Methacholine-induced volume dependence of respiratory resistance in preschool children

Enhanced negative volume dependence of airway resistance is associated with bronchoconstriction in tracheostomized paralysed open-chest animals. Signi ficant upper airways responses may be associated with bronchoconstriction a nd could thereby alter the pattern of volume dependence in spontaneously br eathing subjects. The aim of the study was to test whether volume dependenc e of respiratory resistance (Rrs) could be demonstrated in preschool childr en undergoing routine methacholine challenge. The volume dependence of respiratory oscillation resistance at 12 and 20 Hz (Rrs,12 and Rrs,20) was examined in eight 4-5.5-yr-old children showing a positive response to methacholine. Multiple linear regression analysis was also used to account for flow dependence during tidal breathing (Rrs,12 or Rrs,20=K1+K2\V'\+K3V). Rrs,12 and Rrs,20 yielded similar results. Negative volume dependence was p resent at baseline and significantly enhanced by methacholine (p<0.01). For instance, the mean+/- SD inspiratory K3 at 20 Hz was -4.1+/-1.3 hPa.s.L-2 at baseline and -15.0+/-4.3 hPa.s.L-2 after methacholine, in which case it was also larger on expiration than on inspiration (p<0.05), possibly as a r esult of upper airway responses. A significant increase in the negative volume dependence of respiratory res istance may thus be shown in preschool children in response to methacholine . The volume dependence (K3) during inspiration may be particularly useful in detecting bronchoconstriction, because it is less likely to be affected by upper airway mechanisms than during expiration.