Evaluation of impulse oscillation system: comparison with forced oscillation technique and body plethysmography. J. Hellinckx, M. Cauberghs, K. De Boeck, M. Demedts. #ERS Journals Ltd 2001. ABSTRACT: The impulse oscillation system (IOS) has been developed recently to measure respiratory system resistance (Rrs) and reactance (Xrs) at different frequencies up to ¢25 Hz. IOS has, however, not been validated against established techniques.This study compared IOS with the classical pseudorandom noise forced oscillation technique (FOT) and body plethysmographic airway resistance (Raw) in 49 subjects with a variety of lung disorders and a wide range of Raw (0.10-1.28 kPa?L -1 ?s). Rrs,IOS was slightly greater than Rrs,FOT, especially at lower frequencies, with a mean ¡ SD difference at 5-6 Hz of 0.14 ¡ 0.09 kPa?L -1 ?s. Comparisons with the wavetube technique applied on two analogues indicated an overestimation by IOS. Xrs,IOS and Xrs,FOT were very similar, with a slightly higher resonant frequency with IOS than with FOT (mean difference¡SD 1.35¡3.40 Hz). Raw was only moderately correlated with Rrs-FOT and Rrs-IOS; although the mean differences were small (0.04¡0.14 kPa?L -1 ?s for Rrs 6 ,FOT and -0.10¡0.14 kPa?L -1 ?s for Rrs 5 ,IOS), IOS and FOT markedly underestimated high resistance values.In conclusion, the impulse oscillation system yields respiratory system resistance and reactance values similar, but not identical to those provided by the forced oscillation technique. Eur Respir J 2001; 18: 564-570. Recently, the Jaeger impulse oscillation system (IOS, Erich Jaeger, Hoechberg, Germany) has been introduced as a user-friendly commercial version of the forced oscillation technique (FOT). IOS offers data-analysis and an elaborate report, containing total respiratory system resistance (Rrs) and reactance (Xrs) at a wide range of frequencies. It also contains estimations of central and peripheral pulmonary mechanics based on a simple model. However, only limited data have been published on this technique and these reports were mainly related to results in asthmatic and healthy children [1][2][3].The FOT was introduced by DUBOIS et al.[4] in 1956 as a method to characterize respiratory impedance and its two components, Rrs and Xrs, over a wide range of frequencies. Briefly, flow oscillations generated by means of a loudspeaker are applied at the subject9s mouth and superimposed on normal breathing. The resulting pressure signal, as well as the flow signal, are recorded and analysed. These signals are, in general, waveforms containing several frequencies. For each of these frequencies, the ratio of pressure to flow can be considered (i.e. the impedance), which is a complex number that contains information about both the ratio of the magnitude of pressure to flow and about the phase shift between these signals. Most often this complex number is represented by its real part, the respiratory resistance
Because of the contradictory statements published about the influence of the shunt properties of the upper airway on the measurements of the respiratory impedence by means of the forced oscillation technique, this influence has been reevaluated. In healthy adults and children and in patients with obstructive lung disease, the total respiratory impedance was measured by applying oscillations at the mouth (conventional technique) or around the head (head generator technique), with the cheeks either supported by the hands or not. In healthy adults the two techniques (conventional cheeks supported and head generator) yield similar results for respiratory resistance (Rrs) and a more pronounced increase of respiratory reactance (Xrs) with frequency with the head generator. In children and in patients with moderate airway obstruction, the negative frequency dependence of Rrs observed with the conventional technique tends to disappear with the head generator. This is not observed in patients with severe airway obstruction. The differences between the two techniques can be explained by the influence of the shunt impedance of the upper airway on Rrs and Xrs. Correction for this influence by subtracting the impedance measured during a Valsalva maneuver is not satisfactory, since the Valsalva maneuver itself modifies the upper airway shunt. The head generator technique reduces the influence of the upper airway shunt but does not suppress it altogether; the residual error is small, however.
Partitioning of pulmonary resistance of 15 excised human and 5 canine lungs by means of a retrograde catheter demonstrated that the share of peripheral airways (with an ID of 2.4 mm or less) and of lung tissue in pulmonary resistance was markedly larger (44-96%) in humans than in dogs (41-59%). Similar percentages were found in patients with chronic obstructive pulmonary disease (COPD). The variations of resistance with volume during deflation and inflation of the lungs were due primarily to variations of peripheral resistance (Rp). The latter systematically increased at high and low lung volumes. Higher Rp values, with a more pronounced frequency dependence, were met in patients with COPD. A morphometrical study showed an inverse relationship between the value of Rp and the mean diameter of the terminal bronchioles, provided the airways density was taken into account.
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