Figure 25-17.

Schematic illustration of volume adjustment to calculate the isovolume FEF_25%–75%, or forced expiratory flow between 25% and 75% of forced vital capacity.

Left, Before administration of bronchodilator, the FEF_25%–75% is calculated from a line connecting two points on the volume-time curve of the forced vital capacity (FVC). One solid circle indicates when 25% of the FVC is exhaled (6.5 L), and the other solid circle indicates when 75% of the FVC is exhaled (3.5 L). This volume change (3.0 L) develops in 3.4 seconds, so the FEF_25%–75% is 0.88 L/sec. Right, After administration of bronchodilator, one open circle indicates when 25% of the FVC is exhaled (6.0 L), and the other open circle indicates when 75% of the FVC is exhaled (2.0 L). This volume change develops in 1.3 seconds, so the FEF_25%–75% is 3.0 L/sec. The values based on the “before” volumes from the pretreatment curve (solid circles) have been extended to the posttreatment (“after”) curve. The volume-adjusted or isovolume FEF_25%–75% is determined from a line connecting the solid circles on the “after” graph. In this case the volume change is the same as that observed in the “before” graph, or 3.0 L, but it took place in only 0.6 second, so the isovolume FEF_25%–75% is 5.0 L/sec, a marked improvement induced by the bronchodilator. This approach was developed because early reports indicated that some patients appeared to have significant improvement in forced expiratory volume in 1 second (FEV₁) but not in FEF_25%–75% when no volume adjustment was made in the calculation of FEF_25%–75%. When a volume adjustment was made in the calculation of FEF_25%–75%, there was improvement in both FEV₁ and FEF_25%–75%, as illustrated.