Acoustic input impedance of excised dog lungs

Abstract

Input impedances of six excised dog lungs were computed from Fourier analysis of their response to a transient pulse pressure excitation. Impedances, between 156 Hz and 10 000 Hz, were measured at transpulmonary pressures of 30, 20, 10, and 5 cm H2O. There was a substantial amount of interindividual variability in the impedance. There were, however, generally three well defined impedance peaks and the frequencies of these peaks seemed to be dependent upon body size and lung volume. If the airways are modeled simply as a constant diameter tube one is led to the paradoxical conclusion that substantial airway closure occurs at relatively large lung volumes. However, conditions of open airways are consistent with our results if the airways are modeled as having a more complex geometrical shape. We conclude that the input impedance at these high frequencies is variable among individuals, and between lung volumes in any given individual, and that estimates of pathlength or the alveolar boundary condition cannot be easily obtained by interpretation based on simple geometrical models.