Pulmonary function tests are usually ordered when patients go to their primary physician or a pulmonary specialist with complaints of shortness of breath, cough, decreased activity, and dyspnea. When the history is taken, patients’ complaints are due to their breathing problem, for example they are short of breath walking up stairs or dressing. As dyspnea begins to affect their daily life more and more, people will often make lifestyle changes, such as selling a two-story house and moving to a ranch house with no stairs or altering the way they dress (e.g., wearing housecoats or jogging pants that are easier to put on and take off). Often, people think that their breathing problems are just part of getting older, and they try to change their lifestyle to accommodate their limitations and avoid doing things that make them short of breath. In many cases, by the time a patient comes to the care provider with these complaints, he or she is already showing signs of moderate to severe chronic obstructive pulmonary disease (COPD). As will be discussed in this chapter, this is a very good reason for considering office spirometry to be a “fifth vital sign.” Suggestions have been made that all smokers and any nonsmoker over 40 years of age should have office spirometry. If the values are abnormal, even though patients may not be experiencing noticeable lifestyle changes, the appropriate drug therapy regimen can be implemented and patients who are smokers will have a definite motivation to quit smoking.

Pulmonary function tests (PFTs) help in the evaluation of the mechanical function of the lungs.¹ They are based on researched norms, taking into account sex, height, and age. For example, there are predicted values for a male, age 65, who is 6 feet tall.^2,3 Race and ethnic differences also play roles in the reference values and need to be taken into account for diagnostic and research purposes.^4–6

When the patient performs the test, actual results (observed) are compared with the predicted value expected of a person of that gender, height, and age to see whether he or she falls within the “normal” range or has a restrictive, obstructive, or a mixed component, based on the tests. If the patient is not within the normal range, a bronchodilator is given, and the test is repeated to see whether there is significant improvement with medication. Basically, the pulmonary function tests are categorized as volume, flow, or diffusion studies. Diagnosis of pulmonary disease or dysfunction and improvement with treatment are evaluated after interpreting a patient’s pulmonary function tests.

Spirometry is the most useful and commonly available test of pulmonary function. Both pulmonologists and primary care physicians commonly use screening spirometry in their offices in order to assess patients and to evaluate the effectiveness of treatment being given to the patient.⁷

Patients with asthma and COPD make up a large portion of the primary care physician’s caseload. COPD is often diagnosed in the moderate to severe stage of the disease. The diagnosis of COPD can be made quite easily with spirometry, taking the patient’s symptoms and history into account. Smokers can be evaluated for early lung disease, and this can serve as a “teachable moment” for them to quit smoking.⁸

Yawn and colleagues (2007)⁹ compared the office spirometry interpretations of pulmonary experts with those of family physicians. They found agreement in 78% of the completed tests. In addition, following spirometry, changes were implemented in the management of 48% of patients. Of interest, there was closer correct interpretation of pulmonary functions between family physicians and pulmonologists in patients who had asthma, versus those with COPD.

Office Spirometry to Improve Early Detection of Chronic Obstructive Pulmonary Disease

The National Lung Health Education Program has recommended that office spirometry be used to screen for subclinical lung disease in adult smokers. For patients who are smokers or for any patients over 40 who have unexplained dyspnea, cough, wheezing, or excessive mucus, spirometric measurements can be considered another vital sign to measure during the routine physical examination, along with blood pressure and cholesterol levels.16,17

In a study of 35- to 70-year-old individuals visiting their general practitioners, patients were given a questionnaire on symptoms of obstructive lung disease.¹⁸ Spirometry was performed in patients with positive answers to the questions and in a random sample of 10% of the group. It was found that 42% of the newly diagnosed cases of obstructive disease would not have been detected without spirometry. The researchers concluded that office spirometry is essential in general practice and can be done by general practitioners who have training in the performance and interpretation of the pulmonary function tests. It is essential that there be good quality assurance and good training when these tests are performed in a general practitioner’s office. Studies have shown variability between the results of pulmonary function tests done in the office versus those done in a lab, so the results should not be considered interchangeable.^19,20

Respiration: Effect of Anatomical and Physiological Dead Space

The most important function of the lungs is to supply the body with oxygen and to the remove carbon dioxide (CO₂) that is produced as a waste product of metabolism. As this continuous gas exchange takes place, sufficient ventilation is needed to move the gases to the alveoli. A number of conducting airways in the lungs, from the trachea down to the terminal bronchi, do not participate in respiration but only move the gases to the alveoli. The volume of this series of airways is known as anatomical dead space. Generally, the anatomical dead space is proportional to the adult body weight. For example, in a 150-pound person, there is an anatomical dead space of approximately 150 mL. A normal tidal volume (TV), the breath normally taken, has to be large enough to reach the alveoli well past the anatomical dead space. In a normal adult, the TV is generally 450 to 600 mL. The anatomical dead space would thus represent about one-third of the TV. The rest of the breath would reach the alveoli and be considered “alveolar ventilation.” In many neurologically impaired patients who have limited TVs, it is important to note that little alveolar ventilation may be taking place when the patient is breathing in a rapid and shallow pattern. For example, if a patient’s TV is 200 mL, 150 mL would be anatomical dead space, and only 50 mL of each breath would be effective alveolar ventilation.

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