all or a part of a DVT, breaks off from the site where it was formed. The mobile clot, now known as an embolus, travels through the venous circulation and blocks or occludes the pulmonary arteries. Blockage of the pulmonary arteries is a serious condition that can lead to death. The clot, or more commonly, the clots, blocks the blood flow to parts of the lung, preventing the uptake of oxygen in those areas and delivery of oxygen to the brain and body.

Iliac and femoral veins are the source of most clinically recognized PE. It is estimated that 50 to 80 percent of iliac, femoral, and popliteal vein thrombi (proximal vein thrombi) originate below the popliteal vein (calf vein thrombi) and propagate proximally.

Classification of PE

Acute PE can be classified as massive or submassive.

Massive PE causes hypotension, defined as a systolic blood pressure <90 mmHg or a drop in systolic blood pressure of ≥40 mmHg from baseline for a period >15 minutes. It is a catastrophic entity that frequently results in acute right ventricular failure and death. When death occurs, it is often within one to two hours of the event, although patients remain at risk for 24 to 72 hours.

All acute PE not meeting the definition of massive PE are considered submassive PE.

Clinical presentation

Patients with acute PE typically develop symptoms and signs immediately after obstruction of pulmonary vessels.

On physical examination, young and previously healthy individuals may simply appear anxious but otherwise see deceptively well, even with an anatomically large PTE. They may not have “classic” signs such as tachycardia, low-grade fever, neck vein distention, or an accentuated pulmonic component of the second heart sound. Therefore, like the DVT, a high index of clinical suspicion is warranted.

Unfortunately, because acute coronary ischemic syndromes (i.e., heart attacks) are so common, one may overlook the possibility of life-threatening PTE and may discharge these patients from the hospital after the exclusion of myocardial infarction with serial cardiac measurements and electrocardiograms.

PE Symptoms and Signs

Common signs and symptoms of pulmonary embolism include the following:

• Shortness of breath or needing to breathe rapidly (dyspnea)
• Sharp, knife-like chest pain while taking a deep breath (plueritic pain)
• Coughing or coughing up blood (hemoptysis)
• A rapid heart rate (tachycardia)
• Rapid breathing (tachypnea)
• Crackling or wheezing when listening to the lungs
• Decreases breath sounds when listening to the lungs
• Accentuated pulmonic component of the second heart sound and jugular venous distension

Diagnosis

Tests to diagnose PE include the following:

D-dimer - D-dimer is a substance in the blood that is often increased in people with PE. D-dimer levels are abnormal in 95 percent of patients with PE; a person with a normal D- dimer level is unlikely to have a PE. If the D-dimer test is negative and the patient is thought to have a low risk of PE based upon their signs and symptoms, PE is unlikely and further testing may not be needed.

Spiral computed tomography - Spiral CT scanning, uses a combination of x-rays and computers to take detailed pictures of the pulmonary vasculature. X-rays are taken after the injection of an intravenous dye, which highlights the vessels in the lung (CT angiography) or the leg veins (CT venography).

Spiral CT is the study of choice in medical centers that have experience performing and interpreting the test.

However, spiral CT may not be suitable for patients with poor kidney function and patients who are unable to have a CT for other reasons (eg, an allergy to contrast dye). These patients usually need an alternate test (eg, ventilation/perfusion lung scan) or special treatment to reduce the risk of an allergic reaction before CT angiography.

Ventilation/perfusion lung scanning - A ventilation/perfusion lung scan, sometimes called a V/Q scan, is commonly used to diagnose patients with a suspected PE. During this test, the patient inhales a small amount of a radioactive substance. A radioactive substance is also injected into the bloodstream.

A machine is used to examine how these radioactive substances are distributed within the lungs. If a person has a PE, the scanner can detect the area of lung in which the blood flow has been blocked by the blood clot.

Pulmonary angiography - There have been significant improvements in non-invasive tests for PE. However, some patients still require a more invasive test, called pulmonary angiography, to confirm or exclude PE with certainty. Angiography is the "gold standard" test to diagnose PE.

Pulmonary angiography is performed by inserting a catheter through a vein in the groin area (the femoral vein). The catheter is guided into a branch of the pulmonary artery. Dye is injected through the catheter, highlighting the blood vessels in the lung on x-ray.

Routine laboratory blood test - Routine laboratory findings are nonspecific. They include leukocytosis, an increased erythrocyte sedimentation rate (ESR), and elevated liver enzymes with a normal serum bilirubin.

Arterial blood gas - Arterial blood gas (ABG) measurements and pulse oximetry have a limited role in diagnosing PE.

Electrocardiography - Electrocardiogram (ECG) abnormalities exist in many patients with PE who do not have preexisting cardiovascular disease. However, ECG abnormalities are also common in patients without PE, limiting the diagnostic usefulness of the ECG.

MR angiography - The use of magnetic resonance angiography (MRA) for the diagnosis of PE is limited by respiratory and cardiac motion artifact, suboptimal resolution, complicated blood flow patterns, and magnetic susceptibility effects from the adjacent air-containing lung. However, technologic advances offer promise for an expanded role of MRA in the future.

Echocardiography - Only 30 to 40 percent of patients with PE have echocardiographic abnormalities suggestive of acute PE.

In cases of massive PE, however, these abnormalities are more likely and echocardiography may be useful if a rapid presumptive diagnosis is required to justify the use of thrombolytic therapy.