Plethysmography

Written by Debra Stang | Published on July 20, 2012
Medically Reviewed by George Krucik, MD

What Is Plethysmography?

Plethysmography measures changes in volume in different areas of your body. It involves blood pressure cuffs or other sensors that are attached to a machine called a plethysmograph. It is especially effective in detecting changes caused by blood flow. Plethysmography can help your doctor determine if you have a blood clot in your arm or leg. It can also be used to calculate the amount of air your lungs can hold.

Limb plethysmography involves having blood pressure cuffs placed on your arms and legs. The cuffs are connected to the plethysmograph. Lung or full-body plethysmography, which measures the capacity of your lungs, involves breathing against a mouthpiece while enclosed in an airtight room.

When Is Plethysmography Ordered?

Doctors may order a limb plethysmography if you show signs of blood clots in your legs. Symptoms of blood clots include redness, warmth, swelling, and tenderness. While plethysmography is not as accurate as an arteriogram, which is more commonly used to identify blood clots, it is less invasive and less expensive. These factors make it more appealing to many individuals.

A lung plethysmography is used to evaluate upper respiratory symptoms such as pain or discomfort with breathing, shortness of breath, or the sensation of not being able to get enough air. Your doctor cannot diagnose the underlying cause of your problem from plethysmography alone. However, abnormal results can confirm his or her suspicion that something is preventing your lungs from holding the volume of air they should be capable of holding.

In the past, penile plethysmography has also been used to measure arousal in sex offenders, but this practice is falling out of favor in light of court rulings that it violates the rights of the offender. (Harlow & Scott, 2007)

Procedure for a Limb Plethysmography

A limb plethysmography can be performed in a doctor’s office or hospital. If you are wearing pants or a long-sleeved shirt, the doctor will ask you to undress to your underwear and put on a hospital gown so that one leg and one arm are bared. You will recline in a comfortable position on an examination table, cot, or gurney.

Your doctor will then place blood pressure cuffs on your leg and on your arm. He or she will be most interested in checking the systolic blood pressure. That is the pressure of blood in your arm and leg when your heart contracts. You may feel a little uncomfortable when the blood pressure cuffs tighten around your arm and leg, but you will not feel any real pain. The test usually lasts about 20 to 30 minutes. During this time, you will be asked to move as little as possible.

Limb plethysmography is not associated with any risks or side effects. Unless your doctor tells you otherwise, you can resume your regular schedule immediately after this test.

Procedure for a Lung Plethysmography

The test may be performed in a specialist’s office or in a hospital. You will sit in a small, airtight room. Clips will be used to close off your nostrils. You will be asked to breathe against a mouthpiece when it is open and when it is closed.

Some people report becoming short of breath or light-headed. Let your doctor know if you experience these symptoms during the test.

Unless your doctor tells you otherwise, you will be able to resume your regular activities immediately after the test is completed.

How to Prepare for Your Plethysmography

There are no special requirements to prepare for a limb plethysmorgraphy.

If you are having a lung plethysmography, you should avoid smoking and doing aerobic exercise for eight hours before the test. You should also eat lightly because heavy meals can affect your ability to breathe deeply. It’s best to wear loose, comfortable clothing.

Because the test requires sitting in a small space, it can be hard for people who have claustrophobia (a fear of small spaces). If you think this may be a problem for you, let your doctor know. It’s important to tell your doctor if you are taking any medications, especially medications for breathing problems.

Interpreting the Tests

Limb Plethysmography

Normally, the systolic blood pressure in the leg and the systolic blood pressure in the arm are within 20 mmHg of each other. (National Institutes of Health) If the difference in systolic blood pressure in the arm and the leg is greater than 20 mmHg, it alerts the doctor that you might have a blockage or a blood clot in your leg. Based on this test result, your doctor can then order further tests to determine the exact nature of the problem.

Lung Plethysmography

Lung plethysmography measures how much air a person can hold in his or her lungs. The normal range depends on the person’s age, gender, body size, and level of fitness. This test is a diagnostic starting point. An abnormal result confirms that there is a problem with lung capacity, but it does not tell your doctor what that problem might be. Your doctor would have to do additional tests to discover why your results were abnormal. Possibilities include a breakdown of lung tissue, problems with the muscles around the chest wall, and problems with the lungs’ ability to contract and expand.

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