ERRORS IN 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 to: (1) variations of the resistance itself; (2) the superposition of rapid oscillatory and slow respiratory signals and (3) the presence in the breathing signals of harmonics the frequency of which is the same as that of the oscillations. The importance of the 3rd cause of variability, in a mechanical system in which causes 1 and 2 were excluded was investigated. 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 oscillations is high (e.g., 30 Hz). At lower frequencies, the influence of harmonics in breathing can be corrected by calculating average resistance or reactance values over 1 or more respiratory cycles.