High-frequency respiratory input impedance measurements in infants assessed by the high speed interrupter technique

Technical standards for respiratory oscillometry

Oscillometry (also known as the forced oscillation technique) measures the mechanical properties of the respiratory system (upper and intrathoracic airways, lung tissue and chest wall) during quiet tidal breathing, by the application of an oscillating pressure signal (input or forcing signal), most commonly at the mouth. With increased clinical and research use, it is critical that all technical details of the hardware design, signal processing and analyses, and testing protocols are transparent and clearly reported to allow standardisation, comparison and replication of clinical and research studies. Because of this need, an update of the 2003 European Respiratory Society (ERS) technical standards document was produced by an ERS task force of experts who are active in clinical oscillometry research.The aim of the task force was to provide technical recommendations regarding oscillometry measurement including hardware, software, testing protocols and quality control.The main changes in this update, compared with the 2003 ERS task force document are 1) new quality control procedures which reflect use of "within-breath" analysis, and methods of handling artefacts; 2) recommendation to disclose signal processing, quality control, artefact handling and breathing protocols (e.g. number and duration of acquisitions) in reports and publications to allow comparability and replication between devices and laboratories; 3) a summary review of new data to support threshold values for bronchodilator and bronchial challenge tests; and 4) updated list of predicted impedance values in adults and children.

Computer assessment of indirect insight during an airflow interrupter maneuver of breathing

The paper answers the questions if it is possible to conclude in objective way on more (than one -Rint - in a classical IT) number of parameters from the time domain post-interrupter signals during the occlusional measurement of respiratory mechanics and also verifies what accuracy can be achieved in such attempt. To obtain reported results, the time-domain enhanced interrupter technique (TD-EIT) was developed in this paper using computer simulations. Three-stage scheme of work was assumed in the project. First, the quality of the model identification was assessed for various combinations of pressure and flow signals recorded during the interruption. Then, the correlation between the working characteristics of the interrupter valve and the precision of the parameter estimation were assessed for the TD-EIT algorithm. Finally, a verification experiment by forward-inverse modeling was organized, in which the mechanical characteristics of a complex model were mapped with reduced analogs and with the use of neural networks for three typical modes: 'Normal state', 'Airway constriction' and 'Cheeks supported'. Obtained results show that to became effective in time-domain post-interrupter data exploration, both pressure and flow signals should be used in assessment of respiratory mechanics, taken in a range of at least 100ms and when both slopes (valve closing and opening) of quasi-step excitation are included. What is more, the faster the valve the smaller error of parameter estimation in proposed TD-EIT was observed, and this uncertainty importantly falls down for the length of time window exceeding the limit of 100ms. The pioneering use of neural network for mapping the mechanical properties of lungs with the use of interrupter experiment methodology proves that it is possible to conclude about more (than one) number of parameters characterizing the complex system and that this insight is biased with the error not exceeding of 10%; only peripheral properties are estimated worse. Such observation has a potential to change the experimental protocol, which was used in interrupter measurements up to date and to make this technique more attractive in comparison to other method, i.e. forced oscillation technique or impulse oscillometry. As regards the practical meaning of reported results for engineers and end-users (physicians and patients), proposed solution can be applied in simple portable devices with a feature of easy operation (important for e-monitoring).

Preliminary study on the accuracy of respiratory input impedance measurement using the interrupter technique

Respiratory input impedance contains information about the state of pulmonary mechanics in the frequency domain. In this paper the possibility of respiratory impedance measurement by interrupter technique as well as the accuracy of this approach are assessed. Transient states of flow and pressure recorded during expiratory flow interruption are simulated with a complex, linear model for the respiratory system and then used to calculate the impedance, including three states of respiratory mechanics and the influence of the measurement noise. The results of computations are compared to the known, theoretical impedance of the model. At 1 kHz sampling rate, the optimal time window lays between 100 and 200 ms and is centred around the pressure jump caused by the flow interruption. The proposed algorithm yields satisfactory accuracy in the range from 10 to 400 Hz, particularly to 150 Hz. Depending on the simulated respiratory system state, the error of calculated impedance (relative Euclidean distance between the vectors of computed and theoretical values), for the window of 190 ms, varies between 5.0% and 7.1%.

Wearable interrupter module for home-based applications in a telemedical system dedicated to respiratory mechanics measurements

The mobile interrupter module, dedicated to the enhanced interrupter (EIT) measurement of respiratory mechanics in a home environment and capable of cooperation with a telemedical system, is presented. Characterized by noninvasiveness and minimal requirements regarding patient cooperation, the EIT algorithm is especially suitable for newborns, preschool children, and patients suffering from respiratory muscle impairment. Furthermore, this device enables access to raw data--without initial preprocessing--in a fully flexible measurement protocol (which is not available in any commercial apparatus), and the EIT procedure improves insight (the number and precision of assessed parameters) into the physiological system with respect to the classical occlusive methods.

