How is PE Diagnosed? Understanding PE Diagnosis.
A number of different things may alert a physician that a person may be experiencing a pulmonary embolism, or blood clot in their lung. When this is suspected, a number of crucial tests may be performed, including:
Pulse Oximetry. Often, the first test performed when PE is suspected is a blood oxygen level. The simplest way to measure the blood oxygen level is with a pulse oximeter. Pulse oximetry is a noninvasive way (does not involve a blood draw or needle stick) to monitor the percentage of hemoglobin that is saturated with oxygen. Hemoglobin is the unique molecule in red blood cells that has the ability to carry oxygen.
The pulse oximeter consists of a probe or sensor plus a computer. The probe, which looks like a padded clothespin, is placed on a relatively thin part of a person’s body, such as a finger or earlobe. Both red and infrared light are then transmitted through the tissue by the probe. Based on the absorption of the red and infrared light caused by the difference in color between hemoglobin that is saturated with oxygen (red) and unsaturated hemoglobin (blue), the computer can estimate the proportion of hemoglobin that is oxygenated. The pulse oximeter then displays this result as a percentage. A blood oxygen saturation level less than 95 percent is abnormal. It may be explained by a lung or heart problem already present, such as emphysema or pneumonia, or by PE (or both).
Arterial Blood Gas. A more precise measurement of blood oxygen level is obtained from a sample taken directly from an artery with a needle or a thin tube (catheter). An arterial blood gas (ABG) measures the levels of both oxygen and carbon dioxide in the blood to determine how well the lungs are working. While most blood tests are performed on samples taken from a vein, an ABG is performed on a sample taken from an artery. In most cases, the artery in the wrist is used for this purpose, but other arteries may be used. The levels of blood gases are measured as partial pressures in units of millimeters of mercury (mm Hg). A partial pressure of oxygen less than 80 mm Hg is abnormal.
Chest X-Ray. A chest x-ray cannot prove that PE is present or absent because clots do not show up on x-ray. Nevertheless, a chest x-ray is a useful test in the evaluation for PE because it can find other diseases, such as pneumonia or fluid in the lungs, that may explain a person’s symptoms. Occasionally, when pulmonary infarction occurs, the x-ray may suggest this diagnosis, although more testing is necessary to prove it with certainty. A normal or negative chest x-ray with a low, otherwise unexplained blood oxygen level, however, raises the suspicion for PE.
Ventilation-Perfusion Scan (VQ Scan). A VQ lung scan may be a useful test to determine whether a person has experienced PE. This test evaluates both air flow (V = ventilation) and blood flow (Q = perfusion) in the lungs. About one hour before the test, a slightly radioactive version of the mineral technetium mixed with liquid protein is administered through a vein to identify areas of the lung that may have reduced blood flow. Multiple images are taken from different angles, using a special camera that detects radioactivity. For half of the images, the person breathes from a tube that contains a mixture of air, oxygen, and a slightly radioactive version of the gas xenon, which reveals air flow in different parts of the lung. For the other half of the images, the camera tracks the technetium, which reveals blood flow in different parts of the lung. PE is suspected in areas of the lung that have significant “mismatches”—that is, good air flow but poor blood flow.
Except for the minor discomfort from having an intravenous catheter placed, a VQ lung scan is painless and usually takes less than an hour. The exposure to radioactivity from the test is very minor and results in no side effects or complications.
A radiologist interprets the images from the VQ lung scan and decides whether the probability of a PE is high, low, or intermediate. If the probability is high, the diagnosis is made. If the probability is low or intermediate (that is, nondiagnostic), or if the VQ scan cannot be interpreted clearly, other testing must be considered. Even when PE is ultimately proven to be present, the VQ scan may be nondiagnostic. If clinical suspicion is low and the VQ scan reveals a low probability of PE, generally no further testing is needed. A normal VQ scan means PE is not present.
Spiral Computed Tomography of the Chest. An alternative to the VQ scan is a spiral computed tomography (CT) of the chest. A spiral CT of the chest uses special equipment to obtain multiple cross-sectional x-ray images of the organs and tissues of the chest. CT produces images that are far more detailed than those available with a conventional x-ray. Many different types of tissues—including the lungs, heart, bones, soft tissues, muscles, and blood vessels—can be seen.
During the spiral CT, radiation is emitted from a rotating tube. Different tissues absorb this radiation differently. During each rotation, approximately 1,000 images are recorded, which a computer then reassembles to produce a detailed image of the interior of the chest. The x-ray rotates as the patient passes through the CT scanner in a spiral path—hence the term “spiral” CT. The amount of radiation exposure is relatively low, and the procedure is not invasive.
Pulmonary Angiogram. If the VQ scan interpretation is low, intermediate, or uncertain probability of PE, or if the spiral CT is normal yet the symptoms are still suspicious, then the definitive test is a pulmonary angiogram. An angiogram is an invasive test that uses x-rays to reveal blockages or other abnormalities within the veins or arteries. Contrast dye (usually iodine dye) helps blood vessels show up clearly on x-rays. During an angiogram, contrast dye is injected into a blood vessel, and its path is tracked by a series of x-rays.
A pulmonary angiogram examines the arteries that carry blood from the heart to the lungs and is performed to see if PE is present. Using x-rays in real-time (fluoroscopy), the radiologist inserts a catheter into a vein and advances it until it reaches the vena cava (the very large vein that carries blood to the heart). Next, the radiologist advances the catheter still farther into the right side of the heart and finally into the pulmonary artery, the large artery that carries blood to the lungs. The radiologist directs the tip of the catheter into the different branches of the right and left pulmonary arteries and injects the contrast dye, which illuminates the arteries on x-ray. If PE is present, it will show up as a blockage.
Risks associated with a pulmonary angiogram include the possibility of damage caused by the catheter, bleeding, and an allergic reaction to the contrast dye. The amount of radiation from the x-rays is too small to cause any harm.
Echocardiogram. An echocardiogram is an ultrasound of the heart. Doppler ultrasound, B-mode ultrasound, and M-mode ultrasound (a rapid sequence of B-mode images that allows motion to be visualized) are combined to give information about the size of the heart, the function of the valves, and the strength of the heart muscle. (Duplex ultrasound is discussed in detail in Question 9.) The echocardiogram can spot areas of the heart that are not working well. When patients with a PE have an echocardiogram, approximately 40 percent will be found to have abnormalities of the right side of the heart, particularly the right ventricle. While an echocardiogram is not actually used to diagnose a PE, it can identify strain on the right side of the heart caused by a large PE as well as certain heart problems that may imitate a PE.