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
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
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.
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Now that we have defined some of the more common lung function measures, let's discuss how this information is used to diagnose breathing problems. Breathing disorders are all categorized as restrictive or obstructive. Restrictive disorders, such as pulmonary fibrosis, result from abnormal connective tissue in the lung tissue. Lung tissue is normally very stretchy, but connective tissue decreases the ability of the lungs to stretch and, therefore, expand. Restrictive disorders are diagnosed by abnormally low lung volumes such as vital capacity. Let's do a quick demonstration using a balloon to represent a lung. If you put tape on the balloon, the balloon won't be able to expand as well, and that will decrease the amount of air it can hold. Think of the tape as being like connective tissue as it hinders the ability of the balloon to expand.
Alright, let's move on. Obstructive disorders, such as asthma and chronic obstructive pulmonary disorder (COPD), are caused by increased airway resistance. Resistance is increased due to narrowing of the airways, and this decreases the rate of airflow. Persons with obstructive disorders can have normal lung volumes; however, their FEV 1.0 is lower than normal. In other words, they can't push air through their airways as quickly as normal. Let's consider the balloon again. Blow it up, and then let the air out while slightly compressing the opening. The compressed opening is like an increase in airway resistance as it decreases the rate of airflow. Asthma and COPD cause the airways to constrict, thus increasing resistance. The characteristic wheezing associated with obstructive breathing disorders is caused by a forced air movement through a narrow opening.
In summary, ventilation is simply inspiration and expiration. Pulmonary function tests (or PFTs) measure ventilation, and spirometry is the most common way to perform a pulmonary function test. A spirometer measures and reports air movement in a chart called a spirogram. Tidal volume (abbreviated TV) is the volume of air moved at rest, vital capacity (abbreviated VC) is the volume of air forced out after maximum inspiration, residual volume is the volume of air remaining in lungs after forced expiration and total lung capacity is the maximum volume of air the lungs can hold. Spirometry can measure airflow rate as well, where FEV 1.0 is the volume of air expired in the first second after inspiration. Restrictive disorders are diagnosed by abnormally low lung volumes, while obstructive disorders are diagnosed by abnormally low flow rates.
Once you have watched this lesson, you should be prepared to:
Describe ventilation, inspiration and expiration
Explain how a spirometer does a pulmonary function test
Define tidal volume, vital capacity, residual volume, total lung capacity and FEV 1.0
Differentiate between restrictive and obstructive breathing disorders
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