Spirometry is the measurement of airflow into and out of the lungs. The patient is given instructions on how to perform the breathing maneuvers. To perform the procedure the nose is pinched off, and the patient breathes as instructed through a mouthpiece attached to the spirometer. The three breathing maneuvers are practiced before recording the procedure, and the highest of three trials is used for evaluation of breathing. The instrument measures air flow by electronic or mechanical displacement principles and uses a microprocessor and recorder to calculate and plot air flow.
The test produces a recording of the patient's ventilation under conditions involving both normal and maximal effort. The recording, called a spirogram, shows the volume of air moved and the rate at which it is moved into and out of the lungs. There are several lung capacities that are measured by spirometry. Accurate measurement of these are dependent upon the patient performing the appropriate maneuver properly. The most common are described below:
- Vital capacity (VC): This is the amount of air in liters that is moved out of the lung during normal breathing. The patient is instructed to breathe in and out normally to full expiration for this maneuver. Vital capacity is normally about 80% of the total lung capacity. Because of the elastic nature of the lungs and surrounding thorax, a small volume of air will remain in the lungs after full exhalation. This volume is called the residual volume (RV).
- Forced vital capacity (FVC): After breathing out normally to full expiration the patient is instructed to breath in with a maximal effort and then exhale as forcefully and rapidly as possible. The FVC is the volume of air that is expelled into the spirometer following a maximum inhalation effort.
- Forced expiratory volume (FEV): At the start of the FVC maneuver, the spirometer measures volume of air that is delivered through the mouthpiece at timed intervals of 0.5, 1.0, 2.0, and 3.0 seconds. The sum of these measurements normally constitutes about 97% of the FVC measurement. The most commonly used FEV measurement is FEV-1, which is the volume of air exhaled into the mouthpiece in one second. The FEV-1 should be at least 70% of the FVC.
- Forced expiratory flow 25-75% (FEF 25-75): This is a calculation of the average flow rate over the center portion of the forced expiratory volume recording. It is determined from the time in seconds at which 25% and 75% of the vital capacity is reached. The volume of air exhaled in liters per second between these two times is the FEF 25-75. This value reflects the status of the medium and small sized airways.
- Maximal voluntary ventilation (MVV): A maneuver in which the patient breathes as deeply and as fast as possible for 15 seconds. The average airflow (liters per second) indicates the strength and endurance of the respiratory muscles.
Normal values for FVC, FEV, FEF, and MVV are dependent on the patient's age, gender and size (height).
Spirometry is the most commonly performed pulmonary function test (PFT). The test can be performed at the bedside, in a physician's office, or pulmonary laboratory. It is often the first test performed when a problem with lung function is suspected. Spirometry may also be suggested by an abnormal x-rays, arterial blood gas analysis, or other diagnostic pulmonary test result. In March 2000, the National Lung Health Education Program recommended that regular spirometry tests be performed on persons over 45 years old who have a history of smoking. Spirometry tests are also recommended for persons having a family history of lung disease, chronic respiratory ailments, and persons of advanced age. Spirometry measures ventilation, the movement of air into and out of the lungs. The spirogram will identify two different types of abnormal ventilation patterns, obstructive and restrictive. Common causes of an obstructive pattern are cystic fibrosis, asthma, broniectasis, bronchitis, and emphysema. These conditions may be collectively referred to using the acronym CABBE. Chronic bronchitis, emphysema, and asthma result in dyspnea and ventilation deficiency, a condition known as chronic obstructive pulmonary disease (COPD). As of 2001, COPD is the fourth leading cause of death among Americans. Common causes of a restrictive pattern are pneumonia, heart disease, pregnancy, lung fibrosis, pnemothorax (collapsed lung), and pleural effusion (compression caused by chest fluid).
Obstructive and restrictive patterns can be identified on spirographs. Volume (liters) is plotted on the y-axis versus time (seconds) on the x-axis. A restrictive pattern is characterized by a normal shape showing reduced volumes for all parameters. The reduction in volumes indicates the severity of the disease. An obstructive pattern produces a spirogram with an abnormal shape. Inspiration volume is reduced. The volume of air expelled is normal, but the air flowrate is slower causing an elongated tail to the FVC.
A flow-volume loop spirogram is another way of displaying spirometry measurements. This requires a FVC maneuver followed by a forced inspiratory volume (FIV). Flow rate in liters per second is plotted on the y-axis and volume (liters) is plotted on the x-axis. The expiration phase is shown on top and the inspiration phase on the bottom. The flow-volume loop spirogram is helpful in diagnosing upper airway obstruction and can differentiate some types of restrictive patterns.
