Separation of airway and tissue properties by transfer respiratory impedance and thoracic gas volume in reversible airway obstruction

Abstract

The aim of this study was to establish the ability to estimate separate airway and tissue properties from transfer respiratory impedance (Zrs,tr) data in the presence of airway obstruction.Zrs,tr, thoracic gas volume (TGV) and airway resistance (Raw,pleth) were measured in the presence of obstruction and after use of a bronchodilator (BD) in 13 normal or asymptomatic asthmatic adults and 28 children with symptomatic asthma. An analytical approach was used to solve the equations of a simplified variant of DuBois' model, including airway resistance (Raw*) and inertance (Iaw), tissue compliance (Ct) and resistance (Rt) and pulmonary gas compliance (Cg).The equations of the model could not be reliably solved in four children before BD. Mean Raw,pleth was not different from mean Raw* in adults before (mean±SEM) (3.4±0.5 vs 3.1±0.3 hPa·s·L -1 ) or after BD (1.4±0.2 vs 1.8±0.2 hPa·s·L -1 ), or in children after BD (2.9±0.3 vs 3.2±0.2 hPa·s·L -1 , respectively). In children before BD, Raw* was significantly underestimated compared with Raw,pleth (3.8±0.4 vs 5.4±0.6 hPa·s·L -1 ). Overall, a significant positive correlation was found between the difference [Raw,pleth -Raw*] and Raw,pleth (r=0.82). In adults, BD induced a decrease in Raw* and Rt, an increase in Ct, and no change in Iaw. In children after BD, there was no significant change in Raw* or Ct, whilst Rt decreased and Iaw increased.Taking Raw,pleth as the gold standard, it is concluded that coherent estimation of parameters of DuBois' model may be obtained from combined Zrs,tr and TGV measurements in normal subjects and moderately obstructed adults, but not in children with significant airway obstruction. This seems to be due to the systematic underestimation of Raw*. Eur Respir J., 1996, 9, 253-261 Respiratory input impedance (Zrs,in) measured by the forced oscillation technique is becoming increasingly popular in lung function testing, and provides a convenient, noninvasive estimation of respiratory system resistive and reactive properties. Another interesting approach is to obtain the transfer impedance of the respiratory system (Zrs,tr) by varying pressure at the chest wall and measuring flow at the mouth. The advantage of this technique is twofold. Firstly, pressure variations in the pneumotachograph are small and the accuracy of flow measurement at high frequency does not critically depend on the symmetry of the flow channel. This is particularly important when large resistances are to be measured [1], which is the case of children with airway obstruction. Secondly, Zrs,tr is significantly influenced by pulmonary gas compressibility (Cg) described in DuBois' model [2].The parameters of airway and tissue impedance were estimated from Zrs,tr in normal adult humans, using Cg value provided by thoracic gas volume (TGV) measurement [3]. With comparative measurements of Zrs,in and Zrs,tr it was found that the second, but not the first, approach could provide a correct estimation of airway and tissue parameters [4]. It was also shown in healthy dogs and ...

Respiratory input impedance measurement: Forced oscillation methods

“…Complex pathophysiological phenomena underlie changes in Z~ in disease states such as COPD (LoRrNO et al, 1997;MARCHAL et al, 1996;LUTCHEN et al, 1994;1996). Numerous anatomical and pathophysiological aspects of respiratory system behaviour are responsible for this (FINUCANE et al, 1975;MACLEOD, 1997).…”

“…Zrs, t r is also less sensitive to the influence of the upper-airway shunt Z ..... because of the less direct importance ofZ,, w in the mechanical network presented by the respiratory system to the forcing signal . Z~,t ~ has good sensitivity to airflow obstruction and bronchodilator responses in chronic obstructive pulmonary disease (COPD) and asthma (MARCHAL et al, 1996). Compared with Z~,i .…”

An Official American Thoracic Society/European Respiratory Society Statement: Pulmonary Function Testing in Preschool Children

“…The technique has the advantage of eliminating most of the upper airway artifact (see below) and separating airways and tissues impedances. Its application in children has been limited mostly to research studies (170,171). In the standard input impedance technique, Zrs is defined as the relationship between transrespiratory pressure and the airway V .…”

Deep Inhalation Prevents the Respiratory Elastance Response to Methacholine in Rats

“…One important end point in this study was to characterize respiratory mechanics using the very same tool as in asthmatic children where the use of a single oscillation frequency is favored in our setup for the following reasons: (1) the signalto-noise ratio is optimal, (2) it provides an easy description for the time course of Zrs change after a DI, (3) studying the response of the chest from 0.5 to 10 -20 Hz, as done in experimental animals (17,23,25) is hampered by the spontaneous breathing of the child, and (4) our prior experience with model estimation of respiratory mechanical parameters from multiple frequency data has shown significant limitation in children with airway obstruction (26). On the other hand, a single frequency does not allow estimation of airway and tissue mechanics as previously described (25), and it may be questioned how much these measurements of Zrs are representative of the changes occurring in the airways and/or lung.…”