Interpretation of spirometry through signal analysis

Chronic obstructive pulmonary disease is attaining alarming proportions that requires more objective and quantitative ways for the diagnosis and the evaluation / stratification of, both, the disease and the therapeutic outcomes. Within this context, the present study explores the possibility to increase the effectiveness of spirometry through signal analysis. Expiratory flow results from converging airflows at different levels of airway branching. Furthermore, along a branching network of air conduits, the characteristics of converging air currents determine those of the resulting air flow. Thus, for the human bronchial tree, the characteristics of air currents within the smaller branches are, ideally, conserved at the expiratory flow recorded at the mouth. This makes it theoretically possible to use signal analysis methodologies in order to identify the characteristics of airflow along the different levels of the respiratory tree. The present study reports on an attempt to identify alterations non-invasively in the frequency spectrum of the first derivative of the Forced Vital Capacity curve of patients presenting with different respiratory conditions. Such alterations can be attributed to the onset and operation of the airway closure phenomenon that limits airflow, during forced expiration. Fundamental to the design of the study was the notion that the forced expiratory output of the respiratory system is determined by the bronchial tree and the upper respiratory tract. These two entities shape the air flow that is expelled from the collective airspace of the bronchial tree subdivisions distal to the terminal bronchi. At the end we were able to identify simple measures that are derived from the power spectrum of the derivative of the spirometric curve that permit the definition of specific filters and allow for the accurate classification of, at least, the basic types of respiratory disease.

Lung function tests in neonates and infants with chronic lung disease: lung and chest-wall mechanics

This is the fifth paper in a review series that summarizes available data and critically discusses the potential role of lung function testing in infants and young children with acute neonatal respiratory disorders and chronic lung disease of infancy (CLDI). This review focuses on respiratory mechanics, including chest-wall and tissue mechanics, obtained in the intensive care setting and in infants during unassisted breathing. Following orientation of the reader to the subject area, we focused comments on areas of enquiry proposed in the introductory paper to this series. The quality of the published literature is reviewed critically with respect to relevant methods, equipment and study design, limitations and strengths of different techniques, and availability and appropriateness of reference data. Recommendations to guide future investigations in this field are provided. Numerous different methods have been used to assess respiratory mechanics with the aims of describing pulmonary status in preterm infants and assessing the effect of therapeutic interventions such as surfactant treatment, antenatal or postnatal steroids, or bronchodilator treatment. Interpretation of many of these studies is limited because lung volume was not measured simultaneously. In addition, populations are not comparable, and the number of infants studied has generally been small. Nevertheless, results appear to support the pathophysiological concept that immaturity of the lung leads to impaired lung function, which may improve with growth and development, irrespective of the diagnosis of chronic lung disease. To fully understand the impact of immaturity on the developing lung, it is unlikely that a single parameter such as respiratory compliance or resistance will accurately describe underlying changes. Assessment of respiratory mechanics will have to be supplemented by assessment of lung volume and airway function. New methods such as the low-frequency forced oscillation technique, which differentiate the tissue and airway components of respiratory mechanics, are likely to require further development before they can be of clinical significance.

Broadband frequency dependence of respiratory impedance in rats

Past studies in humans and other species have revealed the presence of resonances and antiresonances, i.e., minima and maxima in respiratory system impedance (Zrs), at frequencies much higher than those commonly employed in clinical applications of the forced oscillation technique (FOT). To help understand the mechanisms behind the first occurrence of antiresonance in the Zrs spectrum, the frequency response of the rat was studied by using FOT at both low and high frequencies. We measured Zrs in both Wistar and PVG/c rats using the wave tube technique, with a FOT signal ranging from 2 to 900 Hz. We then compared the high-frequency parameters, i.e., the first antiresonant frequency (far,1) and the resistive part of Zrs at that frequency [Rrs(far,1)], with parameters obtained by fitting a modified constant-phase model to low-frequency Zrs spectra. The far,1 was 570 ± 43 (SD) Hz and 456 ± 16 Hz in Wistar and PVG/c rats, respectively, and it did not shift with respiratory gases of different densities (air, heliox, and a mixture of SF6). The far,1 and Rrs(far,1) were relatively independent of methacholine-induced bronchoconstriction but changed significantly with increasing transrespiratory pressures up to 20 cmH2O, in the same way as airway resistance but independently of changes to tissue parameters. These results suggest that, unlike the human situation, the first antiresonance in the rat is not primarily dependent on the acoustic dimensions of the respiratory system and can be explained by interactions between compliances and inertances localized to the airways, but this most likely does not include airway wall compliance.

