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Pulmonary Function Tests
Definition and Overview
Disease
Patterns of Pulmonary Function Impairment
Interpretation of Results
Fire Fighters and Spirometry
Definition and Overview
Spirometry Versus Full Pulmonary Function Tests
Pulmonary function tests are a method of assessing the function of the
lungs. They are a way of detecting and quantifying abnormal lung function in
a noninvasive manner. They are one of the most common diagnostic tests used
for measuring lung function. The simplest form of pulmonary function testing
is called spirometry which measures how quickly air can be expelled from the
lungs. Spirometry is performed by having an individual blow into a device
called a spirometer. This machine makes tracings of the rate at which air
leaves the lungs. That is, it measures the volume of air that is inhaled or
exhaled as a function of time. Spirometry is most useful for measuring
diseases that cause obstruction to airflow. Spirometry is unable to
measure absolute lung volumes. That is, it does not measure the amount
of air in the lungs, just the amount of air entering or leaving the lungs.
A more extensive measurement of lung function can be done with full
pulmonary function tests which measure lung volumes (also called
static lung volumes) and diffusion capacities (discussed below), as well as
flow rates. Static lung volumes can be measured by two methods: (1) by
plethysmography and (2) by a gas dilution technique. In
plethysmography, a subject sits inside an air tight box. The subject
breathes in and out through a breathing tube. When the lungs reach a
certain volume (called the functional residual capacity) a shutter closes
off the breathing tube. The subject attempts to breathe in against the
closed shutter. This causes the chest volume to expand. The increase
in chest volume causes a decrease in the box volume and a corresponding
increase pressure in the box. Using these data and Boyles law (P1V1=P2V2),
air volumes within the lung can be calculated. As noted, the second
method for determining lung volumes is called the gas dilutional technique.
In this method, the subject is connected to a spirometer with a known volume
of inert gas such as helium. The subject breathes in the helium which,
as the picture below illustrates, equilibrates in the lungs. Lung
volumes can be calculated using the law of conservation of matter.
Characteristics of Full Pulmonary Function Tests
There are three categories of information that can be obtained from full
pulmonary function tests. They are lung volumes, flow rates and diffusion
capacities. Lung volumes provide information on the size of the different
compartments of the lungs. Flow rates provide information on the rates of
airflow within the airways. Diffusion capacity provides information on the
ease with which gas flows from the lungs (alveolus) to the capillaries. Each
category will be discussed below.
Lung volumes
The lung can be divided into four
different compartments which are illustrated below . These are:
(1) Total lung capacity (TLC) which is the amount of air in the lungs after
a deep breath in.
(2) The residual volume (RV), which is the amount of air left in the lungs
after maximal expiration (a deep breath out).
(3)The vital capacity (VC) which is the amount of gas expired when going
from total lung capacity to residual volume.
(4) The functional residual capacity (FRC) which is the amount of gas within
the lungs at end expiration.
Lung volumes provide information regarding
diseases which affect the size of the lungs. Specifically, there are a group
of diseases called interstitial lung diseases which result in stiff lungs
that are characterized by smaller lung volumes on pulmonary function tests.
Examples of interstitial lung diseases include pulmonary fibrosis, silicosis
and asbestosis.
Flow rates or Forced Expiration
Flow rates involve assessing the rate of flow
during maximal forced expiration. An individual is asked to blow out as fast
and as hard as possible from maximal inspiration (TLC) to maximal expiration
(RV). The volume expelled is called the forced vital capacity (FVC), and the
volume expelled in the first second is the forced expiratory volume in one
second (FEV1). These flow rates are useful indicators of diseases
that cause obstruction to airflow. Specifically, the FEV1/FVC
ratio is a useful index of obstruction. A decreased ratio might
indicate the presence of obstructive disease. A second flow rate that is
often measured is called the maximum mid-expiratory flow rate (MMFR). This
is the maximum flow rate between 25 and 75 percent of the volume expired. It
is also called the forced expiratory flow (FEF25-75). This result
reflects early airway obstruction and is a more sensitive indicator of mild
airway obstruction than the FEV1/FVC ratio. Examples of diseases
that cause obstruction are asthma, chronic bronchitis, and emphysema.
