Introduction:
A lung perfusion or ventilation-perfusion (V/Q) scan is a nuclear test that uses a perfusion scan to measure the blood flow distribution and a ventilation scan to measure the airflow distribution. It is used in diagnosing blood clots in the lungs, as seen in the case of pulmonary embolism.
Assessment of lung ventilation and perfusion can be done with the help of a ventilation-perfusion scan.
A few radioactive particles are used, known as the tracer elements. These radioactive particles emit gamma rays which get picked up by the scanner. The radioactive tracer is distributed evenly throughout the lungs where the blood flow is normal, but the tracer does not get absorbed in the areas that are not getting blood.
Similarly, the tracer gets absorbed only in the areas with normal airflow and does not get absorbed into areas without airflow. These radioactive aerosol particles are cleared by crossing the airway membrane and entering the circulation, where the kidney removes them. The biological half-life is approximately 80 +/-20 minutes in healthy individuals and 24 +/-9 minutes in smokers. The ideal particle size is in the range of 0.1 to 0.5 micrometers.
What Are the Indications for Lung Perfusion?
The most common indications of lung perfusion are:
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To diagnose a suspected case of acute or chronic pulmonary embolism.
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To monitor pulmonary function after lung transplant.
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To provide pre-operative estimates of lung function in lung cancer patients where pneumonectomy is planned.
Why Is a Lung Perfusion Scan Done?
Typically, lung perfusion is done to measure the blood flow throughout the lungs. The conditions like abnormal circulation or shunts in the pulmonary blood vessels and chronic obstructive pulmonary disease can be diagnosed with this scan. This imaging scan is usually performed in case of pulmonary embolism. The most common symptoms of pulmonary embolism are :
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Shortness of breath or dyspnea.
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Chest pain that worsens while inhaling.
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Hemoptysis or coughing up blood.
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Leg pain or swelling.
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Excessive sweating.
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Lightheadedness.
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Dizziness.
How Is the Test Performed?
Usually, a chest x-ray is performed twenty-four hours before the ventilation-perfusion scan. The ventilation test is performed using a radioisotope like xenon or technetium 99m diethylenetriaminepentaacetic acid (DTPA) in an aerosol form. It is delivered to the patient through a non-breathing mask. In contrast, the patient lies in a supine position. The micro-aerosol particles are small enough to reach the distal-most part of the tracheobronchial tree. The patient is also imaged upright in three phases: initial breath, equilibrium, and washout.
The perfusion test is performed intravenously, injecting technetium 99m macro aggregated albumin (MMA) in the supine position. These particles are of enough size to get lodged into the precapillary arterioles. A high-resolution gamma camera acquires images of the lungs for both phases of the test.
The areas in which the radioisotope gets accumulated in more significant amounts are known as "hot spots," whereas the areas that do not absorb the tracer and appear to be less bright on the image scan are known as "cold spots."
Why Is Ventilation-Perfusion SPECT Better Than Normal Ventilation-Perfusion?
A ventilation perfusion single photon emission computed tomography (SPECT) allows the generation of three-dimensional ima. In contrast, a normal ventilation-perfusion scan provides a single planar or a two-dimensional image. It has higher specificity and sensitivity than the normal conventional method. It is usually considered the second-line diagnostic modality if computed tomography pulmonary angiography is contraindicated.
What Are the Typical Results of the Test?
Typically, the ventilation-perfusion test is done, and a chest x-ray is done to evaluate it. Every part of the lung should take up the radioisotope evenly.
What Are the Abnormal Results of the Test?
If the lungs take up less than the normal amount of radioisotope during the scan, it may be due to the following aspects:
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Airway obstruction.
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Narrowed pulmonary artery.
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Pulmonary emboli.
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Pneumonitis or inflammation of the lungs.
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Reduced breathing and ventilation ability.
What Has Replaced the Ventilation-Perfusion Scan?
The utility of this technique is limited in patients with underlying pulmonary disease, in whom mostly up to seventy percent of scans are indeterminate.
Images taken with a contrast medium in the main pulmonary artery are rarely used, particularly now that computed tomography pulmonary angiography (CTPA) is widely available. Right heart catheterization remains useful in investigating patients with pulmonary hypertension, providing information on pulmonary and right heart pressures. However, patients with renal disease or allergy to contrast dye may use the ventilation-perfusion scan more safely.
Types of Defects Found on a Ventilation-Perfusion Scan:
The types of defects are
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Matched ventilation-perfusion.
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Mismatched ventilation-perfusion.
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Reverse mismatched defect.
Matched Ventilation-Perfusion Defect: The normal regional response to acute hypoxia is vasoconstriction, which shunts blood flow away to other aerated areas of the lungs. This results in reduced ventilation and reduced perfusion regions that are "matched." It can be commonly seen in asthma, bronchitis, pneumonia, and emphysema cases.
Mismatched Ventilation: Perfusion Defect: It can be commonly observed in the case of pulmonary embolism, where the pulmonary arterial perfusion is reduced. However, the ventilation remains unaffected in such cases. The ventilation perfusion is uncoupled or mismatched in territories affected by pulmonary embolism.
What Are the Results of a Ventilation-Perfusion Scan?
The results of a ventilation-perfusion scan are
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Normal Perfusion Scan: The patient is unlikely to have an acute pulmonary embolism.
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Low Probability Ventilation Perfusion Scan: The patient has less than a twenty percent probability of having an acute pulmonary embolism.
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Intermediate Probability ventilation Perfusion Scan: The patient has between twenty to eighty percent probability of having an acute pulmonary embolism.
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High Probability Ventilation Perfusion Scan: The patient has a greater than eighty percent probability of having an acute pulmonary embolism.
Conclusion:
Lung perfusion is a type of medical imaging using scintigraphy to evaluate air and blood circulation within a patient's lungs. It is a relatively non-invasive method and is usually the initial test of choice for diagnosis.
