Finally, the measurement of hyperreactivity has been used as a research tool to evaluate the mechanisms underlying the airway responses in occupational asthma. The role of the late asthmatic reaction rather than the immediate reaction in altering bronchial hyperreactivity has been well established. More recently this change in airway reactivity has been shown to follow the immediate asthmatic response but precede the onset of the late asthmatic response. This finding has important implications with regard to the mechanisms and treatment of asthma.
Bronchial hyperresponsiveness may be demonstrated with a variety of constrictor stimuli, including histamine, methacholine, cold air, fog, exercise, sulfur dioxide, and PGP*,. Of these various stimuli, the most widely used and best evaluated are histamine and methacholine. Three protocols have been devised for measuring hyperresponsiveness using either of these stimuli.
Chai Method. The dosimeter method of ChaP uses incremental, cumulative doubling doses of methacholine or histamine, originally delivered via a De Vilbiss no 42 nebulizer and more recently via a De Vilbiss no 646 nebulizer. The dilutional increments for methacholine range from 0.075 mg/ml to 25.0 mg/ml, and for histamine, from 0.03 mg/ml to 10 mg/ml. Five breaths of each dilution are administered at 5-minute intervals, and a positive response is designated as a greater than 20% reduction in FEV, sustained for three minutes following the challenge.