Variability analysis of oscillatory airway resistance in children

Testing pulmonary function in preschool children is problematic since individual cooperation is often insufficient to yield adequate results. This is especially the case when repetitive measurements are carried out, e.g., during provocation tests in order to diagnose bronchial hyperresponsiveness. We therefore sought to determine whether the variability of the oscillatory airway resistance, which can be measured with minimal patient cooperation, might serve as an approach to separate asymptomatic children with asthma from age-matched healthy controls. Monofrequent forced oscillation technique was used to measure the oscillatory resistance (Ros) continuously at a repetition rate of 10 Hz. Forty consecutive values of Ros at the end of expiration (Ros(pe)) and at the end of inspiration (Ros(pi)) were used to calculate variability measures [standard deviation (SD), averaged deviation (LD), root mean square successive differences (RMSSD)] known from heart rate variability analysis to describe the airway diameter variability in 36 asthmatics and 21 healthy controls. Airway diameter variability of Ros(pi) and Ros(pe) as expressed by SD from the mean (P = 0.01 and 0.05, respectively), from LD (P = 0.01 and 0.21, respectively) and from the RMSSD (P = 0.006 and 0.03, respectively) was greater in patients with asthma. Mean values of Ros(pi) and Ros(pe) were not different between groups. Airway diameter variability is higher in asymptomatic children with asthma than in healthy controls. Future research should evaluate whether the measurement of the variability of Ros(pi), which does not require the patient's cooperation, can give similar information as challenge tests with respect to bronchial hyperresponsiveness.

The response to β-agonists in wheezy infants: three methods compared

BACKGROUND

Studies into the effects of salbutamol in the treatment of wheeze in infancy have been conflicting, possibly due to differences in outcome variables. We aimed to assess the response to salbutamol using indices derived from passive and forced expiration.

METHODS

We recruited 39 infants who had a history of wheezing (mean age 43 weeks) and measured maximum flow at functional residual capacity (V'(max FRC)) by rapid thoracoabdominal compression (RTC), and forced expired volume at 0.4s (FEV0.4) using the raised-volume RTC technique (RV-RTC). We calculated passive compliance (C(rs)), resistance (R(rs)) and time constant (tau) from relaxed expirations that followed the augmented inspirations delivered during RV-RTC. Measurements were repeated after aerosol salbutamol (800 mcg).

RESULTS

Data were obtained in 32 infants for V'(max FRC), 22 for FEV0.4 and 19 for passive mechanics. There were no mean changes in any index of forced expiration after salbutamol. Some individuals showed significant changes (improvement or worsening) in one or other index. Overall, there was a small increase in C(rs) after salbutamol but no change in R(rs) or tau.

CONCLUSIONS

We found no consistent pattern of response in either index of forced expiration. Validated clinical scores or alternative physiological techniques may be preferable to respiratory mechanics in assessing bronchodilator response.

Clinical application of forced oscillation

This review summarizes current clinical use of the forced oscillation technique (FOT) for analysis of lung function. It presents an intuitive approach to FOT pattern recognition for interpretation of results in human subjects, and the view that FOT is now well established and, clinically, eminently useful in patients with airflow obstruction. The focus of this review is on findings that relate directly to clinical utility, with less emphasis on theoretical mechanisms. The major thrust for clinical application of FOT derives from a number of European clinical research centers. Farre and Navajas and their colleagues in Barcelona, Harf and the Lorinos and their coworkers in Paris, Peslin and Duvivier and their coworkers in Vandoeuvre-les-Nancy, Pride and coworkers in London, and Van de Woestijne, Clement, Demedts, Landser, Van Noord, and their colleagues in Leuven have essentially been responsible for clinical development of FOT over the past 25 years. Publishing space does not permit an exhaustive listing of the many contributions of these investigators, but it is intended that the present review will provide a useful infrastructure from which the reader may progress to other research citations as desired.

Lung function tests for pre-school children

There are a few devices for measuring lung function in pre-school children. Neither the interrupter technique nor the forced oscillation techniques have been standardised. We highlight some of the issues around the measurement of airway resistance using the interrupter technique and emphasise that, as with other lung function measurements, operators should have a proper understanding of the methods before they can be applied. Both methods for measuring airway resistance have potential for clinical and research application.

Alterations in airway wall properties in infants with a history of wheezing disorders

Airway diameter and airway wall mechanics (compliance) are important determinants of flow limitation and wheezing. We have previously used the high-speed interrupter technique (HIT) to measure input impedance (Zin) in infants at frequencies up to 900 Hz, including antiresonance phenomena, which are known to be related to wave propagation velocity, and have shown that the frequency at which the first antiresonance occurs (f(ar,1)) is a function of airway wall compliance. We aimed to determine whether f(ar,1) (and thus airway wall compliance) was different in infants with a history of wheezing disorders. We compared 23 asymptomatic infants (aged 36 to 81 wk) with a history of wheezing with an age-matched group of 19 healthy control infants. We found that f(ar,1) was significantly lower in infants with wheezing disorders than in the control group (p < 0. 005), implying differences in airway wall compliance, even when they were clinically asymptomatic. Developmental differences in airway wall mechanics may be important in the pathogenesis of wheezing disorders or, alternatively, alterations in airway wall mechanics might be a consequence of postinflammatory remodeling.