Background: The mid-portion of the maximal expiratory flow-volume (MEFV) curve is often described by values of the mean forced expired flow as lung volume decreases from 75% to 25% of vital capacity (ie, forced expiratory flow, midexpiratory phase [FEF_{25–75}]). It is common practice to report also forced expired flow at 50% of vital capacity (FEF_{50}).
Study objective: To investigate whether FEF_{50} and FEF_{25–75} are highly correlated or whether the difference between them reflects a degree of airways obstruction. Also, we wanted to investigate the correlation between the two in cases of irregularly shaped MEFV curves (ie, “saw-toothing”).
Design: Analysis of the correlation between FEF_{50} and FEF_{25–75} in a single determination. We assessed the relationship between the FEF_{50}/FEF_{25–75} ratio and the degree of airways obstruction, as reflected by other traditional parameters such as FEV_{1}, FEV_{1}/FVC ratio, and specific airway conductance (SGaw).
Patients: There were 1,350 forced expiratory maneuvers performed by children with a broad range of pulmonary abnormalities.
Results: FEF_{50} correlated with FEF_{25–75} as follows: FEF_{50} (L/s) = 0.041 + 1.136*FEF_{25–75}(L/s); r^{2} = 0.956; standard error of the estimate = 0.013; p < 0.0001. The FEF_{50}/FEF_{25–75} ratio remained stable and did not correlate with FEV_{1} (r = 0.12), FEV_{1}/FVC ratio (r = 0.11), or SGaw (r = 0.02; difference not significant). The correlation between FEF_{25–75} and FEF_{50} was similar for both the smooth curve (r = 0.97) and the irregular curve (r = 0.96).
Conclusions: Although not identical, FEF_{25–75} and FEF_{50} are highly correlated, and the ratio of the two is fairly constant. Therefore, the practice of reporting both of them is unnecessary. We suggest that it is reasonable to prefer FEF_{50}.