Inhalation Challenge

Inhalation challenge with methacholine is now a standardized procedure using a drug specifically approved for the purpose of determining the nature and extent of airways hyperreactivity. The methodology described involves the intermittent administration of graduated concentrations of methacholine by aerosol inhalation followed by routine spirometry after each incremental dose thereby enabling the construction of a dose response curve. The PD₂₀FEV₁, the provocation dose of agonist necessary for a 20% decrease in FEV₁, is interpolated from the curve and is the accepted index of airways sensitivity. Clinically, the procedure has been widely used to implement the diagnosis of asthma in such atypical cases as when the physical exam and pulmonary function tests are equivocal. In such a setting, a positive test often justifies the use of therapeutic bronchodilators whereas a negative test would lead the examiner away from the alternative of hyperactive airways. Other applications include the evaluation of antigen sensitivity for diagnostic and research purposes including occupational asthma, epidemiology, and investigational drug trials.