Shunt properties of large intrathoracic airways.

The impedance of the wall of human intrathoracic trachea and central airways was measured by submitting preparations of excised airways to forced oscillations at various frequencies from 2 to 32 Hz. Both real (resistance) and imaginary (reactance) parts of wall impedance demonstrate a marked frequency dependence, varying with transmural pressure. These variations of resistance and reactance are related and are linked to the static elastic properties of the airways. The data allow us to calculate the total shunt impedance of the central intrathoracic airways. When the latter shunt values are used to correct measurements of impedance values of excised emphysematous lungs, it turns out that the shunt does not modify markedly the observed frequency dependence of resistance and compliance of those lungs, at least at transpulmonary pressures > 0.2 kPa. A model study suggests, in addition, that the latter frequency characteristics cannot be explained satisfactorily by parallel mechanical inhomogeneities. We submit that the frequency dependence of resistance and compliance of excised emphysematous lungs is determined mainly by the visco- and/or plastoelastic properties of lung tissue itself.