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

The characterization of respiratory air flow patterns is a subject of continuous interest. The pattern of the air flow curve is an individual characteristic which remains constant over long periods of time. Basically two different approaches have been proposed to determine the individual parameters of the respiratory flow, namely the graphical characterization, which originated from visual indices and subsequently led to harmonic analysis, and the 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 reproduce the shape of the cycle and remain constant in different environmental conditions for one individual. This paper evaluates the possibilities of the Fourier approximation approach to characterize the respiratory personality. The Fourier sine coefficients which were assumed to be invariable were found to be very sensitive to changes in respiratory times. Both the amplitude and the angle of the Fourier sine transformation varied when duration of inspiration and expiration changed.