Journal of Applied Physiology, Vol 68, Issue 2 665-671, Copyright © 1990 by American Physiological Society
Respiratory impedance to ambient pressure changes at low frequencies
R. Peslin, C. Duvivier, B. Suki, R. Farre, E. Oostveen and C. Gallina
Unite 14 Institut National de la Sante et de la Recherche Medicale, Physiopathologie Respiratoire, Vandoeuvre-les-Nancy, France.
Respiratory impedance may be studied by measuring airway flow (Vaw) when
pressure is varied at the mouth (input impedance) or around the chest
(transfer impedance). A third possibility, which had not been investigated
so far, is to apply pressure variations simultaneously at the two places,
that is to vary ambient pressure (Pam). This provides respiratory impedance
to ambient pressure changes (Zapc = Vaw/Pam). In that situation airway
impedance (Zaw) and tissue impedance (Zt) are mechanically in parallel, and
both are in series with alveolar gas impedance (Zg): Zapc = Zaw + Zg +
Zaw.Zg/Zt. We assessed the frequency dependence of Zapc from 0.05 to 2 Hz
in nine normal subjects submitted to sinusoidal Pam changes of 2-4 kPa peak
to peak. The real part of Zapc (Rapc) was of 6.2 kPa.1(-1).s at 0.05 Hz and
decreased to 1.9 kPa.1(-1).s at 2 Hz. Similarly the effective compliance
(Capc), computed from the imaginary part of Zapc, decreased from 0.045
1.kPa-1 at 0.05 Hz to 0.027 1.kPa-1 at 2 Hz. Breathing against an added
resistance of 0.46 kPa.1(-1).s exaggerated the negative frequency
dependence of both Rapc and Capc. When values of airway resistance and
inertance derived from transfer impedance data were introduced, Zapc was
used to compute effective tissue resistance (Rt) and compliance (Ct). Rt
was found to decrease from 0.32 to 0.15 kPa.1(-1).s and Ct from 1.11 to
0.64 1.kPa-1 between 0.25 and 2 Hz. Ct was slightly lower with the added
resistance. These results are in good agreement with the data obtained by
other approaches.
