When comparing the differences between obstructive and restrictive lung diseases, spirometry testing plays an important diagnostic role. Since each disease is characterized by a unique, physiologic pattern, careful interpretation of spirometry results helps doctors differentiate between the two conditions. Once the pattern is recognized, the right diagnosis should follow, but only after taking into consideration a patient's history, physical examination and additional diagnostic studies.
The following guide takes you on a journey through each disorder, then provides a wealth of information about what's really important when rendering and interpreting a spirometry test:
What Characterizes an Obstructive Lung Disease?
When a person has difficulty expelling all the air from her lungs, she's said to have an obstructive lung disease. An obstructive pattern exists when air moves out of the lungs at a slower rate than that of a healthy person. This occurs because inflammation and swelling (secondary to long-term, cumulative exposure to airway irritants) cause the airways to become narrow and blocked, making it difficult to completely rid the lungs of air. This leaves an abnormally high volume of air in the lungs after a full exhalation, which is referred to as "increased residual volume." In obstructive lung defects, increased residual volume leads to air getting trapped in, and hyperinflation of, the lungs — two changes in the lungs that may contribute to worsening symptoms.
The following lung diseases are categorized as obstructive:
- COPD (mainly a combination of emphysema and chronic bronchitis)
- Chronic bronchitis
What Characterizes a Restrictive Lung Disease?
Restrictive lung diseases are characterized by reduced total lung capacity (TLC). TLC represents the amount of air present in the lungs after taking the deepest breath possible. When a restrictive pattern is present, the lungs are "restricted" from fully expanding, making it difficult for a person to take a full breath. Measuring TLC is of critical importance to the diagnosis of restrictive lung disease because it confirms the presence of a true restriction, as well as quantifying the degree of that restriction. Restrictive lung diseases are categorized as intrinsic, extrinsic or neurological.
Intrinsic Restrictive Lung Disorders
Intrinsic, as it applies to lung disorders, refers to that which is inherent to the lungs themselves. Take a look at some common, intrinsic restrictive lung disorders:
Extrinsic Restrictive Lung Diseases
Extrinsic, as it applies to lung disease, refers to that which originates from outside the anatomical boundaries of the lungs. The following lists several examples of extrinsic restrictive lung disorders:
Neurological Restrictive Lung Diseases
Restrictive lung diseases can also stem from neurological causes, including:
- Paralysis of the diaphragm
- Myasthenia gravis
- Muscular dystrophy
- Amyotrophic Lateral Sclerosis (ALS or Lou Gehrig's Disease)
Test Values Important to Spirometry Interpretation
Spirometry measures the rate of lung volume changes during forced breathing maneuvers. There are several values measured during spirometry that are critically important when diagnosing and distinguishing between obstructive and restrictive lung diseases. Following are the most critical:
- Forced vital capacity (FVC) — The FVC maneuver begins with the patient taking as deep a breath as possible, and then exhaling as forcibly, and for as long, as possible. Because lung capacity is reduced in both obstructive and restrictive diseases, the FVC measure alone does not diagnose either disorder.
- Forced Expiratory Volume in One Second (FEV1) — This value represents the total amount of air that can be forcibly exhaled in the first second of the FVC maneuver. Healthy individuals generally expel about 75% to 85% of their FVC in the first second of the test. The FEV1 is decreased in obstructive lung diseases and normal or minimally decreased in restrictive lung diseases.
- Ratio of FEV1 to FVC — The ratio of FEV1 to FVC is referred to as FEV1/FVC, %FEV1 or FEV1%. It is indicative of the percentage of the total FVC expelled from the lungs during the first second of a forced exhalation. This ratio is decreased in obstructive lung disorders and normal or increased in restrictive lung disorders.
- Total lung capacity and residual volume — Lung volumes are critical to the diagnosis of restrictive lung disease. When a patient exhales as forcibly and for as long as he can, there will still be some air remaining in the lungs. The remaining amount represents the residual volume. When you add the residual volume plus the forced vital capacity together, their sum equals the total lung capacity (TLC). TLC is normal or increased in obstructive defects, and decreased in restrictive defects. TLC and residual volume are not measured by spirometry. Rather, their values are obtained through additional testing, including a test called body plethysmography.
