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.
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.
All graphics were provided by Johns Hopkins Medical Institution web site.
