This chapter examines the many practical applications of pulmonary function testing (PFT) for evaluating lung function and disease progression in individual patients, as well as identifying broader public health concerns across populations. The full range of PFT procedures can assess airway resistance, functional residual capacity (FRC) and residual volume, pulmonary diffusing capacity, arterial blood gases (ABGs), exercise capacity, and even energy expenditure. While spirometry was introduced in Chaps. 4 and 6, the more detailed descriptions here include specific diagnostic criteria for grading the severities of obstructive and restrictive lung disease types, as developed by the American Thoracic Society and European Respiratory Society (ATS/ERS) (Table 16.1).

Spirometry is non-invasive, dynamic pulmonary function testing that indirectly assesses airway resistance, notably in medium-sized bronchi. Among the lung volumes estimated by spirometry to distinguish obstructive and restrictive disorders are: vital capacity (VC), forced vital capacity (FVC), forced expiratory volume in one second (FEV₁), peak expiratory flow (PEF) rate, and forced expiratory volume over the middle half of expiration (FEF_25-75). There are three distinct steps to obtaining a reliable FVC. First, maximal inspiration is measured by the subject inhaling as deeply as possible toward TLC before pausing for 1 second at that volume. Second, the subject exhales maximally as quickly as possible toward RV, continuing for at least 6 seconds, yielding an expiratory flow volume curve or loop [Fig. 16.1 (a)]. Third, the subject immediately inhales again toward TLC to obtain an inspiratory flow volume loop. At least three such efforts, but not more than eight, are performed in a given PFT session, with brief resting intervals using a normal V_T between efforts if needed.

With these data, the subject’s largest FVC and FEV₁ from any of the attempts are recorded, even if the highest values for FVC and FEV₁ do not occur in the same expiratory effort. However, the highest two estimates of FVC and FEV₁ should not differ by more than 150 mL to assure an appropriate degree of repeatability. If the patient’s best results differ by more than 150 mL, it is usually recommended that another full test be conducted. Similarly, once the expiratory flow-volume loops have been plotted, the patient’s PEF results should be evaluated to ensure that peak rates (usually near the start of expiration) do not vary by more than 5% between the highest two values. If patient variance exceeds this benchmark, another full test should be completed.

The shape of the flow-volume loop during expiration [Fig. 16.1 (b)] is modified by dynamic airway compression that is effort-independent during expiration (Chap. 6). A “choke point” may exist throughout expiratory flow, so that whenever P_IP exceeds P_AW, airway collapse can occur (Fig. 16.2), making expiratory flow-volume loops concave. Dynamic compression of airways does not occur during inspiration because P_IP < P_AW throughout that phase of breathing, stenting the airways and alveoli open.

Comparing a subject’s measured spirometry results and subsequent PFT data (eg, lung volumes and DL_CO described below) to appropriate reference values is essential to proper interpretation. Reference values are derived from studies of large, well-defined healthy populations of children and nonsmoking adults, expressed as normal intervals or frequency distributions representing 95% of the sample population. The most common method of analysis is to compare the observed data to the mean reference value, with results expressed both as absolute numeric values and as the percent of predicted value. The lower limit of normal (LLN) describes the lowest 5th percentile according to reference equations based on age, gender, and height, and sometimes ethnicity. Any value below the LLN is deemed clinically abnormal. To determine the severity of obstruction, the FEV₁ expressed as percentage predicted is often used (Table 16.2).