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 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%.