F. J. Landser

Measurement of total respiratory impedance in calves by the forced oscillation technique

The resistance (Rrs) and reactance (Xrs) of the total respiratory system were determined at various frequencies in 14 healthy conscious beagle dogs. A pseudorandom noise pressure wave was produced at the nostrils of the animals by means of a loudspeaker adapted to the nose by a tightly fitting mask. A Fourier analysis of the pressure and flow signals yielded meanRrs andXrs, over 16 s, at frequencies from 2 to 26 Hz. The influence of the posture of the dog, the position of its head, the linearity of the respiratory system, the reproducibility of the method and the effects of upper and lower airway obstructions were studied. In sitting and standing healthy dogs with the head in the extended position,Rrs values increased progressively with frequency from 5.4±0.4 (SEM) cmH2O L−1 s at 6 Hz up to 8.8±0.7 cmH2O L−1 s at 26 Hz, the mean resonant frequency being 6.1±0.5 Hz. No significant differences were observed between measurements performed with the head in the normal or the extended position. In a recumbent posture, allRrs values were increased butRrs was still dependent on the frequency in the same way (7.1±0.7 cmH2O L−1 s at 6Hz up to 10.0±0.5 cmH2O L−1 s at 26 Hz). Tracheal compression also induced higherRrs values without changes in the frequency dependence or in the resonant frequency.In anaesthetized dogs, airway obstruction was induced by inhalation of histamine (4 mg/ml for 5 min; theRrs values tended to decrease with increasing frequency, and the resonant frequency was markedly increased.

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.