ARE TIDAL BREATHING INDEXES USEFUL IN INFANT BRONCHIAL CHALLENGE TESTS

Tidal breathing indices have been used to assess histamine-induced airway obstruction in adults and children. The aim of this study was to see whether they could be used to assess histamine challenge in infants. Tidal Row during quiet breathing was measured using a face mask and pneumotachograph and maximum flow at functional residual capacity (V-maxFRC) was measured from partial forced expirations in 18 sleeping, sedated infants who responded to histamine challenge and in 18 nonresponders. The tidal indices calculated were inspiratory and expiratory time (t(i) and t(e)), tidal peak expiratory flow (PEF), mean tidal expiratory flow rate (V-T/te) and the expiratory time constant of the respiratory system (t(rs)). The time to maximal expiration divided by expiratory time (t(me)/te) and 2 revised forms of this index (t(me(a))/t(e) and t(me(b))/t(e)) were also calculated. Recordings of t(me(a)) and t(me(b)) were taken at 95% of peak tidal expiratory flow, before and after the peak, respectively. In nonresponders, there was an insignificant mean rise in V-maxFRC of 11.8% but no change in any tidal index. in responders, the mean percentage fall in V-maxFRC was 43.3% (range, -31 to -81%); t(rs) fell from 0.61 s to 0.51 s (P<0.05) and breathing frequency and mean tidal expiratory flow rate increased from 34.0 to 37.5 min(-1) (P<0.01) and from 66.6 to 72.6 mL.s(-1) (P<0.05), respectively, suggesting that infants had adopted a strategy of active expiration in response to bronchial challenge. There was no change either in t(me)/t(e) or in the revised indices after challenge. It is therefore concluded that t(me)/t(e) is an insensitive index of airflow obstruction compared to V-maxFRC and cannot be used to assess the response to bronchial challenge. changes in other indices were small and related to a presumed pattern of active expiration after challenge. (C) 1994 Wiiey-Liss, Inc.