By Michael V. Frey, RRT-NPS
Spirometry is an increasingly popular tool that can be used to detect, manage, and follow patients who have a variety of lung disorders. Through ongoing medical and technological advances, spirometry has become a highly reliable diagnostic method that is used not just by respiratory professionals, but all kinds of health care practitioners. What may not be widely understood, however, is that the interpretation of the spirometric results can be a challenge for at least two reasons. First, test results are largely determined by patient effort and cooperation during the testing process. Second, practitioners may not always have a clear interpretation or understanding of the results.1
Spirometry is used for both screening and clinical evaluation. Tests are performed in a variety of clinical sites ranging from small clinical offices to large testing facilities and acute care centers. Physicians and other health care professionals may conduct spirometry tests themselves or supervise others conducting the tests, or they may be involved only in interpreting test results.2
However, spirometry is often underutilized and misunderstood by many health care practitioners. Even the basics can be misunderstood. A prime example of this is using basic peak flow measurements before and after bronchodilator treatments. From shift to shift, the practitioners have to be on the same page in regard to the consistency of the maneuvers, whether or not they are done in the same time frame, or with the same efforts. The results can often be misleading or, worse, can cause the wrong diagnosis or the use of wrong treatment modalities.3
What does basic spirometry measure, and why is it important? Spirometry is a breathing maneuver in which a patient inhales as deeply and completely as possible, then exhales all the volume of air out of their lungs as forcefully and completely as possible, in one complete breath. The maneuver measures forced vital capacity (FVC). We can extrapolate certain bits of information from this maneuver, such as the FEV1 (forced expiratory volume in the first second) and the FEV1/FVC ratio. These measures can determine the presence of any airway abnormalities.
The simplicity of spirometry hides the subtleties of the physiologic mechanisms at work, especially the processes that result in airflow limitation. These factors are responsible for the consistency that spirometry can provide when done appropriately. This applies once the practitioner verifies a certain level of regularity in the patient’s efforts. These maneuvers can be very difficult to falsify, thus assuring proper results from the testing process once validity is determined.4 This makes spirometry an increasingly effective tool to help determine the patient’s condition with some feeling of certainty.
Now let’s look more closely at both FVC and FEV1, and their ratio.
Forced vital capacity is the measurement of the patient’s volume of air that they can exhale after a full inspiration with maximum speed and effort. A normal adult has a vital capacity of 3 to 5 liters of air volume. This represents the greatest breathing capacity in the patient. Various factors that affect this measurement include the configuration of the chest cage, physical fitness, posture, and gender. The FVC may be reduced by a decrease in the amount of functioning lung tissue, which results from a variety of ailments including atelectasis, edema, fibrosis, pulmonary resection, tumors, pneumonia, ascites, chest deformity, neuromuscular disease, and pneumothorax. Other airway obstructions also reduce FVC.
Forced expiratory volume in the first second represents the maximum amount of air that the patient can forcefully exhale during the first second of the effort. This value will then be converted to a percentage. The normal range for this value is 80% of predicted.
FEV1 60% to 79% of predicted = Mild obstruction
FEV1 40% to 59% of predicted = Moderate obstruction
FEV1 less than 40% of predicted = Severe obstruction
This value is a marker for the obstructive disease sets.
The FEV1/FVC ratio is also known as the Tiffeneau-Pinelli index. It is a measurement that can lead the practitioner to the diagnosis of an obstructive or restrictive lung disease process. This is a calculated number that represents the patient’s vital capacity and what they are able to expire in the first second. A normal value would be 80% or greater and is based on age, weight, height, ethnicity, and sex. In an obstructive disease state, this ratio will be reduced, because the trapping of air tends to reduce the amount of air the patient is able to blow out. There are differing values and ranges of diagnostic cutoff depending on the organizing body. Some accrediting bodies also require a bronchodilator to be given, then the test administered for correct values. The facility needs to decide which method to follow to determine which values they will use.
As I have mentioned, patient effort is the biggest variable throughout the testing process. In an important study, Krowka and colleagues demonstrated a positive relationship between peak expiratory flow (PEF) and effective effort in five normal subjects. They used the flow-volume curves as a noninvasive measure of expiratory effort with the correlation of effort obtained via esophageal balloon monitoring. They then measured the difference between the largest FEV1 and the FEV1 from the maneuver with the highest PEF during 10 test sessions in 10 normal subjects. The FEV1 difference was always greater than 0 mL, with a mean FEV1 difference of 110 mL for all sessions. This decreased to 80 mL when the maneuvers with poorly reproducible PEF or FVC values were discarded.5
The researchers concluded that the FEV1 is inversely dependent on the patient’s effort and that maximal effort decreases FEV1 because of the effect of thoracic gas compression on lung volume during standard spirometry. The flow-volume curves of superimposed efforts may show the practitioner that the patient had less than maximal efforts. This means the practitioner must be cognizant of these factors and work with and alleviate patient anxiety and weakness to come up with reproducible results.”5
Spirometry can help with the diagnosis of numerous disease processes including chronic bronchitis, COPD, pulmonary fibrosis, and asthma. The medical practitioner may use the test results in conjunction with other factors to determine the patient’s condition, for example, what treatment regimen to start and also to follow up with the patient to see what adjustments of the regimen are indicated. Spirometry also is used preoperatively to determine whether the patient may tolerate surgery. For example, lung cancer patients may benefit from this test prior to surgery.
Spirometry has become so widely and commonly used that it is in danger of being taken for granted. To ensure a balanced approach to patients’ needs, a basic understanding of spirometry is necessary for all health care professionals who use the technique.
Michael V. Frey, RRT-NPS, is clinical consultant, alternate care, CareFusion, Yorba Linda, Calif. For further information, contact [email protected].
1. Spirometry 360. Retrieved from www.spirometrytraining.org.
2. Barreiro T, Perillo I. An approach to interpreting spirometry. Am Fam Physician. 2004;69(5):1107-1115.
3. Townsend MC; Occupational and Environmental Lung Disorders Committee. Spirometry in the occupational health setting—2011 update. J Occup Environ Med. 2011;53(5):569–584.
4. Hayes D, Kraman S. The physiologic basis of spirometry. Respir Care. 2009;54(12):1717–1726.
5. Krowka MJ, Enright PL, Rodarte JR, Hyatt RE. Effect of effort on measurement of forced expiratory volume in one second. Am Rev Respir Dis. 1987;136(4):829-833.