Measurement of responsiveness to inhaled histamine: comparison of FEV1 and SGaw.

The histamine provocation concentration producing a 20% reduction in FEV1 (PC20-FEV1) and that producing a 35% reduction in SGaw (PC35-SGaw) were determined on two different days in 16 asthmatics and 27 normal subjects. The PC20-FEV1 showed good separation between asthmatics and normals. All asthmatics had PC20 less than 8 mg/ml and only one normal had a PC20 slightly below 8 mg/ml. Separation was not quite as good for PC35-SGaw. All asthmatics had a PC35 less than or equal to 3 mg/ml, but four (15%) normals also had a PC35 less than 3 mg/ml. Comparison of individual points revealed that the PC35-SGaw was consistently approximately four-fold lower than the PC20-FEV1 in normals, mildly hyperreactive asthmatics and moderately hyperreactive asthmatics, but was greater than or equal to PC20-FEV1 in markedly hyperreactive asthmatics. This is consistent with inspiration-induced inhibition of bronchoconstriction during PC20-FEV1 testing in the former groups and inspiration-induced enhancement of bronchoconstriction in the latter group. We conclude that the FEV1 is clinically acceptable to measure the response to a bronchoconstricting agent and that FEV may be preferable to SGaw because of better separation of asthmatics from other groups.