Concise review for clinicians
Pulmonary Function Tests for the Generalist: A Brief Review

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Abstract

One of the frequent reasons patients see their primary care physicians is for the symptom of dyspnea. Among the objective tests to quantify this symptom is the pulmonary function test, which includes several different studies: spirometry with flow-volume loop, lung volumes, and diffusing capacity of lung for carbon monoxide. The results may indicate both respiratory and nonrespiratory disorders, including helping in the diagnosis of cardiac or neuromuscular diseases. This review, intended for the generalist, describes common findings of pulmonary function tests and provides a road map for interpretation.

Section snippets

Spirometry

The most basic and useful PFT is spirometry. It includes measurement of exhaled or inhaled air during forced maneuvers. The forced vital capacity (FVC) is the amount of air that can be forcefully expelled, beginning with the lungs completely full (at total lung capacity [TLC]) and blowing maximally until as empty as possible (at residual volume [RV]). The forced expiratory volume in the first second (FEV1) is the amount expelled during the first second of the FVC maneuver. The ratio of FEV1 and

Lung Volumes

Although spirometry provides much useful information about lung function, it does not measure the total amount of air in the lungs at maximal inspiration, known as TLC. It also does not measure the amount of air left in the lungs after maximal expiration, known as RV, or the volume of the lungs at which the outward recoil of the resting chest wall is counterbalanced by the inward recoil of the lungs, known as functional residual capacity. Although many laboratories report functional residual

Diffusing Capacity of Lung for Carbon Monoxide

The third component of the PFT is Dlco, which is used as a measure of gas exchange, sometimes in conjunction with pulse oximetry at rest or during exercise. This is determined using the single-breath method, in which the patient exhales to RV and then inhales a vital capacity of air with small concentrations of carbon monoxide and helium or other inert gas. At maximal inspiration, the patient holds his or her breath for 10 seconds and then exhales quickly. Exhaled gas is sampled to determine

Special Cases

Typical obstructive and restrictive patterns are commonly identified by PFTs. However, there are frequent cases that do not fit the standard patterns of obstruction and restriction, such as the previously discussed “nonspecific pattern.” A second recently described process is called complex restriction. This occurs when TLC is low (ie, a restrictive process), and FVC is reduced disproportionately compared with TLC (>10% lower than TLC).20 A study of 200 such cases found that patients with

Basic Guide to Interpretation

Now that the basic mechanisms behind the PFT have been explained, one way to consider interpretation of these tests is provided in Figure 2 and the accompanying text.6

Conclusion

Pulmonary function tests are essential to the diagnosis of many lung conditions and help to identify numerous nonpulmonary disease processes. Understanding the basic interpretation of the components of this valuable test is crucial for primary care physicians to aid in the diagnosis of patients with respiratory symptoms.

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Potential Competing Interests: Dr Scanlon receives royalties for sales of Interpretation of Pulmonary Function Tests: A Practical Guide, published by Lippincott Williams & Wilkins.

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