Forced expiration is a simple but extremely useful pulmonary function test. A spirometry tracing is obtained by having a person inhale to total lung capacity and then exhaling as hard and as completely as possible. These tracings are a very effective way of separating normal ventilatory states from obstructive and restrictive states.
In a normal forced expiration curve, the volume that the subject can expire in one second (referred to as FEV1) is usually about 80% of the total forced vital capacity (FVC), or something like four liters out of five.
In an obstructive condition, however, such as asthma, bronchitis or emphysema, the forced vital capacity is not only reduced, but therate of expiratory flow is also reduced. Thus, an individual with an obstructive defect might have a forced vital capacity of only 3.0 liters, and in the first second of forced expiration, exhale only 1.5 liters, giving a FEV1/FVC of 50%. With a restrictive disease, such as fibrosis, forced vital capacity is also compromised. However, due to the low compliance of the lung in such conditions, and the high recoil, the FEV1/FVC ratio may be normal or even greater than normal. For example, a patient with a restrictive condition might have a FVC of 3.0 liters, as was seen in the obstructive cases, but the FEV1 might be as high as 2.7 liters, giving a FEV1/FVC ratio of 90%.
Forced expiration curves are particularly useful because they are so reproducible. At every lung volume there exists a maximal rate of flow which cannot be exceeded. When an individual tries to exceed his maximal flow rate, he forcefully contracts his abdominal muscles to increase his already positive pleural pressure. This increases the driving pressure for air flow from the alveoli to the mouth but also causes the bronchi (whose pressure lies somewhere between that in the alveoli and that at the mouth, but is less than pleural pressure) to collapse. Thus the airways become occluded and flow is slowed until the pressure difference across the airways drops a bit, the airways can reopen, and flow can continue.