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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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