Lung (or pulmonary) plethysmography is a test used to measure how much air you can hold in your lungs. Lung plethysmography helps healthcare providers assess patients with lung diseases, which are often associated with a decrease in total lung capacity (TLC).
Although another test called spirometry is the standard test for measuring lung volumes, lung plethysmography has been found to be more reliable. It can measure a person’s TLC, which is the total volume of air in the chest after they have inhaled as deeply as possible.
Plethysmographic measurements are based on Boyle’s Law, a principle that describes the relationship between the pressure and volume of a gas.
Lung plethysmography can be done in pulmonary function laboratories, cardiopulmonary laboratories, clinics, and pulmonology offices. In most cases, the test is performed by a respiratory care technician, who will monitor you at all times.
You will sit or stand in a small, airtight chamber and breathe or pant against a mouthpiece. Clips will be put on your nose to shut off air to your nostrils. You’ll be asked to breathe against the mouthpiece when it’s both opened and closed. This will provide your doctor with important information.
As your chest moves while you breathe or pant, it changes the pressure and amount of air in the chamber and against the mouthpiece. From these changes, your doctor can get an accurate measure of TLC, the amount of air in your lungs.
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 only three minutes to perform. A tracer gas such as carbon dioxide may be included in the air you breathe during the test.
Be sure to let your healthcare provider know if you’re taking any medications, especially those for breathing problems. You may have to temporarily stop taking certain medications before the test.
You’ll be more comfortable if you wear loose clothes during the test. Avoid smoking, strenuous exercise, and heavy meals for six hours before the test, as these can all affect your ability to take deep breaths.
Risks associated with lung plethysmography include:
- dizziness or lightheadedness
- shortness of breath
- anxiety for those who are uncomfortable in tight spaces
- too much carbon dioxide (hypercapnia) or too little oxygen (hypoxia) in your blood can occur if you’re in the chamber for a longer than typical amount of time, which is uncommon
- transmission of infection is possible if the equipment (mouthpiece) is not properly cleaned before you use it
Your healthcare provider may order lung plethysmography to:
- help diagnose restrictive lung disease
- 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
Lung plethysmography should not be performed if you:
- are mentally confused
- experience muscle incoordination (poor muscle control causing jerky or unpredictable movements)
- have a condition that prevents you from entering the chamber or properly performing the required steps
- are claustrophobic
- require continuous oxygen therapy that cannot be discontinued, even temporarily
Your doctor will assess whether or not your results are normal. Normal values depend on many factors, such as age, height, ethnic background, and gender.
Abnormal results indicate that there is a problem in your lungs. Although lung plethysmography cannot determine the cause of the problem, it can help your doctor narrow down the possibilities, which include 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.