Influence of upper airway shunt on total respiratory impedance in infants

When input impedance is determined by means of the forced oscillation techn ique, part of the oscillatory flow measured at the mouth is lost in the mot ion of the upper airway wall acting as a shunt. This is avoided by applying the oscillations around the subject's head (head generator) rather than at the mouth (conventional technique). In seven wheezing infants, we compared both techniques to estimate the importance of the upper airway wall shunt impedance (Zuaw) for the interpretation of the conventional technique resul ts. Computation of Zuaw required, in addition, estimation of nasal impedanc e values, which were drawn from previous measurements (K. N. Desager, M. Wi llemen, H. P. Van Bever, W. De Backer, and P.A. Vermeire. Pediatr: Pulmonol . 11: 1-7, 1991). Upper airway resistance and reactance at 12 Hz ranged fro m 40 to 120 and from 0 to -150 hPa.l(-1).s, respectively. Varying nasal imp edance within the range observed in infants did not result in major changes in the estimates of Zuaw or lung impedance (ZL), the impedance of the resp iratory system in parallel with Zuaw. The conventional technique underestim ated ZL, depending on the value of Zuaw. The head generator technique sligh tly overestimated ZL, probably because the pressure gradient across the upp er airway was not completely suppressed. Because of the need to enclose the head in a box (which is not required with the conventional technique), the head generator technique is difficult to perform in infants.