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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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