Effect of salbutamol on gas compression in cystic fibrosis and asthma

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 (V̇(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 FEV₁ and V̇(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 and V̇(max25) and FVC were lower in CF. Both groups showed significant (p < 0.05) increases in FVC, FEV₁, and V̇(25iso) after BD. The percentage of change in FEV₁ correlated with the percentage of change in V̇(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 V̇(25iso) was not related to the change in V(comp25iso) for either group. Only four patients with CF showed a paradoxical decrease in V̇(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 V̇(25iso) are rare, are associated with small changes in flow, and are not related to changes in V(comp). This would suggest that FEV₁ can be used to assess the BD response in patients with CF.