Interpretation of Pulmonary Function Tests A Practical Guide, 3. ed

List of Abbreviations

1. Introduction

Pulmonary function tests can provide important clinical information, yet they are vastly underused. They are designed to identify and quantify defects and abnormalities in the function of the respiratory system and answer questions such as the following: How badly impaired is the patient's lung function? Is airway obstruction present? How severe is it? Does it respond to bronchodilators? Is gas exchange impaired? Is diffusion of oxygen from alveoli to pulmonary capillary blood impaired? Is treatment helping the patient? How great is the surgical risk?

Pulmonary function tests can also answer other clinical questions: Is the patient's dyspnea due to cardiac or pulmonary dysfunction? Does the patient with chronic cough have occult asthma? Is obesity impairing the patient's pulmonary function? Is the patient's dyspnea due to weakness of the respiratory muscles?

The tests alone, however, cannot be expected to lead to a clinical diagnosis of, for example, pulmonary fibrosis or emphysema. Test results must be evaluated in light of the history; physical examination; chest radiograph; computed tomogram scan, if available; and pertinent laboratory findings. Nevertheless, some test patterns strongly suggest the presence of certain conditions, such as pulmonary fibrosis. Also, the flow-volume loop associated with lesions of the trachea and upper airway is often so characteristic as to be nearly diagnostic of the presence of such a lesion (see Chapter 2).

As with any procedure, pulmonary function tests have shortcomings. There is some variability in the normal predicted values of various tests. In some studies, this variability is in part due to mixing asymptomatic smokers with nonsmokers in a ''normal'' population. Some variability also occurs among laboratories in the ways the tests are performed, the equipment is used, and the results are calculated.

This text assumes that the tests are performed accurately, and it focuses on their clinical significance. This approach is not to downplay the importance of the technician in obtaining accurate data. Procedures such as electrocardiography require relatively little technician training, especially with the new equipment that can detect errors such as faulty lead placement. And, of course, all the patient need do is lie still. In marked contrast is the considerable training required before a pulmonary function technician becomes proficient. With spirometry, for example, the patient must be exhorted to put forth maximal effort, and the technician must learn to detect submaximal effort. The patient is a very active participant in several of the tests that are discussed. Many of these tests have been likened to an athletic event—an apt analogy. In our experience, it takes several weeks of intense training before a technician becomes expert in administering common tests such as spirometry. If at all possible, the person interpreting the test results should undergo pulmonary function testing. Experiencing the tests is the best way to appreciate the challenges faced when administering the test to sick, often frightened patients.

However, the main problem with pulmonary function tests is that they are not ordered often enough. Population surveys generally document some abnormality in respiratory function in 5% to 20% of subjects studied. Chronic obstructive pulmonary disease (COPD) is currently the fourth leading cause of death in the United States. It causes more than 100,000 deaths per year. It is estimated that 16 million people in the United States have COPD. All too often the condition is not diagnosed until the disease is far advanced. In a significant number of cases, lung disease is still not being detected. If we are to make an impact on COPD, it needs to be detected in the early stage, at which point smoking cessation markedly reduces the likelihood of progression to severe COPD. Figure 1-1 shows the progression of a typical case of COPD. By the time dyspnea occurs, airway obstruction is moderately or severely advanced. Looked at differently, spirometry can detect airway obstruction in COPD 5 to 10 years before dyspnea occurs.

Nevertheless, few primary care physicians routinely order pulmonary function tests for their patients who smoke or for patients with mild to moderate dyspnea. For patients with dyspnea, however, in all likelihood the blood pressure has been checked and chest radiography and electrocardiography have been performed. We have seen patients who have had coronary angiography before simple spirometry identified the true cause of their dyspnea.

FIG. 1-1. Typical progression of the symptoms of chronic obstructive pulmonary disease (COPD). Only spirometry enables the detection of COPD years before shortness of breath develops. (From PL Enright, RE Hyatt [eds]: Office Spirometry: A Practical Guide to the Selection and Use of Spirometers. Philadelphia: Lea & Febiger, 1987. Used with permission of Mayo Foundation for Medical Education and Research.)

Why are so few pulmonary function tests done? It is our impression that a great many clinicians are uncomfortable interpreting the test results. They are not sure what the tests measure or what they mean, and, hence, the tests are not ordered. Unfortunately, very little time is devoted to this subject in medical school and in residency training. Furthermore, it is difficult to determine the practical clinical value of pulmonary function tests from currently available texts of pulmonary physiology and pulmonary function testing. The 2007 Joint Commission Disease-Specific Care program for the management of COPD may prompt primary care practitioners to adopt more sensitive and specific diagnostic methods.

The sole purpose of, and justification for, this text is to make pulmonary function tests user-friendly. The text targets the basic clinical utility of the most common tests, which also happen to be the most important. Interesting but more complex procedures that have a less important clinical role are left to the standard physiologic texts.



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