What Are Pulmonary Function Tests?

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  • 0:07 Pulmonary Function Tests
  • 0:46 Spirometry
  • 3:54 Breathing Disorders
  • 5:54 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

Pulmonary function tests are used to measure air movement into and out of the lungs. Spirometry is the most common way to measure airflow. This lesson will describe the use of spirometry to measure lung volumes and flow rates as well as how breathing disorders are diagnosed.

Pulmonary Function Tests

Ventilation refers to the process of air movement into and out of the lungs. Inspiration specifically refers to movement of air into the lungs, while expiration refers to movement of air out of the lungs. Pulmonary function tests (or PFTs), sometimes referred to as lung function tests, are used to determine how much air our lungs can hold, and how quickly they can move air in and out. In short, PFTs assess ventilation. In this lesson, we'll describe how PFTs are performed, and how they are used to diagnose breathing problems.

A handheld spirometer
Handheld Spirometer


Spirometry is the most common way to perform a pulmonary function test. A specialized technician has a patient breathe through a mouth piece into an instrument called a spirometer.

The spirometer, in turn, measures air movement and records the information on a chart called a spirogram. Each curve that you see on the screen represents a single act of ventilation. The left side of the curve represents inspiration, while the right side of the curve represents expiration. Furthermore, the more air we breathe, the larger the curve - in other words, the higher it goes up and the further it goes down.

Example of a spirogram recorded by a spirometer
Spirogram Example


Now, let's examine some of the more common lung function values that are measured with spirometry. Tidal volume (or TV) is the volume of air moved in and out of the lung during a resting ventilation. Here's a helpful way to remember this fact: we are usually at rest when we watch television, which has the same abbreviation as tidal volume - that is TV. Vital capacity (or VC) is the volume of air that can be forced out after a forced inspiration.

Let's demonstrate this one. After a resting expiration, inhale as much air as possible and then force out as much as possible. It might sound something like this (deep breath); that's a lot of air! I've got a question for you. After that forced expiration, did you force all of the air out of your lungs? As it turns out, we can't force all of the air out of our lungs, even if we try. Take a look at the spirogram again. You can see that residual volume (abbreviated RV) is the volume of air remaining in your lungs after a forced expiration. It's a good thing our lungs don't empty. If they did, then the walls of the lungs would stick together, and we wouldn't be able to breathe again. We'd die!

Let's look at another volume. Total lung capacity (or TLC) is the maximum volume of air that our lungs can hold, and the formula is TLC = VC + RV. Let me make a quick comment here. In the spirogram, you see volumes and capacities. Well, what's the difference? Well, the capacities are simply the sums of two or more volumes.

Pulmonary function tests are used to measure the rate of velocity and airflow as well. Forced expiratory volume (or FEV 1.0) is the volume of air you can force out in the first second after a normal inspiration. In other words, FEV 1.0 measures how fast we can move air through our airways.

Vital capacity plus residual volume equals TLC.
Total Lung Capacity Formula

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