What are lung volume measurements?

Spirometry measures airflow. By measuring how much air you exhale, and how quickly you exhale, spirometry can evaluate a broad range of lung diseases. In a spirometry test, while you are sitting, you breathe into a mouthpiece that is connected to an instrument called a spirometer. The spirometer records the amount and the rate of air that you breathe in and out over a period of time. When standing, some numbers might be slightly different.

For some of the test measurements, you can breathe normally and quietly. Other tests require forced inhalation or exhalation after a deep breath. Sometimes, you will be asked to inhale a different gas or a medicine to see how it changes your test results.

How do you measure lung volumes?

Lung volume measurement can be done in two ways:

• The most accurate way is called body plethysmography. You sit in a clear airtight box that looks like a phone booth. The technologist asks you to breathe in and out of a mouthpiece. Changes in pressure inside the box help determine the lung volume.
• Lung volume can also be measured when you breathe nitrogen or helium gas through a tube for a certain period of time. The concentration of the gas in a chamber attached to the tube is measured to estimate the lung volume.