KJ Desmond, DL Demizio, PD Allen, ND MacDonald and AL Coates
Department of Paediatrics, Children's Hospital of Eastern Ontario, Ottawa, Canada.

In cystic fibrosis (CF), it has been suggested that increases in FEV1 postbronchodilator (BD) can be accompanied by paradoxical decreases in isovolume maximal flow at 25% of vital capacity (V25iso) measured from maximum expiratory flow-volume curves (MEFVC), raising concerns about determining the benefits of BD in CF. MEFVC measured using expired volume has been shown to be subject to errors due to gas compression. In the present study, BD response was assessed in 91 patients with asthma and 78 with CF using the percentage of change in FEV1 and V25iso determined using MEFVC from both mouth (m) and plethysmograph (p) volumes. From the two curves, volume of compression (Vcomp) was measured. Baseline measurements were similar for both groups, except that the residual volume to total lung capacity ratio (RV/TLC) was higher and Vmax25 and FVC were lower in CF. Both groups showed significant (p < 0.05) increases in FVC, FEV1, and V25iso after BD. The percentage of change in FEV1 correlated with the percentage of change in V25iso (r = 0.53 for CF and 0.66 for asthma, p < 0.001). Baseline Vcomp25 was higher in asthma than in CF. The percentage of change in V25iso was not related to the change in Vcomp25iso for either group. Only four patients with CF showed a paradoxical decrease in V25iso, and the differences in flow were small. Two had an increase in Vcomp and two had a decrease. We conclude that paradoxical decreases in V25iso are rare, are associated with small changes in flow, and are not related to changes in Vcomp.(ABSTRACT TRUNCATED AT 250 WORDS)