Lung Perfusion - An Overview

What Is Meant by a Lung Perfusion Scan?

A ventilation/perfusion scan is a type of nuclear medicine test that can show airflow and circulation in a child's lungs. Ventilation refers to how well the air reaches all regions of the lungs, and perfusion studies the motion of blood through the lungs. For the ventilation section, a mask attached to a bag containing oxygen and the radiopharmaceutical gas known as Xenon-133 is placed over your child's nose and mouth, and he or she will breathe normally. For perfusion, in the perfusion section, a radiopharmaceutical is injected into a vein called Technetium-99m MAA. Later, the gamma camera captures images of the lungs to know how wide the radiopharmaceutical can spread.

How Does a CT Scan Differ From a VQ Scan?

CT and VQ scans are imaging techniques of the lungs whereby one is trying to see if there is a blood clot. A CT scan makes some images inside your body using a camera by injecting your body with some contrast dye, which the healthcare provider then analyzes to try and detect some issues that might have been caused in your body. The two-phase VQ scan is used. It involves a camera capturing pictures of radioactive particles inside your body. One set of images can be reviewed, or both can be compared to determine how well your lungs function.

What Are the Indications for Lung Perfusion?

The most common indications of lung perfusion are:

• To diagnose a suspected case of acute or chronic pulmonary embolism.

• To monitor pulmonary function after lung transplant.

• 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. 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 :

• Shortness of breath or dyspnea.

• Chest pain that worsens while inhaling.

• Hemoptysis or coughing up blood.

• Leg pain or swelling.

• Excessive sweating.

• Lightheadedness.

• 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, with Technetium-99m macroaggregated albumin (MMA) injected into the prone position. The particles are large enough to lodge in the precapillary arterioles. For both test phases, a high-resolution gamma camera acquires images of the lungs.

The areas where the radioisotope accumulates in greater amounts are known as "hot spots," whereas the areas that do not absorb the tracer and appear 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 a three-dimensional image. 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 performed, and a chest X-ray is taken 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:

• Airway obstruction.

• Chronic obstructive pulmonary disease (COPD).

• Pneumonia.

• Narrowed pulmonary artery.

• Pulmonary emboli.

• Pneumonitis or inflammation of the lungs.

• 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

• Matched ventilation-perfusion.

• Mismatched ventilation-perfusion.

• 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: This defect is commonly observed in pulmonary embolism, where the pulmonary arterial perfusion is reduced. However, ventilation remains unaffected in such cases. 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

• Normal Perfusion Scan: The patient is unlikely to have an acute pulmonary embolism.

• Low Probability Ventilation Perfusion Scan: The patient has less than a twenty percent probability of having an acute pulmonary embolism.

• Intermediate Probability Ventilation Perfusion Scan: The patient has between twenty to eighty percent probability of having an acute pulmonary embolism.

• High Probability Ventilation Perfusion Scan: The patient has a greater than eighty percent probability of having an acute pulmonary embolism.

What Are the Complications of a Lung Perfusion Scan?

A ventilation-perfusion scan usually has minimal complications or side effects and is well tolerated. Among them are an allergic reaction to the tracer injected and pain, redness (erythema), and swelling (edema) associated with injection sites.

Conclusion:

Lung perfusion is the movement of blood in the lungs for the interchange of gases; hence, ensuring that oxygenation is maintained while carbon dioxide is removed from the body is quite essential. Proximal perfusion ensures proper respiratory function, while distress can cause respiratory failure, among other medical disorders. Hence, adequate knowledge about lung perfusion ensures proper diagnosis and treatment of pulmonary embolism, chronic lung conditions, and heart failure, thereby delivering the best health care to the patient.