Chest
Volume 98, Issue 2, August 1990, Pages 358-364
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Variations of Respiratory Impedance with Lung Volume in Bronchial Hyperreactivity

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The total respiratory impedance was measured at various frequencies (4 to 52 Hz) with a pseudo-random-noise forced oscillation technique (FOT). The apparatus (Oscillaire) was connected with a spirometer forming a closed respiratory circuit in which gas concentrations were kept constant. Measurements were made in 15 healthy subjects (group 1) and in 30 asthmatic patients with bronchial hyperreactivity, subdivided into group 2 treated only with inhaled β2-mimetics (n = 15) and in group 3 using both β2-mimetics and steroids in inhalation (n = 15). No significant differences were found between the impedance data obtained with the Oscillaire alone and those obtained with the Oscillaire connected with the spirometer circuit. The impedance was measured at FRC level, and at FRC +1 L and -1 L. The relative changes of the resistance at 8 Hz were -23.2 percent (13.8) at FRC+1 L and +40.9 percent (29.3) at FRC-1 L relative to the values at resting FRC. This inverse relation between airway resistance and lung volume was similar in all groups. The average reactance decreased at FRC-1 L in all groups. However, at FRC + 1 L the average reactance increased 50.6 percent in group 2 and 94.2 percent in group 3, but decreased in group 1. Concomitant changes were observed in the resonant frequency and in the frequency dependence of resistance. Because of these qualitatively different responses of the impedance data to changes in lung volume (both for the whole group and for each individual) between healthy subjects and asthmatic patients, this test might be useful for the diagnosis of bronchial hyperreactivity.

Section snippets

Equipment

The measurements were performed with the apparatus (Oscillaire, Jones, United States) developed by Landser et al.9 A pseudo-random-noise signal (4 to 52 Hz in steps of 4 Hz) is superimposed on spontaneous breathing. A Fourier analysis allows the calculation of impedance values from the recordings of pressure and flow at the mouth of the subject, at each of the investigated frequencies. The impedance data are separated in a real part (resistance) and an imaginary part (reactance). To express the

RESULTS

The average values of the impedance data for each group at resting FRC level are given in Table 2. Table 3 shows the coefficients of variation of the one-day or day-to-day intraindividual reproducibility of the impedance and forced expiratory volume and flow data.

The impedance data were correlated with the flow and volume measurements that were acquired a few minutes later. Only the data with a correlation coefficient of 0.50 or higher are shown in Table 4 for each group.

No significant

DISCUSSION

The oscillatory impedance measurements are quite reproducible, especially the real part or resistance and the resonant frequency (CV: 8 to 15 percent). The imaginary part of the impedance or reactance has a lesser reproducibility that is still acceptable. The frequency dependence, however, is poorly reproducible, certainly in the patient groups.

The within-day variability expresses primarily the variability of the man-machine combination (measurement error). The intraindividual reproducibility

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