Correlation of forced expiratory volume and vital capacity
Lung function testing with forced expiratory volume and vital capacity
Forced expiration is a simple but extremely useful pulmonary function test. Following maximal inhalation a person exhales as hard and as completely as possible. Measuring forced expirations is very effective for separating normal ventilation from obstructive and restrictive conditions.
Two values are obtained with this manoeuvre: the volume exhaled in one second (FEV1) and the vital capacity (VC), which is – by definition – identical with the forced vital capacity (FVC). In healthy subjects the volume that can be expired in one second is usually about 80% of the forced vital capacity, or something like four litres out of five (male adult).
In an obstructive condition, however, such as asthma, bronchitis or emphysema, not only the forced vital capacity, but the rate of expiratory flow is also decreased. With restrictive disease, like 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 higher than normal.
Forced expiration curves are particularly useful because they are clearly reproducible. At every lung volume a maximal flow rate exists, which cannot be exceeded. An attempt to exceed maximal flow rate, increases the already positive pleural pressure. Elevation of driving pressure not only increases air flow but also causes collapse of bronchi. Thus airways become occluded and flow is slowed until the pressure difference across the airways drops again reopening the airways so that flow can continue.
Literature:
Klinke R, Silbernagel S. Lehrbuch der Physiologie. 2001. Thieme: Stuttgart
Shier D, Butler J, Lewis R. Hole’s human anatomy and physiology. 2004. McGraw Hill: New York
