FORCED EXPIRATORY VOLUMES
Vital capacity is the volume that can be expired following a maximal inspiration. Forced vital capacity (FVC) is the total volume of air that can be forcibly expired after a maximal inspiration, as shown in Figure 5-6. The volume of air that can be forcibly expired in the first second is called FEV1. Likewise, the cumulative volume expired in 2 seconds is called FEV2, and the cumulative volume expired in 3 seconds is called FEV3. Normally, the entire vital capacity can be forcibly expired in 3 seconds, so there is no need for "FEV4."
FVC and FEV1 are useful indices of lung disease. Specifically, the fraction of the vital capacity that can be expired in the first second, FEV1/FVC, can be used to differentiate among diseases. For example, in a normal person, FEV1/FVC is approximately 0.8, meaning that 80% of the vital capacity can be forcibly expired in the first second (see Fig. 5-6A). In a patient with an obstructive lung disease such as asthma, both FVC and FEV1 are decreased, but FEV1 is decreased more than FVC is. Thus, FEV1/FVC is also decreased, which is typical of airway obstruction with increased resistance to expiratory airflow (see Fig. 5-6B). In a patient with a restrictive lung disease such as fibrosis, both FVC and FEV1 are decreased, but FEV1 is decreased less than FVC is. Thus, in fibrosis, FEV1/FVC is actually increased.[((bibcite Physiology 3E by Costanzo))]
