The effect of initial airway calibre on the response to bronchial provocation is unclear. Theoretically, geometric relationships within the airways might influence the measurement of responsiveness, particularly since a given change in calibre will produce a disproportionately greater reduction in flow in airways which are already narrowed. We have examined the relationship between serial measurements of prechallenge forced expiratory volume in one second (FEV1) and responsiveness to methacholine (PD20) in 8 children and 12 adults with asthma. Measurements were made every 2-3 wks for 12-18 months and all patients kept a daily record of symptoms and twice daily measurements of peak expiratory flow (PEF). Spearman's rank correlation coefficient (rho) for the relationship PD20 versus pre-challenge FEV1 was derived for each patient and varied widely within the group (range -0.22 to 0.73, mean 0.31); the strength of this correlation was not related to a patient's mean FEV1 % predicted, but was related to the degree to which PD20 and pre-challenge FEV1 themselves reflected concurrent asthma severity (mean morning PEF and mean symptom scores for the three days around each test). This suggests that the observed relationship between pre-challenge FEV1 and PD20 may be due less to the influence of airway geometric factors, which might be expected to be present in all patients, but rather that pre-challenge FEV1 is reflecting the severity of the underlying disease. Larger studies will be needed to test this hypothesis further.
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