Comparison of maximal midexpiratory flow rate and forced expiratory flow at 50% of vital capacity in children

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 [FEF25-75]). It is common practice to report also forced expired flow at 50% of vital capacity (FEF50). Study objective: To investigate whether FEF50 and FEF25-75 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 FEF50 and FEF25-75 in a single determination. We assessed the relationship between the FEF50/FEF25-75 ratio and the degree of airways obstruction, as reflected by other traditional parameters such as FEV1, FEV1/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: FEF50 correlated with FEF25-75 as follows: FEF50 (L/s)=0.041+1.136*FEF25-75(L/s); r(2)=0.956; standard error of the estimate=0.013; p<0.0001. The FEF50/FEF25-75 ratio remained stable and did not correlate with FEV1 (r=0.12), FEV1/FVC ratio (r=0.11), or SGaw (r=0.02; difference not significant). The correlation between FEF25-75 and FEF50 was similar for both the smooth curve (r=0.97) and the irregular curve (r=0.96). Conclusions: Although not identical, FEF25-75 and FEF50 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 FEF50.