When patients undergo spirometry testing, especially if the results were not what they expected, they often ask if there was something they did — or didn't do — to influence their test results. Indeed, there are factors associated with inaccurate spirometry results, the most common of which are listed in the following:
5-Step Approach to Spirometry Interpretation
Although your doctor is the best person to talk to about your spirometry results and the only one who can diagnose you with either an obstructive or a restrictive lung disease, there are several methods available that point clinicians in the right direction. The following is just one of many approaches (you can also reference the table at the bottom of this page):
- Step 1: Begin by looking at the forced vital capacity (FVC) to determine if it's within a normal range.
- Step 2: Next, look at the forced expiratory volume in one second (FEV1) to see if it's within normal limits.
- Step 3: If the FVC and the FEV1 are both normal, stop at this step — the spirometry test is normal.
- Step 4: If the FVC and/or the FEV1 are decreased, there is a strong possibility of lung disease.
- Step 5: If Step 4 suggests the presence of lung disease, look closely at the %predicted for FEV1/FVC. If %predicted for FEV1/FVC is 69% or less (less than 0.70 according to the Global Initiative for Obstructive Lung Disease), an obstructive lung disease is highly likely. A value of 85% or greater is suggestive of restrictive lung disease.
Meeting the Bronchodilator Challenge
Spirometry test results almost always include measurements obtained from both before and after administration of a bronchodilator. This is called a bronchodilator challenge. A bronchodilator challenge tells doctors the degree of broncho-constriction present and how well (or how poorly) a patient responds to a bronchodilator.
What constitutes as improvement varies from clinic to clinic, but the American Thoracic Society (ATS) defines it, in both COPD and asthma, as having an increase in post-bronchodilator FEV1 of at least 12% from baseline and of at least 200 milliliters. A significant bronchodilator response is favorable to both obstructive and restrictive lung diseases, indicating a positive response to treatment and in some cases, a better prognosis.
How is Disease Severity Determined?
There are several approaches to determining disease severity in both obstructive and restrictive lung diseases. Whichever method is used, FEV1 as a percentage of the predicted FEV1, quantifies the severity of obstruction in obstructive lung diseases. Similarly, forced vital capacity (FVC) or total lung capacity (TLC), as a percentage of the predicted FVC or TLC, quantifies the degree of restriction present in restrictive lung diseases.
NOTE: Remember, a variety of systems are used to interpret pulmonary function tests and determine the severity of disease. The information included in this article is just one approach. The article should only serve as a guide to help you better understand your test results. It is not meant to diagnose lung disease, nor should it replace sound medical advice from a health care professional. Please review your test results in detail with your primary care provider or your pulmonologist for further information.
Obstructive and Restrictive Lung Patterns Chart
|Measurement||Obstructive Pattern||Restrictive Pattern|
|Forced Vital Capacity||Decreased or normal||Decreased|
|Forced Expiratory Volume in One Second||Decreased||Decreased or normal|
|FEV1/FVC Ratio||Decreased||Normal or increased|
|Total Lung Capacity||Normal or increased||Decreased|
Al-Ashkar, F. MD., et. al. Interpreting pulmonary function tests: Recognize the pattern, and the diagnosis will follow. Cleveland Clinic Journal of Medicine. Volume 70 Number 10. October, 2003.
Barreiro, Timothy J., D.O., Perillow, Irene, M.D. An Approach to Interpreting Spirometry. American Academy of Family Physicians. Volume 69, Number 5/March 1, 2004.
Gildea, Thomas R., McCarthy, Kevin. Pulmonary Function Tests The Cleveland Clinic Center for Continuing Education. Updated 2011.
Northern Arizona University. Pulmonary Function Testing.
Spirometry for Healthcare Providers. Global Initiative for Obstructed Lung Disease. Updated 2011.
The Global Initiative for Obstructive Lung Disease. Global Strategy for the Diagnosis, Management and Prevention of Chronic Obstructive Pulmonary Disease. Updated 2011.