Measurement of ventilatory mechanical impedance in infants using a head pressure generator

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Abstract
Two methods of measuring ventilatory mechanical impedance (Z) by forced oscillations between 6 and 20 Hz were compared in 24 infants aged 2 to 49 months: (1) the application of pressure oscillations at the airway opening (Z1); and (2) the application of pressure oscillations around the head (Z2). The latter has been recently proposed to minimize the influence of compliant upper airway walls (Peslin et al., J Appl Physiol. 1985, 59:1790–1795). Ventilatory resistance and compliance (Rsb, Csb) were also obtained with the single breath method. The real part of Z1 (R1) was markedly lower than that of the corresponding Z2 (R2), at any frequency. R1 exhibited a systematic negative frequency dependence, in contrast with R2. At any frequency, the slope of the regression equation on Rsb was closer to unity for R2 than for R1. The imaginary part of Z1 (X1) was negative over the whole frequency interval, and negative values of inertance were derived from X1. X2 was negative at low and positive at high frequencies. Resonant frequency (mean ± SD = 10.5 ± 3.5 Hz) was always reached with Z2 and correlated negatively with body weight (r = −0.61). Inertance estimated from X2 was positive and correlated negatively with body height (r = −0.66). The compliance derived from Z1 (C1 = 3.35 ± 2.32 103 L·cm H2O−1) was not significantly different from that derived from Z2 (C2 = 2.99 ± 2.02 10−3 L·cm H2O−1). The marked difference observed between Z1 and Z2 is related to the importance of the upper airway shunt and may be explained by inaccuracies of both methods. The ventilatory system impedance (Zrs) is underestimated by Z1 because some of the flow through the flowmeter does not reach the airway, and is overestimated by Z2 because some of the flow from the subject does not reach the flowmeter. The latter effect is negligible in mouth breathers, but becomes quite substantial in nose breathers. Although computer simulations suggest that the error in estimating Zrs from Z2 appears to be lower than that resulting from Z1, the potential benefit offered by the routine use of a head generator in infants remains to be established. Pediatr Pulmonol 1989; 7:209–216.