Pulmonary Embolism (cont.)

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d-Dimer blood test

If the healthcare professional's suspicion for pulmonary embolism is low, a d-Dimer blood test can be used for reassurance that a blood clot may not be present. The d-Dimer blood test measures one of the breakdown products of a blood clot. If this test is normal, then the likelihood of a pulmonary embolism is very low. Unfortunately, this test is not specific for blood clots in the lung. It can be positive for a variety of reasons including pregnancy, injury, recent surgery, cancer, or infection. D-dimer is not helpful if the potential risk for a blood clot is high.

CT scan

If there is greater suspicion, then computerized tomography (CT scan) of the chest with angiography can be done. Contrast material (dye) is injected into an intravenous line in the arm while the CT is being taken, and the pulmonary arteries can be visualized. There are some limitations of the test, especially if a pulmonary embolism involves the smaller arteries in the lung. However similar problems are seen with the more invasive pulmonary angiogram. As CT scan has become more and more sophisticated, not identifying significant emboli is very unusual. It is very important that the contrast material used during the CT angiogram be timed appropriately so that the bolus of dye is not diluted as it travels through the lungs.

There are some risks with this test since a patient may be allergic to the contrast material, and the contrast material may damage kidney function, especially if the patient's baseline kidney function (as measured by a creatinine blood test) is abnormal. It may be wise to limit the patient's exposure to radiation, especially in pregnant patients. However, since pulmonary embolus can be fatal, even in pregnancy this test can be performed, preferably after the first trimester.

Ventilation-perfusion scans

Ventilation-perfusion scans (VQ scans) use radioactive labeled molecules(often xenon gas, and albumin protein). Gas is inhaled and the low level radiation is detected throughout the lung fields in the distribution of the pulmonary airways(ventilation). This radioactivity is referred to as gamma radiation and is similar in intensity to sun light rays. The duration of this radioactivity is often measured in hours. Radiolabeled albumin is also injected and the lungs are scanned where these molecules are trapped in the lung following the pulmonary arterial blood path (perfusion). The radiologist then compares multiple different views of perfusion and ventilation looking for areas that are not identical. If blood flow is lacking to a portion of lung often a pie shaped wedge defect is seen, ventilation to this area is usually perserved. This is referred to as a ventilation perfusion mismatch. If a mismatch occurs, meaning that there is lung tissue that has good air entry but no blood flow, it may be indicative of a pulmonary embolus. These tests are read by a radiologist as having a low, moderate, or high probability of having a pulmonary embolism. There are limitations to the test, since there may be a 5%-10% risk that a pulmonary embolism exists even with a low probability V/Q result.Ventilation-perfusion scans (VQ scans) use labeled chemicals to identify inhaled air into the lungs and match it with blood flow in the arteries. If a mismatch occurs, meaning that there is lung tissue that has good air entry but no blood flow, it may be indicative of a pulmonary embolus. These tests are read by a radiologist as having a low, moderate, or high probability of having a pulmonary embolism. There are limitations to the test, since there may be a 5% to 10% risk that a pulmonary embolism exists even with a low probability V/Q result.

Medically Reviewed by a Doctor on 11/20/2015


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