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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.
Why is the Test Performed?
*The ventilation scan is used to evaluate the ability of air to reach all portions of the lungs.
*The perfusion scan measures the supply of blood through the lungs.
*A ventilation and perfusion scan is most often performed to detect a pulmonary embolus. It is also used to evaluate lung function in people with advanced pulmonary disease such as COPD, and to detect the presence of shunts (abnormal circulation) in the pulmonary blood vessels.
How is the Test Performed?
*A Pulmonary ventilation/perfusion scan is actually two tests that may be performed separately or together.
*The perfusion scan is performed by injecting albumin into a vein. The patient is immediately placed on a movable table that is positioned under the arm of a scanner. The lungs are scanned to detect the location of the radioactive particles as blood flows through the lungs.
*The ventilation scan is performed by scanning the lungs while having the person inhale gas. A mask is placed over the nose and mouth, and the patient is asked to breathe the gas while sitting or lying on the table beneath the arm of the scanner.
How to Prepare for the Test?
*No special diet, medications, or fasting are necessary before the test.
*A chest x-ray is usually performed prior to or following a ventilation and perfusion scan.
*The patient usually signs a consent form and is asked to wear a hospital gown or comfortable clothing without metal fasteners.
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.
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.
How Will the Test Feel?
The table may feel hard and/or cold. There may be a sharp prick during injection of material into the vein for the perfusion portion of the scan. The mask used during the ventilation scan may give you a claustrophobic feeling. You must lie still during scanning. The injection usually does not cause discomfort.
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.
*Risks are essentially the same as for x-rays (radiation) and needle pricks.
*There is a small exposure to radiation from the radioisotope. The radioisotopes used during scans are short lived, with almost all radiation gone in a few days. However, as with any radiation exposure, caution is advised during pregnancy or if a woman is breastfeeding. No radiation is emitted from the scanner; it detects radiation and converts it to a visible image.
*There is a slight risk for infection or bleeding at the site of the needle insertion. The risk with perfusion scan is no different than for placement of an intravenous needle for any other purpose.
*It is extremely rare, but a person may develop an allergy to the radioisotope, which may include serious anaphylactic reaction.
Must you do anything special after the test is over?
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 yourdoctor 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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