Lung Perfusion and Ventilatio... Health Article

Definition

A lung perfusion scan and ventilation study are two diagnostic imaging studies. A lung perfusion scan assesses blood flow to the lungs. A lung ventilation study reveals the distribution of air space within the lungs. These are two separate studies that are often performed sequentially. The tests are called by different names, including perfusion lung scan, aerosol lung scan, ventilation lung scan, xenon lung scan, ventilation/perfusion scanning (VPS), pulmonary scintiphotography, or most commonly, V/Q scan.

Purpose

Lung scans may be performed for patients with chest pain, for those coughing up blood (hemoptysis), or for those having difficulty breathing (dyspnea). A perfusion scan alone or both tests are frequently performed for patients with a suspected pulmonary embolism (blood clot in the lung) or for follow-up in patients with known pulmonary embolism. Lung scans are a sensitive method for demonstrating the presence of pulmonary disease but are not often specific for a certain disease. For example, an abnormal scan may also be caused by chronic obstructive pulmonary disease (COPD), asthma, pneumonia, venous hypertension, pleural effusion, and cardiomegaly.

Precautions

The amount of radioactivity a person is exposed to during these tests is very low and is not harmful. However, if the patient has had other recent nuclear medicine tests, it may be necessary to wait until other radiopharmaceuticals have been cleared from the body so that they do not interfere with these tests.

Description

These tests are typically done in a hospital nuclear medicine department or out-patient radiology facility. Scans to diagnose pulmonary embolism are often done on an emergency basis. Most often, both studies are needed. Sometimes a perfusion scan is done without a ventilation scan. Rarely, a ventilation scan is done alone.

For a lung perfusion scan, the patient is injected intravenously with radioactive particles, known as Tc 99m MAA (macroaggregated albumin). The particles pass through the larger blood vessels and become temporarily trapped in small blood vessels. The images thus reflect blood perfusion in the lungs. Images are obtained anteriorly, posteriorly, laterally, and obliquely.

For a lung ventilation scan, the patient inhales a radioactive gas through a mask placed over the nose and mouth. Images of the ventilation lung scan show the distribution of the gas in the lungs. The test typically consists of three phases. The first stage is the initial, or ventilation stage, which reflects the rate of ventilation of the different lung segments. Second is the equilibrium stage, which represents gas volume of the lungs. The third stage is the wash-out phase, which demonstrates any gas trapping that may occur in obstructive diseases. Images are typically obtained posteriorly, although additional views may also be performed. Each test takes approximately 15 to 30 minutes. If possible, the patient usually sits up while the images are taken.

Preparation

To accompany the lung scan, the patient should have a chest x ray within 12 to 24 hours of the study. Otherwise, there is no special preparation needed for these tests. The patient may eat and drink normally before the procedure.

Aftercare

No special aftercare is needed. The patient may resume normal activities immediately.

Complications

There are no complications associated with these tests.

Results

Normally, there is a physiological relationship between the perfusion of the pulmonary blood vessels and their regional alveolar ventilation. An imbalance of this relationship as demonstrated by these studies reflect various respiratory diseases. Other diagnostic tests are often required to confirm a diagnosis.

Normal results for both tests show an even distribu tion of radioactive material in all parts of the lungs. For the lung perfusion scan, diminished or absent perfusion suggests decreased blood flow to that part of the lung, and possibly a pulmonary embolism. However, pneumo nia, emphysema, or lung tumors can create readings on the lung perfusion scan that falsely suggest a pulmonary embolism is present. For the ventilation study, areas that show an increased accumulation of radioactive gas, par ticularly after the wash-out phase, suggests obstructive lung disease. Areas where there is decreased or absent radioactive gas flow suggests mechanical obstruction of air flow, such as an embolus. Certain combinations of abnormalities in lung perfusion and ventilation scans suggest pulmonary embolism.

Health care team roles

Both the lung perfusion and ventilation scans are performed by a nuclear medicine technologist. The technologist is trained to handle radioactive materials, oper ate the equipment, and process the data. The tests are interpreted by a radiologist who may specialize in nuclear medicine. Patients receive the results from their personal physician or the doctor who ordered the test.

KEY TERMS

Pulmonary embolism—A blood clot or other blockage in the arteries leading to the lungs.