Correction for nonlinearity of body flow plethysmograph.

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.