DIFFERENCES IN SENSITIVITY, MAXIMAL RESPONSE AND POSITION OF THE CONCENTRATION-RESPONSE CURVE TO METHACHOLINE BETWEEN ASTHMATICS, PATIENTS WITH ALLERGIC RHINITIS AND HEALTHY-SUBJECTS

The aim of this study was to detect differences in maximal response an d position of the concentration-response curves to methacholine betwee n asthmatics and subjects with allergic rhinitis. A total of 228 adult s (107 mild asthmatics, 96 allergic rhinitics and 25 healthy control s ubjects) were challenged with methacholine. The test was interrupted w hen FEV1 dropped by more than 40% or when the highest concentration of methacholine (200 mg ml(-1)) had been administered. Concentration-res ponse curves were characterized by their PC20 (concentration of methac holine that produced 20% fall in FEV1 = airway sensitivity), and if po ssible, by their EC50 (concentration of methacholine that produced 50% , of the maximal response = position) and level of plateau. The propor tion of subjects with plateau was significantly lower in asthmatics (1 8.7%) than in either allergic rhinitics (57.3%) or healthy subjects (9 2%). It was also significantly lower in allergic rhinitics than in hea lthy subjects. The level of plateau for asthmatics was (means +/- SD) 31.5 +/- 5.5%, compared with 20.8 +/- 8.1% in allergic rhinitics and 1 3.7 +/- 6.7% in healthy subjects (P<0.01). It was also higher in aller gic rhinitics than in healthy subjects (P<0.01). The EC50 values were decreased in asthmatics when they were compared with either allergic r hinitics or healthy subjects (geometric mean EC50: asthmatics = 2.7 mg ml(-1), allergic rhinitics=6.2 mg ml(-1), healthy subjects = 8.7 mg m l(-1); P<0.01), but no significant differences were detected between a llergic rhinitics and healthy subjects. These results demonstrate that in subjects with allergic rhinitis, the prevalence and level of the p lateau on the methacholine concentration-response curve is intermediat e between that of asthmatics and normals. Furthermore, while the asthm atic curves differ from normal in having both an increased maximal res ponse and a leftward shift, the rhinitic curves differ only in terms o f plateau level. These results suggest that airway responsiveness in a sthma and allergic rhinitis may be a consequence of mechanisms that ar e at least partially different.