Some conditions produce specific signs on the spirogram. Irregular inspirations with rapid frequency are caused by hyperventilation associated with stress. Diffuse fibrosis of the lung causes rapid breathing of reduced volume that produces a repetitive pattern known as the penmanship sign. Serial reduction in the FVC peaks indicates trapped air inside the lung. A notch and reduced volume in the early segments of the FVC is consistent with airway collapse. A rise at the end of the expiration is associated with airway resistance.
Spirometry is used to assess lung function over time and is often used to evaluate the efficacy of bronchodilator inhalers such as albuterol. It is important that the patient not use a bronchodilator prior to the evaluation. Spirometry is performed before and after inhaling the bronchodilator. In general, a 12% or greater improvement in both FVC and FEV-1 and/or an increase in FVC by 0.2 liters is considered a significant improvement in an adult patient.
The physician ordering the test should be aware of any medications and medical conditions which may affect the validity of the test. The patient's smoking habits and history should be documented thoroughly. The subject must be able to understand and respond to instructions for the breathing maneuvers. Therefore, the test may not be appropriate for very young, unresponsive, or physically impaired persons. Spirometry is contraindicated in patients whose condition will be aggrevated by forced breathing. Hemoptysis, pneumothorax, recent heart attack, unstable angina, aneurysm (cranial, thoracic, or abdominal), thrombotic condition, recent thoracic or abdominal surgery, nausea or vomiting are conditions that may contraindicate spirometry. The test should be terminated, if the patient shows signs of significant head, chest, or abdominal pain while the test is in progress.
Spirometry is dependent upon the patient's full compliance with breathing instructions especially his or her willingness to extend a maximal effort at forceful breathing. Therefore, the patient's emotional state needs to be considered when performing the procedure.
Bronchodilator—A drug, usually self-administered by inhalation, that dilates the airways.
Forced expiratory volume (FEV)—The volume of air exhaled from the beginning of expiration to a set time (usually 0.5, 1, 2, and 3 seconds).
Forced vital capacity (FVC)—The volume of air that can be exhaled forceably after a maximal inspiration.
Vital capacity (VC)—The volume of air that can be exhaled following a full inspiration.
The patient's age, sex, and race are recorded, and height and weight are measured before starting the procedure. The patient should not have eaten heavily within three hours of the test. He or she should be instructed to wear clothing that is loose fitting over the chest and abdominal area. The respiratory therapist or other testing personnel should explain and demonstrate the breathing maneuvers to the patient. The patient should practice breathing into the mouthpiece until he or she is able to duplicate the maneuvers successfully on two consecutive tries.
No special care is usually required following spirometry. The occassional patient may become light-headed or dizzy. Such patients should be asked to rest or lie down, and they should not be discharged until after the symptoms subside. In rare cases, the patient may experience pneumothorax, intracranial hypertension, vertigo, chest pain, or uncontrolled coughing. In such cases, additional care directed by a physician may be required.
The results of spirometry tests are compared to predicted values based on the patient's age, gender, and height. For example, a young adult in good health is expected to have the following FEV values:
- FEV-0.5 50-60% of FVC
- FEV-1 75-85% of FVC
- FEV-2 95% of FVC
- FEV-3 97% of FVC
In general, any value falling between 80% and 100% of the predicted value is considered normal. Values between 60% and 79% indicate mild lung dysfunction. Values between 40% and 59% indicate moderate lung dysfunction, and values below 40% indicate severe dysfunction.
Health care team roles
Spirometry tests are ordered by a physician, and results are evaluated by a pulmonologist, a physician with special training in pulmonary function. Spirometry testing is performed most often by a registered respiratory therapist (RRT), certified respiratory technician (CRTT), certified pulmonary function technologist (CPFT), or registered pulmonary function technologist (RPFT).
White, G. Basic Clinical Lab Competencies for Respiratory Care, 3rd ed. New York: Delmar Publishers, 1998.
Blonshine, S. and J.B. Fink. "Spirometry: Asthma and COPD guidelines creating opportunities for RTs." AARC Times (January 2000): 43-47.
National Lung Health Education Program (NLHEP). 1850 High Street, Denver, CO 80218. <http://www.nlhep.org>.
Gary, T., et al. "Office Spirometry for Lung Health Assessment in Adults: A Consensus Statement for the National Lung Health Education Program." (March 2000): 1146-1161.
"Spirometry - AARC clinical practice guide." American Association for Respiratory Care. 1130 Ables Lane, Dallas, TX 75229. <http://www.muhealth.org/~shrp/rtwww/rcweb/aarc/spirocpg.html>.
Robert Harr Paul Johnson