The magnitude (1Z,81) and phase angle (0r8) of the total respiratory impedance (Zr,), from 3 to 45 Hz, were rapidly obtained by a modification of the forced oscillation method, in which a random noise pressure wave is imposed on the respiratory system at the mouth and compared to the induced random flow using Fourier and spectral analysis. No significant amplitude or phase errors were introduced by the instrumentation. 10 normals, 5 smokers, and 5 patients with chronic obstructive lung disease (COPD) were studied. Measurements of Zr, were corrected for the parallel shunt impedance of the mouth, which was independently measured during a Valsalva maneuver, and from which the mechanical properties of the mouth were derived. There were small differences in Zr, between normals and smokers but both behaved approximately like a second-order system with 0r8 = 00 in the range of 5-9 Hz, and Or, in the range of +40°at 20 Hz and +60°at 40 Hz. In COPD, Or, remained more negative (compared to normals and smokers) at all frequencies and crossed 0 between 15 and 29 Hz. Changes in Zr,, similar to those in COPD, were also observed at low lung volumes in normals. These changes, the effects of a bronchodilator in COPD, and deviations of Zr, from second-order behavior in normals, can best be explained by a two-compartment parallel model, in which time-constant discrepancies between the lung parenchyma and compliant airways keep compliant greater than inertial reactance, resulting in a more negative phase angle as frequency is increased.
INTRODUCTIONThe description of the respiratory system in terms of electrical analogs (1) by Otis et al. (2) and others (3-15),
