No information is available on the plateau in response to adenosine 5′-monophosphate(AMP). The aims of the present study were (1) to determine whether plateau can be detected with AMP and the relation with the plateau in response to methacholine, and (2) to identify the relation between the plateau and indirect markers of airway inflammation, such as exhaled nitric oxide (ENO) and exhaled breath condensate (EBC) pH.
Airway responsiveness to high concentrations of methacholine and AMP, ENO levels, and EBC pH values were obtained in 31 subjects with well-controlled asthma. Concentration-response curves were characterized by their concentration of agonist that produces a decrease in FEV1 of 20% and, if possible, by the level of plateau.
Although the prevalence of plateau with methacholine (48%) and AMP (58%) was similar, the two challenges did not identify plateau in exactly the same individuals. In 14 subjects who showed plateau with both bronchoconstrictor agents, the mean plateau level for methacholine was 26.0% (95% confidence interval [CI], 21.3 to 30.8), compared with 16.5% (95% CI, 12.2 to 20.8; p < 0.0001) for AMP. Both ENO and EBC pH values were similar in subjects with plateau and in those without plateau.
In well-controlled asthmatics, the plateau in response to AMP can be identified at a milder degree of obstruction than the plateau in response to methacholine, but the two agonists are not identifying the same airway abnormalities. Furthermore, if ENO and EBC pH are markers of inflammation, the determination of the presence or level of plateau is not a reliable method to identify airway inflammation in asthma.