Ventilation/perfusion scan

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


Overview

Ventilation/perfusion scan, also called a V/Q scan, is a medical test to measure the circulation of air and blood within a patient's lungs. The ventilation part of the test evaluates the ability of air to reach all parts of the lungs, while the perfusion part measures how well the blood circulates within the lungs. This test is most commonly done in order to check for the presence of a blood clot or abnormal blood flow inside the lungs (pulmonary embolism or PE), although computed tomography with radiocontrast is now more commonly used for this purpose. A V/Q scan may also be performed in the case of serious lung disorders such as COPD or pneumonia as well as a lung performance quantification tool pre and post lung lobectomy surgery.

Procedure

The ventilation and perfusion segments of a V/Q scan are to be performed together and must include an X-ray of the chest (posterior-anterior projection) in order to properly diagnose. A defect in the perfusion images requires a mismatched ventilation to be indicative of PE.

In the ventilation phase of the test, a gaseous radionuclide Xenon or technetium DTPA in an aerosol form is inhaled by the patient through a mouthpiece or mask that covers the nose and mouth. The perfusion phase of the test involves the intravenous injection of radioactive technetium macro aggregated albumin (Tc99m-MAA). The patient lies beneath a gamma camera which allows radioactive agents to be visualized within the lungs.

Significance of results

Decreased uptake of the inhaled radioisotope (perfusion-ventilation match) may indicate one of the following:

  • The impaired ability to breathe
  • Airway obstruction
  • Possible pneumonia.

Decreased circulation of the injected MAA (perfusion-ventilation mismatch) indicates a problem with blood flow into or within the lungs. A localized area of decreased uptake, usually in a pie shaped wedge configuration when mismatched with a normal ventilation image suggests a pulmonary embolus or blood clot in the lungs.

Risks

Although this test uses radioactive materials, total amount of radiation exposure is low. However, this test should still be administered with caution to women who are pregnant, especailly during the first trimester. If breastfeeding, patient must be councelled to refrain from this activity for approximately 24 hours.

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