Journal of Applied Physiology, Vol 50, Issue 3 658-662, Copyright © 1981 by American Physiological Society
Correction for nonlinearity of body flow plethysmograph
A. Harf, G. Atlan, H. Lorino, S. Deshayes, C. Morin and D. Laurent
A modification of conventional signal processing for the
pressure-compensated flow plethysmograph is proposed to correct the
nonlinearity of the flow element that appears for high flows as encountered
during forced expiration. Woven screens behave as porous media with a
viscous and an inertial component in the resistance; this explains the
nonlinearity (23% at 15.1.s-1 with a 400-mesh wire screen area = 50 cm2).
It was shown that the pressure drop-flow relationship can be described by a
second-degree equation, which can be included in the computation of the
thoracic signal from the box pressure. The need for such a correction is
evidenced by testing the plethysmograph with a flow step input (0-15 l.s-1)
equivalent both in amplitude and in frequency to the thoracic flow during a
forced expiration. Such correction for nonlinearity avoids an
overestimation of the thoracic forced vital capacity of up to 0.5 liter in
normal subjects.
