MEASUREMENT OF HUMAN RESPIRATORY IMPEDANCE BY USING FORCED OSCILLATION METHOD WITH AUTO-REGRESSIVE MOVING AVERAGE MODEL

Abstract

Objective evaluation of the patient's condition is important in a clinical diagnosis. Since the frequency characteristics of the respiratory impedance represent the airway constriction, they are often used to diagnose the obstructive lung diseases. Forced oscillation technique is appropriate to measure the respiratory impedance, and has been often used. But due to the limit of the frequency response of conventional device, the frequency characteristics of human respiratory impedance could be measured only up to 20Hz. In this study, by differentiating displacement signal of the piston which excites the flow, the frequency characteristics of human respiratory impedance could be measured over the frequency range of 2-120Hz, and found to be characterized by two resonances at 8-10Hz and at 80-100Hz. Conventionally, to obtain the frequency characteristics, FFT technique was frequently used, and it required long measurement time. But since it is difficult to confine patients with advanced obstructive lung diseases to a measurement instrument for a long time, we tried to shorten the measurement time by employing the Auto-Regressive Moving Average model (ARMA model) parameter estimation technique, which made it possible to measure the frequency characteristics with a data of one second, corresponding to about one sixteenth of the conventional FFT method. In addition, by using the estimated ARMA model parameters, a 4-element equivalent electrical circuit model is derived for the human respiratory system. Four element values were measured for ten normal males, and found to be slightly lower than those of the previous reports, which had noticed only the resonance at 8-10Hz. Since the 4-element model represents the condition of the human respiratory system, the estimated values can be applied to the clinical diagnosis of lung diseases. The respiratory function test was carried out to determine the degree of constriction of human airway passages induced by cigarette smoke particle irritant aerosols. Nine normal male smokers were asked to inhale cigarette smoke deeply 15 times over 5 minutes (nicotin 1.35mg, and tar 21mg). After smoking, increases in resistance, which indicated the acute effect of airway passage constriction, were observed.