A pulmonary ventilation/perfusion (or VQ) scan is a series of two lung scans. The scans are either performed together or one after the other, but are often referred to as one procedure.

One of the scans measures how well air is able to flow through your lungs. The other scan shows where blood flows in your lungs.

Both scans involve the use of a low-risk radioactive substance that can be traced by a special type of scanner. The substance will show up in the scanned image and can tell your doctor information about how well your lungs are working. The substance will gather at areas of abnormal blood or airflow, which may indicate a blockage in the lung.

A VQ scan is used most frequently to screen for a pulmonary embolus, which is also known as a blood clot in the lungs. Symptoms of pulmonary embolus may include:

A VQ scan is also used sometimes to test the lung function before lung surgery.

Your doctor will explain the procedure of the VQ scan to you, as well as the risks associated with the test. You will be asked to sign a consent form after the possible risks have been explained and you’ve had a chance to ask any questions.

Before the test, you should tell your doctor about any known allergies you have, particularly to contrast dyes or latex. This will make sure your doctor and the rest of the medical staff are prepared for the possibility of an allergic reaction during the test.

It’s also important that you tell your doctor if you’re pregnant or breastfeeding. The contrast dye used may be passed on to the fetus or through breast milk.

You should inform your doctor if you’ve had a test that included the use of radioactive materials — called a nuclear test — in the past 48 hours. If you have, there may be radioactive dye remaining in your body, which can affect the results of the test.

You should try to wear loose-fitting clothing without metal fasteners to the test, or you may be asked to change into a patient gown. It’ll also be necessary to remove any metal jewelry, including piercings, so you may wish to also avoid wearing jewelry to the test. Generally, there is no special dietary preparation, such as fasting, before the scan.

You may also be asked to have an X-ray of your chest done 24 to 48 hours before your test.

For the entire scan, which takes about 45 minutes, you’ll be asked to lie down on an examination table. For the perfusion scan, a technician will set up an intravenous line (IV). Radionuclide dye will be introduced into your bloodstream through a needle, typically using a vein on the inside of your elbow or on the back of your hand. This dye usually contains small amounts of radioactive technetium. You may feel mild to moderate pain from the IV or a pricking sensation.

When the dye has been injected, the technician will remove the IV, and you will then be moved under a special scanner. This scanner will detect the dye and look at how it flows into your lungs via your bloodstream.

You will need to lie still while the images are being captured. However, the technician may ask you to change positions to get pictures from different angles.

For the pulmonary ventilation scan, you will be given a mouthpiece while you’re still lying underneath the scanner. You will be asked to breathe through the mouthpiece, which contains a gas with a radioactive substance, usually radioactive xenon or technetium.

The scanner will take images of your lungs while you’re breathing in the gas. You should try not to swallow this gas. It could interfere with the images that need to be taken of the lungs. You may be asked to hold your breath to capture certain images.

When the technician has taken all the necessary pictures, the mouthpiece will be removed, and you will be able to leave the scanner. Your breathing will gradually remove the gas from your lungs.

There is a low risk associated with a VQ scan. The amount of radiation a person is exposed to in this process is roughly equal to or even slightly less than the amount they are naturally exposed to in a year, according to the Health Physics Society.

However, problems can arise due to the radioactive substances and the insertion of the IV. Possible risks include:

  • excessive bleeding at the IV site
  • infection at the IV site
  • allergic reaction to the radioactive dye

You may be monitored for a short time after the test to check for any allergic reactions. Someone will also check the IV site for redness and swelling. You may feel slightly dizzy from lying down during the test.

It’s important that you drink plenty of fluids after your test to help flush the radioactive substances from your body. If you notice any redness, swelling, or pain at the IV site once you return home, notify your doctor as this may be sign of an infection.

You can eat and drink as usual unless your doctor says otherwise. Also, avoid having any other nuclear procedures for the next 24 to 48 hours.