Complete Pulmonary Function Tests
Clement L. Ren, MD, MS
Introduction
•Assessment of respiratory function is an important component of the care of children with pediatric respiratory disease.
•There are multiple options available for pulmonary function tests (PFTs), but in most cases, primary care physicians will use spirometry, with or without a bronchodilator.
•However, it is still useful for any clinician involved in the care of children with respiratory disease to be familiar with other PFTs.
Lung Volumes
•The space in the lungs can be divided into several different volumes and capacities (capacities are the sums of 2 or more volumes). Figure 5-1 shows the subdivision of lung volumes and capacities.
•Some of the important subdivisions include the following.
—Tidal volume is the volume inspired and expired with each normal breath.
—Vital capacity is the maximal volume that can be expired after starting at full inspiration. If this is done during forced expiration, then it is called the forced vital capacity (FVC).
—Residual volume is the amount of gas remaining in the lungs at the end of a vital capacity maneuver. Because of collapse of the small airways during expiration and the compliance of the chest wall, there will always be some residual volume remaining in the lungs, even after a maximal forced expiratory maneuver.
—Total lung capacity (TLC) is the sum of vital capacity and residual volume, representing the maximal volume of gas that can be held in the lungs at end-inspiration.
—Functional residual capacity (FRC) is the volume of gas in the lungs at the end of relaxed expiration.
Figure 5-1. The subdivisions of lung volume. From Loughlin GM, Eigen H, eds. Respiratory Disease in Children: Diagnosis and Management. Baltimore, MD: Williams & Wilkins; 1994:79.
Spirometry
•Spirometry, the most commonly used PFT, involves measurement of flows and volumes during a maximal forced expiratory maneuver.
•Although spirometry is a simple test, quality data acquisition is still critical to ensure that the test results can be appropriately interpreted.
Spirometric Measurements
•The following are the most commonly used measurements obtained through spirometry. There are other values that can be reported, but they add little to clinical management.
—FVC is the forced vital capacity.
—FEV1 is the volume forcefully exhaled in the first second.
—FEV1/FVC ratio is the emptying constant of the respiratory system.
—FEF25%-75% is the forced expiratory flow between 25% and 75% of vital capacity. Because FEF25%-75% is dependent on FVC, it is much more variable than FEV1 and FVC. FEF25%-75% was previously thought to reflect small-airway flow, but in reality, it is dependent on many other factors; it should not be considered a direct measure of small-airway function. Additionally, it may not provide any additional information beyond that obtained with FEV1 and FVC.
Interpretation of Spirometry Results
•Traditionally, lung diseases have been divided into 2 categories: obstructive and restrictive.
•Obstructive diseases are more commonly encountered and are characterized by reduced flow rates. Examples include asthma and cystic fibrosis.
•Restrictive diseases are characterized by reduced lung volumes, because of either musculoskeletal factors or decreased lung compliance. Examples of restrictive disease include kyphoscoliosis and interstitial lung disease.
•Table 5-1 summarizes the usual PFT findings in obstructive versus restrictive disease.
•In children, FEV1 is frequently normal in obstructive diseases early on in the disease process, and FEV1/FVC ratio is a more sensitive measure of obstruction.
•FEV1/FVC ratio varies with age, and it can be as high as 0.85 in young children and as low as 0.65 in elderly patients.
—Therefore, interpreting the FEV1/FVC ratio requires identifying the lower limit of normal based on the patient’s age, sex, race, and height.
—A ratio less than the 5th percentile (the lower limit of normal) or >2 standard deviations (SDs) below the mean for age signifies obstruction.
FVC is usually normal in obstruction—especially in mild obstruction—but with severe obstruction, it may be reduced. FEV1, forced expiratory volume in 1 second; FVC, forced vital capacity.
Reference Equations
•Just as with growth curves, the values obtained with spirometry and other
PFTs must be normalized for the patient’s age, sex, race, and height.
•The selection of reference equations can have a substantial effect on the interpretation of PFT results.
—Older reference equations included relatively small numbers of nonwhite subjects and tended to cause overestimation of values in younger, shorter children.
—Some equations did not include data on subjects younger than 8 years, and extrapolation below that age resulted in substantial error.
—Other equations contained discontinuities in the adolescent age range.
•More recently, the Global Lung Function Initiative has developed reference equations that span the age range of 3 to 95 years and include both male and female subjects and multiple racial groups.
•According to historical convention, spirometry results are often reported as the percentage predicted of the mean—that is, an FEV1 of 90% predicted means that the value is below the mean of patients matched for height, race, and sex.
•While percentage predicted provides a useful comparison to the normal mean, the threshold for the lower limit of normal will depend on the variability of the spirometric measurement in question.
—For example, an FEV1 of 80% predicted is approximately 2 SDs below the mean, and values below that are considered abnormal.
—In contrast, measurements such as FEF25%-75% are much more variable, and values of 70% or even 65% predicted may still fall within 2 SDs of the mean.
—It is therefore important to be familiar with the variability of the measurement when interpreting percentage predicted spirometric values.
•An alternative to using percentage predicted is to report results as the number of SDs (z scores) from the mean predicted value.
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