Input respiratory impedance to estimate airway hyperreactivity in children: standard method versus head generator.

We previously found that a significant underestimation of respiratory mechanical impedance (Zrs) at high frequency may result from the upper airway artefact in children, when pressure is directly varied at the mouth. To determine the importance of this artefact in estimating lung response to bronchomotor agents with the forced oscillation technique, input respiratory mechanical impedance was measured using 6-32 Hz pseudorandom pressure oscillations applied directly to the mouth (standard generator (SG)) and around the subject's head (head generator (HG)) in 35 children aged 2.5-13 yrs. Changes in resistance were generally larger with HG than SG. The mean +/- SEM changes in resistance of the respiratory system (Rrs) at 20 Hz induced by acetylcholine or allergen challenge were 15 +/- 4% for SG and 67 +/- 12% for HG, and changes induced by bronchodilators were -25 +/- 2% for SG, and -46 +/- 4% for HG (p < 0.01). Challenge induced negative frequency dependence of Rrs with SG and positive frequency dependence with HG. There was significant increase in inertance after salbutamol with SG, but no significant change occurred with HG. With both methods, respiratory compliance decreased significantly after challenge. Computer simulations showed that the difference in change in Rrs induced by airway challenge with HG and SG could be explained by the effect of the upper airway wall impedance (Zuaw). Zuaw could also account for the change in inertance and compliance observed with SG, but not for the change in compliance with HG. The latter could be reproduced by simulating unequal distribution of mechanical time constants within the lung, increased peripheral lung resistance with compliant central airways.(ABSTRACT TRUNCATED AT 250 WORDS)