Pulmonary Function Tests

There are three basic types of pulmonary function tests. Lung volume measurementsdone quietlymeasure the volume of air in your lungs at different points during breathing. Ventilation measurementsdone under relatively dynamic conditionsare concerned with the air flowing into and out of your lungs. Gas exchange measurements determine how the alveoli are functioning. (Although there are many more tests than the ones described below, we discuss only those that have proved over time to be the most informative.)

Most of these pulmonary function tests provide more detailed evidence for the presence or absence of airway obstruction, and document the severity if obstruction exists. One of these pulmonary function tests also provides definitive diagnostic information for asthma. Asthma is confirmed if this test shows dramatic reversibility of airway obstruction when bronchodilator medication is used. If test results exclude asthma, then the finger points to emphysema and/or chronic bronchitis. And if bronchodilator medication improves airflow to some small degree, then we know that a small component of that patient's airway obstruction is reversible.

Your performance on each of these tests will be compared to the predicted values appropriate for you. These are average measurements form large groups of healthy people. Each group combines people of the same race, sex, age, and height. Your measurements will most likely be considered normal if they fall within the range from 80 to 120% of the average for your particular group. But predicted values are guidelines, not rigid judgments, and final interpretation of the pulmonary function tests is undertaken in the context of information gained from your history, physical exam, and chest X-ray.

Lung Volume Measurements: In all these tests you breathe through a mouthpiecemuch like those on snorkels and scuba tanksattached to the spirometer. You wear a nose clip to make sure that all the air moving in and out of your lungs goes through your mouth.

You will be asked to breathe quietly for a few breaths, and then be told to squeeze as much air out of your lungs as you possibly can. The amount of additional air squeezed out after quiet expiration would have ended is your expiratory reserve volume. After this maximum expiration, you will be asked to fill your lungs slowly with as much air as they can hold, and then again squeeze out as much as you can. The amount of air breathed out in going from you maximum inspiration to your maximum expiration is your vital capacity. Next you will be told to take another maximum inspiration, then just breathe out quietly. The amount of air breathed out as you go from your maximum inspiration to the end of a quiet expiration is your inspiratory capacity.

Some air always remains in the lungs after even a maximum expiration, an amount called the residual volume. Since it can never be breathed out, this residual amountand therefore the two lung volumes that include it in their totalmust be determined indirectly with the spirometer. These other two volumes are the functional residual capacity (the amount of air remaining in the lungs at the end of a quiet expiration, that is, the residual volume + the expiratory reserve volume), and the total lung capacity (the total amount of air contained in the lungs after a maximum inspirationthat is, summing all of the individual volumes).

With one indirect technique, you breathe in a specific amount of helium gas and its concentration in your lungs is measured. Then the amount of air that has to be in your lungs to produce that concentration can be calculated. The other indirect technique measures the amount of oxygen that it takes to wash all of the nitrogen gas out of your lungs. (The air we breathe contains about 80% nitrogen.) For either method, all that you must do is breathe quietly until the recording equipment indicates either that (1) the helium being breathed in is not being diluted any further, or (2) all of your lungs' nitrogen has been expelled.

These static lung volumes document the presence and severity of airway obstruction rather than identifying its cause. When the airways are obstructed, all of the volumes measured directly (expiratory reserve volume, inspiratory capacity, vital capacity) tend to be smaller than normal. How much smaller is usually related to the severity of the obstruction. The volumes that must be determined indirectly (residual volume, functional residual capacity, total lung capacity) are larger than normal. All of these changes occur because of hyperinflation and air trapping.