Figure 25-5.

Determination of forced expiratory flow between 25% and 75% of total lung capacity (FEF_25%–75%).

A heavy dashed line connects two points on the volume-time curve of the forced vital capacity (FVC) maneuver. One point is marked when 25% of the FVC has been exhaled (2 L); the other point is marked when 75% of the FVC (6 L) has been exhaled from the level of maximal inhalation indicated by the solid line (a). In this example, the elapsed time between these two points is 1 second; thus the calculated FEF_25%–75% is 4 L/sec. FEV₁, forced expiratory volume in 1 second.