Figure 2 .

Flow-volume curve of a child with asthma, showing the characteristic concave expiratory pattern with markedly reduced mid-expiratory flow rates. The reference values for peak expiratory flow (PEF), mid-expiratory flows at 25%, 50%, and 75% (MEF_25–75) of forced vital capacity (FVC), and the FVC itself are represented by squares, and are connected by a dashed line representing a hypothetical "normal" expiratory flow-volume curve. The FEV₁ can not be read directly from a flow-volume curve because there is no time axis, but the spirometer software will provide it. In this case, the FEV₁ was 71% of the predicted value. Note that despite considerable airways obstruction, PEF is normal.