HYPERVENTILATION DURING BRONCHIAL CHALLENGES IN ASTHMATICS - REPRODUCIBILITY AND ASSESSMENT OF CONTRIBUTING FACTORS

Among asthmatics, the ventilatory response is heterogeneous during bro nchial challenge. This study aimed to investigate the reproducibility of the response and to assess possible causes for hyperventilation. Re peated bronchial histamine and methacholine challenges (HiCh/MeCh) wer e performed in 10 asthmatic adolescents. Ventilation was monitored by respiratory inductive plethysmography (RIP), in order to minimally aff ect the spontaneous breathing pattern. FEV(1) and the volume of trappe d gas (measured as the volume of air mobilized by five maximal breaths after a multiple breath nitrogen washout to 2% N-2), were used to ass ess mainly central and peripheral airways obstruction, respectively. W hen FEV(1) had decreased by at least 20%, mean inspiratory flow (V-TI/ T-I) increased by 21% and minute ventilation (V'(I)) by 21% and 23% du ring HiCh and MeCh, respectively (both P<0.05). No correlation was fou nd between the magnitude of the ventilatory response and either: the d egree of FEV(1) decline, the increase in gas trapping, SaO(2) decline or the increase in dyspnoea score. Histamine challenge after beta(2)-a gonist pre-treatment was associated with increased ventilatory drive i n one patient despite the absence of bronchial obstruction, indicating that histamine might directly stimulate afferent airway nerves which cause hyperventilation. The intra-individual variability of the ventil atory response (increase in V'(I) and V-TI/T-I was more than 100% of t he mean ventilatory response, while the variability of the bronchomoto r response was about 25% of the mean bronchomotor response. Thus, duri ng induced bronchial obstruction in asthmatics, the occurrence of hype rventilation and its intensity are not related to either the degree of central or peripheral airways obstruction, or to the degree of dyspno ea. The reproducibility of the ventilatory response is poor. The venti latory response appears to be the result of a complex interaction betw een several afferent stimuli and central ventilatory control.