Diffusion capacity or DLCO
Diffusion capacity measures the rate of transfer
of gas from the lungs (alveoli) to the blood vessel (capillary). Diffusion
capacity is often decreased in emphysema, interstitial lung diseases and
pulmonary vascular disease. In disorders that only affect the airways such
as asthma or chronic bronchitis, diffusion capacity is often not reduced.
Accuracy and validity
Some of the factors that make pulmonary function testing less accurate
are unreliable equipment, lack of cooperation from the individual being
tested, and poor testing methods. The American Thoracic Society contains
criteria for the performance of spirometry. The National Institute of
Occupational Safety and Health (NIOSH) provides information regarding
certification of technicians. Ideally, there should be less than 5%
variability between the spirometry trials within one testing session for a
test to be considered valid.
Disease Patterns
of Pulmonary Function Impairment
Patterns of Impairment
Abnormalities in pulmonary function can be categorized into three basic
patterns: obstructive ventilatory defect, restrictive ventilatory defect or
a mixed pattern which is a combination of the two.
Obstructive Ventilatory Defect
A purely obstructive ventilatory defect consists
of a decrease in flow rates. Specifically, a decrease in the FEV1/FVC
ratio, the FEF25-75, FEV1 and FVC is observed.
Asthma, emphysema and chronic bronchitis are all examples of obstructive
disease.
Restrictive Ventilatory Defect
Restrictive disease is characterized by reduced
lung volumes. Though spirometry is most useful for the measurement of
obstructive disease, restrictive disease also can be suggested by
spirometry. Specifically, a reduced FEV1
and FVC but a preserved FEV1/FVC ratio may indicate the
presence of restrictive disease. Because a reduced FEV1 and FVC
can also indicate severe obstruction, full pulmonary function tests should
be obtained to clarify this abnormality. If full pulmonary function tests
are obtained, then restrictive disease would be characterized by a decrease
in lung volumes. Restrictive pulmonary disorders can be caused by a
pulmonary disease such as interstitial pulmonary fibrosis or a non-pulmonary
disease such as rigidity of the chest wall, paralysis or muscle weakness.
Mixed pattern
A combination of restrictive and
obstructive disease (mixed pattern) is suggested by a reduced FEV1,
a reduced FVC and a reduced FEV1/FVC ratio.
Other Diagnostic Tests
Other tests that are commonly performed are those which assist in the
diagnosis of asthma. These tests are called the methacholine challenge
test and the pre- and post-bronchodilator test. In order to understand these
tests, it is helpful to briefly review the pathophysiology of asthma.
Asthma is a disease characterized by a hyperresponsive airway. The
smooth muscles in the airway (bronchi) contract causing a decrease in
airflow. In addition, the airway can become inflamed, further
constricting the airway. The characteristics of asthma on spirometry
are a decrease in the FEV1, a decrease in the FVC and a decrease
in the FEV1/FVC ratio.
One important feature of asthma is that it is reversible. This
reversibility can often be demonstrated with the use of medications.
If an individual has an abnormal (obstructive pattern) on spirometry,
reversibility may be demonstrated by administering a medication that causes
the bronchi to dilate (bronchodilator). This relieves the airflow
obstruction and can be demonstrated on repeat spirometry. This is a useful
approach if an individual has symptoms at the time of testing. Because
asthma is reversible, sometimes individuals do not have symptoms at the time
of testing at which time an alternative approach must be utilized. If
no symptoms are present, but there is a need to test for asthma, a
medication can be administered to provoke asthma symptoms. One
medication commonly used is called methacholine. Methacholine is
a nonspecific stimulus of bronchoconstriction. A measure of airway
obstruction such as FEV1 is obtained after administering
methacholine repeatedly. Individuals with asthma are usually very sensitive
to methacholine and drop their FEV1 at relatively low doses of
methacholine.
