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Effect of an H1 blocker, chlorpheniramine, on inhalation tests with histamine and allergen in allergic asthma.

In 15 asthmatic subjects, flow volume loops were recorded before and after quantitative nebulization of increasing concentration of histamine and allergen, until a threshold response (-delta FEV1 > 10%) was obtained. Nine subjects with reproducible threshold doses to these two agents were included in the study. After pretreatment with 10 mg of intravenously administered chlorpheniramine, the threshold dose of histamine increased fourfold in eight subjects and eightfold in one subject, whereas the threshold doses to allergen doubled in seven and remained unaffected in two. One of these two subjects required 15 mg chlorpheniramine intravenously for comparable protection. In three subjects, the threshold dose to inhaled acetylcholine was determined with a similar procedure and found reproducible. Pretreatment with 10 to 15 mg chlorpheniramine increased this threshold dose two to four times, similarly to the effect produced by 0.4 mg of subcutaneously administered atropine. However, 0.4 mg atropine sc. did not alter the airway responses to allergen. The results suggest that intravenously administered chlorpheniramine provided it has a dose ratio of at least four against inhaled histamine, can prevent attacks of allergen-induced asthma. The effect is probably mediated through the H1 rather than the acetylcholine receptor.