Your doctor can order a lung plethysmography test to measure how much air you can hold in your lungs. Lung plethysmography is also called pulmonary or body plethysmography. It helps doctors assess the condition of people with lung disease, which can occur with a decrease in total lung capacity (TLC). TLC is the total volume of air in your chest after you’ve inhaled as deeply as possible.

Although spirometry is the standard way to measure lung volumes, lung plethysmography is more accurate. Measurements from this test are based on Boyle’s Law, a scientific principle that describes the relationship between the pressure and volume of a gas. This law says that if temperature remains the same, you can use measurements of the volume of a gas to find out its pressure and vice versa.

A lung plethysmography test can take place in:

  • pulmonary function laboratories
  • cardiopulmonary laboratories
  • clinics
  • pulmonology offices

A respiratory care technician usually performs it. They’ll monitor your condition at all times.

You’ll sit or stand in a small, airtight chamber that is partially or completely see-through and may resemble a phone booth. Babies may have a special type of test that allows them to lay down. They’ll put clips on your nose to shut off air to your nostrils. They’ll ask you to breathe or pant against a mouthpiece when it’s both opened and closed. This will provide your doctor with important measurements, including:

  • TLC
  • the amount of air left in your lungs when you breathe out normally, which is called functional residual capacity (FRC)
  • how much air is left when you breathe out as much as possible, or residual capacity (RC)

As your chest moves while you breathe or pant, it changes the pressure and amount of air in the chamber. Your breathing also changes the pressure against the mouthpiece. From these changes, your doctor can get an accurate measure of TLC, FRC, and RC.

The mouthpiece may feel uncomfortable against your mouth. If you typically struggle in tight spaces, sitting in the chamber might make you anxious. However, you’ll be able to see outside the chamber at all times, and the test usually takes three minutes to perform. They may include a tracer gas such as carbon dioxide in the air you breathe during the test.

Tell your doctor about any medications you’re taking, especially those for breathing problems, such as asthma. You may have to stop taking certain medications temporarily before the test. You might also need to reschedule the test if you have a cold or the flu.

You’ll be more comfortable if you wear loose clothes during the test. You should also avoid:

  • drinking alcohol at least four hours before the test
  • heavy meals at least two hours before the test
  • smoking at least one hour before the test
  • strenuous exercise at least 30 minutes before the test

All of these activities can affect your ability to breathe and may result in inaccurate test results.

Risks associated with lung plethysmography include:

  • dizziness
  • lightheadedness
  • shortness of breath
  • anxiety for if you’re uncomfortable in tight spaces
  • transmission of infection if the equipment, such as the mouthpiece, isn’t properly cleaned before you use it

Too much carbon dioxide in your blood is called “hypercapnia.” Too little oxygen in your blood is called “hypoxia.” Either can occur if you’re in the chamber for a longer than typical amount of time. This is uncommon.

Your doctor may order a lung plethysmography test to:

  • help diagnose restrictive lung disease, which is a type of disease that restricts lung expansion
  • evaluate obstructive lung diseases, such as bullous emphysema and cystic fibrosis
  • follow the course of a disease and its response to treatment
  • measure your resistance to airflow
  • measure your response to bronchodilator medications
  • assess whether your lung capacity will be affected by such treatments as methacholine, histamine, or isocapnic hyperventilation

You shouldn’t have lung plethysmography if you:

  • are mentally confused
  • lack muscle coordination, or you have poor muscle control that causes jerky or unpredictable movements
  • have a condition that prevents you from entering the chamber or properly performing the required steps
  • are claustrophobic, or you have a fear of being in small places
  • require continuous oxygen therapy

Normal values depend on a combination of many factors, such as:

  • age
  • height
  • ethnic background
  • sex

A normal value for you may be different than a normal value for someone else. Your doctor will assess whether or not your results are normal. If you have a restrictive lung disease, your lung volumes will likely be lower than expected. In some cases, however, unexpectedly high measurements can indicate a condition that’s trapping air in the lungs, such as emphysema.

Abnormal results indicate that there’s a problem in your lungs. Your doctor can’t use lung plethysmography to determine the cause of the problem, but they can use it to narrow down the possibilities, such as a breakdown of the lung structure, a problem with the chest wall and its muscles, or an inability of the lungs to expand or contract.