Interpretation of
Results
Definition of “Normal”
Normal results are defined by comparing individual test results to
predicted values from reference populations. These individual results are
expressed as a percentage of the predicted value as defined by the reference
population, adjusting for age, height and gender. Spirometry results in the
healthy adult population are normally distributed and the 95% confidence
intervals range from 80% to 120%. The cutoff for abnormal results are in the
lower 5%, which is equivalent to an FEV1and FVC of less than 80%.
Pitfalls of a Cutoff
An 80% cutoff should not be used in isolation as a pass/fail criteria to
automatically disqualify an individual from his or her job. Spirometry is a
good diagnostic tool when used in the context of other diagnostic tests and
patient information. It is most useful when used to compare an individual’s
test results over time. It is potentially dangerous, however, to interpret a
test in isolation with the assumption that 79% represents pulmonary disease
and 81% indicates that an individual is free of disease.
Longitudinal Approach
Rather than focusing on one specific number as a cutoff between normal
and abnormal, a more effective and useful method of utilizing spirometry
results is to compare individual test results over time. Regardless of where
an individual starts out, a decline in the FEV1 or FVC of 10% or
greater should warrant further attention. For example, if an individual
starts out at 120% of predicted, then he or she would need to lose 40% of
his or her lung function before being considered abnormal. There is a much
greater opportunity for effective medical intervention prior to the
progression of severe pulmonary disease if individuals are evaluated when a
10% loss of function has occurred.
Fire fighters and
Spirometry
Risk to Fire Fighters
Fire fighters may be exposed to smoke and other toxicants on a regular
basis. Smoke contains particulates and gases that are irritating to the
lungs and upper respiratory tract. These irritants are the products of
combustion from both synthetic as well as natural products. Monitoring data
have indicated that fire fighters can be exposed to a whole host of
respiratory toxicants including hydrogen chloride, phosgene, sulfur dioxide,
aldehydes and particulates. Exposure to smoke and other chemicals may
produce both acute and chronic pulmonary effects. Spirometry is a useful
screening test to determine the presence of pulmonary disease as early as
possible.
OSHA and Spirometry
Because fire fighters wear respirators on a regular basis, employers must
comply with the respiratory protection regulations under OSHA. Under this
mandate, employers must develop a written respiratory protection program to
protect the fire fighters who wear respirators. Among other things, this
program requires that a physician or licensed health care professional
perform medical evaluations for respirator use. This medical evaluation must
include, at a minimum, the completion of a mandatory questionnaire or
equivalent information. Follow-up medical examinations must be provided to
all employees whose questionnaires or initial medical evaluation indicates
such a need. Follow-up evaluations should include any medical testing deemed
necessary by the medical professional. OSHA does not specifically require
spirometry testing.
IAFF and Spirometry
The International Association of Fire Fighters believes that the unique
nature of a fire fighter's work environment necessitates a thorough physical
examination and spirometry. The components addressed in the IAFF/IAFC Fire
Service Joint Labor Management Wellness/Fitness Initiative should be
implemented for the medical and fitness evaluation of fire fighters. This
initiative's requirements meet the provisions of the OSHA standard.
The Wellness/Fitness initiative recommends spirometry testing because it
may reflect early changes in the lungs, prior to the onset of symptoms. This
would allow for earlier intervention and treatment of a potential medical
problem. The wellness/fitness initiative does not recommend the use of
spirometry to automatically exclude a fire fighter from work but rather, to
monitor changes over time and to treat abnormalities before they become
clinically significant.
All graphics were provided by Johns Hopkins Medical Institution web site.
References:
Bascom R, Ford E. Don't Just "Do Spirometry"-Closing the Loop in
Workplace Spirometry Programs. Occup Med State Art Review 7:347-363,
1992.
Rosenstock, L. Introduction: The Chest Radiograph and Pulmonary Function
Testing. In: Rosenstock L, Cullen R, eds. Textbook of Clinical Occupational
and Environmental Medicine. Philadelphia:194-197.
Lees PSJ. Combustion Products and Other Firefighter Exposures. Occup Med
State Art Review 10:691-706, 1995.
Weinberger SE, Principles of Pulmonary Medicine, 2nd. ed. Philadelphia,
W.B. Saunders Co., 1992.
Johns Hopkins Medical Institution web site..
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