Are tidal breathing indices 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 flow during quiet breathing was measured using a face mask and pneumotachograph and maximum flow at functional residual capacity (VmaxFRC) 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 (ti and te), tidal peak expiratory flow (PEF), mean tidal expiratory flow rate (VT/te) and the expiratory time constant of the respiratory system (trs). The time to maximal expiration divided by expiratory time (tme/te) and 2 revised forms of this index (tme(a)/te and tme(b)/te) were also calculated. Recordings of tme(a) and tme(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 VmaxFRC of 11.8% but no change in any tidal index. In responders, the mean percentage fall in VmaxFRC was 43.3% (range, -31 to -81%); trs 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 tme/te or in the revised indices after challenge.(ABSTRACT TRUNCATED AT 250 WORDS)