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Ventilation-Perfusion Scan or "V-Q Scan"

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What is the test?

The ventilation-perfusion scan is a nuclear scan so named because it studies both airflow (ventilation) and blood flow (perfusion) in the lungs. The initials V-Q are used in mathematical equations that calculate airflow and blood flow. The purpose of this test is to look for evidence of a blood clot in the lungs, called a pulmonary embolus, that lowers oxygen levels, causes shortness of breath, and sometimes is fatal.

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How do I prepare for the test?

About one hour before the test, a technician places an IV in your arm. A slightly radioactive version of the mineral technetium mixed with liquid protein is injected through the IV to identify areas of the lung that have reduced blood flow.

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What happens when the test is performed?

The test is performed in the radiology department of a hospital or in an outpatient facility. You are asked to put on a hospital gown. Once you are ready, multiple pictures of your chest are taken from different angles, using a special camera that detects the radionuclide. For half of these pictures, you are asked to breathe from a tube that has a mixture of air, oxygen, and a slightly radioactive version of a gas called xenon, which can be detected by the camera, and which measures airflow in different parts of the lung. For the other half of the pictures, the camera tracks the injected radionuclide to determine blood flow in different parts of the lung. A blood clot is suspected in areas of the lung that have good airflow but poor blood flow. Except for the minor discomfort of having the IV placed, the test is painless. It usually takes less than one hour.

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What risks are there from the test?

Many people worry when they hear that the liquid and gas used in this test are slightly radioactive. In truth, the radioactivity you are exposed to in this test is so small that there are no side effects or complications, unless you are pregnant.

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Must I do anything special after the test is over?

No.

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How long is it before the result of the test is known?

The results are usually available within a few hours, because the test is done only when you are suspected of having a potentially life-threatening condition (pulmonary embolus). Your doctor can interpret the pictures to determine whether your probability of having a blood clot in your lungs is high, low, or intermediate. If the probability is high, usually your doctor will order bloodthinning medicine. If it is low, he or she may not give immediate treatment, but will want to examine you again in a short time. If you face an intermediate risk, or if the V-Q scan cannot be clearly interpreted, your doctor may order a pulmonary angiogram (see page 8) to help determine whether you have a blood clot. This test is more definitive than a V-Q scan, but because it is more difficult and risky, the V-Q scan usually is done first.

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