airway reactivity measurement dose of methacholine at which pulmonary conductance is half its pretreatment value 2012-03-13T02:15:00Z CHEBI:50142 CHEBI:6804 CMO:0000713 JSmith The administered quantity of methacholine, a quaternary ammonium ion in which the nitrogen is substituted with three methyl groups and a 2-acetoxypropyl group and which acts as a parasympathomimetic bronchoconstrictor, at which the ability of the repiratory tract to allow airflow during breathing (inspration and expiration) is half its value in an untreated subject, or at which the resistance of the respiratory tract to airflow is doubled. VT:0002338 clinical_measurement_ontology methacholine median effective dose (ED50) methacholine response/sensitivity measurement