Figure 1.

Variables used to quantify global and peripheral concavity (see text). Measured FEF_50% and measured FEF_75% are the forced expired flows when 50% and 75% of the FVC has been expired. Reference FEF_50% and Reference FEF_75% are the reference flows that would be obtained if the flow-volume curve had zero curvature i.e., a linear descending limb (dotted line). The variable, y, is the volume to peak expiratory flow (PEF); a value of 0.6 L can be assumed for this. In this example, global concavity is approximately 50 Units and peripheral concavity is approximately 65 Units.