Journal of Applied Physiology, Vol 70, Issue 1 143-151, Copyright © 1991 by American Physiological Society
Shunt effect of gas compression inside pneumotachographs during forced oscillations
B. Louis, A. Harf, H. Lorino and D. Isabey
Institut National de la Sante et de la Recherche Medicale, U.296, Creteil, France.
Determination of the frequency response of pneumotachographs is needed
whenever they are used to measure high-frequency flows, such as in the
forced oscillation method. When screen and capillary pneumotachographs are
calibrated using an adiabatic compression in a closed box as a reference
impedance, they can be adequately described by a series of
inertial-resistive elements. However, this type of reference impedance
strongly differs from the actual respiratory impedance (ZL). We studied the
frequency response of pneumotachographs up to 250 Hz in reference to the
impedance of a compressible gas oscillating in a long tube, taken as a more
generalizable model of actual ZL. We found that, with this device, the
series resistance-inertance models fail to describe the frequency response
of the pneumotachograph. However, when compressible effects in the
pneumotachograph are taken into account by adding to the resistive models a
compliance (Cpn) corresponding to the compression in half of the inner
volume of the pneumotachograph, the agreement with experiments becomes
satisfactory. Gas compression-related phenomena were demonstrated to be
negligible only when the parameter omega Cpn magnitude of ZL is much
smaller than 1 (omega pulsation). Results obtained in normal humans have
shown that such a correction is required above 100 Hz. Similar correction
at lower frequency might also be necessary in cases of large respiratory
impedance (e.g., babies, subjects with pathological lungs, and intubated
subjects).
