EFFECT OF SALBUTAMOL ON GAS-COMPRESSION IN CYSTIC-FIBROSIS AND ASTHMA

In cystic fibrosis (CF), it has been suggested that increases in FEV(1) postbronchodilator (BD) can be accompanied by paradoxical decreases in isovolume maximal flow at 25% of vital capacity (V-25iso 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 FEV(1) and V-25iso determined using MEFVC from both mouth (m) and plethysmograph (p) volumes. From the two curves, volume of compression (V-comp) was measured. Baseline measurements were similar for both groups, except that the residual Volume to total lung capacity ratio (RV/TLC) was higher V-max25 and FVC were lower in CF. Both groups showed significant (p < 0.05) increases in FVC1 FEV(1), and V-25iso after BD. The percentage of change in FEV(1) correlated with the percentage of change in V-25iso (r = 0.53 for CF and 0.66 for asthma, p < 0.001). Baseline V-comp25 was higher in asthma than in CF. The percentage of change in V-25iso was not related to the change in V-comp25iso for either group. Only four patients with CF showed a paradoxical decrease in V-25iso and the differences in flow were small. Two had an increase in V-comp and two had a decrease. We conclude that paradoxical decreases in V-25iso are rare, are associated with small changes in flow, and are not related to changes in V-comp.. This would suggest that FEV(1) can be used to assess the BD response in patients with CF.