Stability of Fourier coefficients in relation to changes in respiratory air flow patterns

The characterization of respiratory air flow patterns is a subject of conti nuous interest. The pattern of the air flow curve is an individual characte ristic which remains constant over long periods of time. Basically two diff erent approaches have been proposed to determine the individual parameters of the respiratory flow, namely the graphical characterization, which origi nated from visual indices and subsequently led to harmonic analysis, and th e optimal control modeling approach. It has been suggested that respiratory personality can be accurately characterized by the Fourier coefficients of the air flow curve. The first four Fourier sine components should reproduc e the shape of the cycle and remain constant in different environmental con ditions for one individual. This paper evaluates the possibilities of the F ourier approximation approach to characterize the respiratory personality. The Fourier sine coefficients which were assumed to be invariable were foun d to be very sensitive to changes in respiratory times. Both the amplitude and the angle of the Fourier sine transformation varied when duration of in spiration and expiration changed. (C) 2001 IPEM. Published by Elsevier Scie nce Ltd. All rights reserved.