Errors in the measurement of total respiratory resistance and reactance by forced oscillations

The total respiratory resistance determined by means of the forced oscillation technique during spontaneous breathing demonstrates a variability which may be due (1) to variations of the resistance itself, (2) to the superposition of rapid oscillatory and slow respiratory signals, (3) to the presence in the breathing signals of harmonics the frequency of which is the same as that of the oscillations. In the present study we investigate the importance of the third cause of variability, in a mechanical system in which causes 1 and 2 have been excluded. It is shown that the presence of high frequency components in breathing is an important source of error in the measurement of instantaneous resistance and reactance. The error is larger at higher frequency and amplitude of breathing, lower frequency and amplitude of forced oscillations, and in the presence of a high respiratory impedance. The error is likely to be negligible when the frequency of the forced oscillation is high (e.g. 30 Hz). At lower frequencies, theinfluence of harmonies in breathing can be corrected by calculating average resistance or reactance values over one or more respiratory cycles.