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 tomography scan, if available; and pertinent laboratory findings. Nevertheless, some test patterns strongly suggest the presence of certain conditions, such as pulmonary fibrosis. In addition, 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 needs to 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.