INTERPRETATION OF RESPIRATORY INPUT IMPEDANCE IN HEALTHY INFANTS

Respiratory input impedance (Z(in)) is a potentially informative test of pulmonary function in infants who are unable to perform standard te sts commonly performed in children and adults. Analysis of Z(in) in do gs using the six-element model of DuBois et al. (J Appl Physiol 8:587, 1956) provides estimates of airways resistance separate from tissue r esistance, as well as an estimate of thoracic gas volume. However, rel iable estimates of these parameters can only be obtained when Z(in) di splays a distinct antiresonance that is associated with the tissue ine rtance and alveolar gas compression compliance. To determine whether i nfants have such an antiresonance, Z(in) was measured in nine healthy infants (4 < f < 160 Hz). An antiresonance was found at 112.8 +/- 10.4 Hz, and the six-element model fit these data well, but the resulting parameters were physiologically unrealistic. We hypothesized that the antiresonance in the measured Z(in) is the result of a shunt complianc e proximal to alveolar gas compression compliance. Gas compression in the face mask and nonrigid upper airway walls could provide such a shu nt compliance. We investigated another model with four parameters, a s ingle shunt compliance (C-im) representing gas compression in the face mask in parallel with the infant's total respiratory resistance (R(rs )), inertance (l(rs)), and compliance (C-rs). This model fits the data well, and the estimated R(rs) (19.3 +/- 4.2 cmH(2)O/L/s) was physiolo gically reasonable. However, C-rs (C-rs = 1.03 +/- 0.58 mL/cmH(2)O) wa s one order of magnitude smaller than reported C-rs. The value for C-i m was slightly larger than that based on the estimated volume of gas i n the face mask, suggesting an additional influence of upper airway wa ll shunting. Computer simulations using a model that includes the face mask and upper airway walls confirmed that C-im and the upper airway wall properties significantly influence Z(in) data over this frequency range. Nevertheless, these simulations suggest that the R(rs) estimat ed from the four-element model is related to airway resistance. (C) 19 96 Wiley-Liss, Inc.