Pulmonary function tests are a broad range of tests that measure
how well the lungs take in and exhale air and how efficiently they
transfer oxygen into the blood.
Spirometry measures how well the lungs exhale. The information
gathered during this test is useful in diagnosing certain types
of lung disorders, but is most useful when assessing for obstructive
lung diseases (especially asthma and chronic obstructive pulmonary
Lung volume measurements detect restrictive lung diseases. In
this set of diseases, a person cannot inhale a normal volume of
air. Restrictive lung diseases may be caused by inflammation or
scarring of the lung tissue (interstitial lung disease) or by abnormalities
of the muscles or skeleton of the chest wall.
Testing the diffusion capacity (also called the
DLCO) permits an estimate of how efficiently the lungs transfer oxygen
from the air into the bloodstream.
How the test is performed
- In a spirometry test, a person breathes into
mouthpiece that is connected to an instrument called a spirometer.
The spirometer records the amount and the rate of air that is breathed
in and out over a specified time. Some of the test measurements
are obtained by normal, quiet breathing and other tests require
forced inhalation or exhalation after a deep breath.
- Lung volume measurement can be performed in
two ways. The most accurate way is for a person to sit in a body
plethysmograph ( a sealed, transparent booth that resembles a telephone
booth) while breathing in and out into a mouthpiece. Changes in
pressure inside the booth allow determination of the lung volume.
Lung volume can also be measured when a person breathes nitrogen
or helium gas through a tube for a specified period of time. The
concentration of the gas in a chamber attached to the tube
is measured, allowing estimation of the lung volume.
- The diffusion
capacity is measured when a person breathes carbon monoxide for
a very short time, often one breath. The concentration of carbon
monoxide and other gases in exhaled air is then measured. The
difference in the amount of carbon monoxide inhaled and the amount
exhaled allows estimation of how rapidly gas can travel from the
lungs into the blood.