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Obstructive pulmonary disease is characterized by an increase in airflow resistance and the presence of air trapping. Work of breathing is increased in these conditions, and an increase in residual volume (RV) and total lung capacity (TLC) is common.
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PULMONARY FUNCTION TESTS
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In patients with early disease, pulmonary function tests characteristically demonstrate a reduction in maximum midexpiratory flow (MMEF), which is the forced expiratory flow between 25% and 75% of vital capacity (FEF25–75%). Normal MMEF is >2.0 L/s for adult males and >1.6 L/s for adult females. A reduction of MMEF to <70% of these values may be the only manifestation of early disease (Figure 45-1). This value is not technique- or effort-dependent.
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As obstructive disease becomes more severe, there is a reduction in both the measured forced expiratory volume in 1 second (FEV1) and the FEV1/FVC (forced vital capacity) ratio. A decrease of these values to below 75% of predicted is typical in obstructive disease. Additionally, the slow vital capacity (SVC) will be greater than the FVC when FVC is abnormally decreased secondary to obstruction.
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Although less accurate and specific than spirometry, peak flow rates using peak flow meters can also be used to evaluate obstructive disease. They do not replace spirometry during the initial evaluation of chronic disease, but can be a helpful point-of-care tool to measure a patient’s response to therapy during an acute exacerbation. Normal peak flow values are based on age, sex, and height, and there are several online calculators available.
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Flow volume loops graphically record maximal inspiratory and expiratory maneuvers with flow on the Y-axis and volume on the X-axis. Unlike the characteristic flow volume loops for restrictive disease, which are fairly consistent for the variable etiologies of restrictive disease, the flow volume loops of patients with obstructive disease can differ greatly and are able to distinguish between various types of obstructions: intra- and extra-thoracic obstruction, or fixed or variable obstruction (Figure 45-2).
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In patients with a fixed obstruction, the degree of obstruction is not altered by changes in pleural or airway pressures. ...