Assessment of bronchial reactivity by forced oscillation admittance avoids the upper airway artefact

Airway obstruction, upper airway artifact and response to bronchodilator in asthmatic and healthy children

BACKGROUND

The forced oscillation technique (FOT) may be affected by the upper airway artifact in children with airway obstruction. The aim was to test the impact of the artifact on FOT ability to diagnose asthma in children as recommended by current guidelines.

METHODS

The FOT was performed in 58 asthmatics and 27 controls before and after salbutamol. Respiratory resistance (Rrs) was measured at 8 Hz with a standard generator (SG), and a head generator (HG) that minimizes the upper airway artifact. The response to salbutamol was computed as change in respiratory resistance (Δ%Rrs) and admittance (Δ%Ars) that almost cancels the effect of the upper airway artifact.

RESULTS

Rrs and Δ%Rrs were significantly larger in patients than controls by HG (respectively P < 0.001 and P = 0.002) but not SG, while Δ%Ars was larger in asthma than control by both (P < 0.04). Best discriminators between patients and controls were Rrs or Δ%Rrs by HG and Δ%Ars by SG.

CONCLUSION

In asthmatic children, the upper airway artifact significantly impacts FOT measurements. The diagnostic value may be improved by minimization of the shunt, such as the computation of Δ%Ars.

Regional differences in bronchial reactivity assessed by respiratory impedance

We used the Impulse Oscillometric System (IOS) to gain information concerning the distribution of hyper-reactivity along the bronchial tree during methacholine challenge test (MCT). 37 subjects underwent MCT until reaching the provocative dose (PD20). At each dose, we estimated respiratory resistance at 5 and 20Hz (R5, R20), and reactance at 5Hz (X5). In non-responsive subjects (N=14) no changes in R5, R20, and X5 were observed during MCT. In responsive subjects, a wide spectrum of responses was found concerning frequency dependence and PD20. We describe two phenotypes representing the extremes of response. For PD20>400μg (N=13), MCT caused equal changes of resistance/reactance on varying oscillation frequencies, suggesting a homogeneous bronchoconstriction along the bronchial tree. For PD20<200μg (N=10), a remarkable frequency dependence was observed, with increase in R5, no change in R20, and decrease in X5, suggesting hyper-responsiveness of the distal airways paralleled by a change in visco-elastic properties of lung parenchyma.

Influence of cheek support on respiratory impedance measured by forced oscillation technique

The forced oscillation technique (FOT) is a useful tool to assess respiratory resistance and reactance during tidal breathing in patients with respiratory diseases, specifically asthma and chronic obstructive pulmonary disease. Although the FOT has been clinically used, results of respiratory impedance can be affected by various factors such as upper airway artifact. We investigated the effects of cheek support on respiratory resistance and reactance measured by a commercially available FOT equipment MostGraph-01. Respiratory resistance at 20 Hz (R20) with support of the cheeks was significantly higher than those without the cheek support in healthy subjects. Two different cheek support protocols, support of the cheeks by subjects themselves and an operator, were compared in healthy volunteers and patients with respiratory diseases. The cheek support protocols significantly affected respiratory resistance at 5 Hz (R5) and reactance at 5 Hz (X5) in the patient group but not in the healthy subjects. Moreover, for X5, there was a significant interaction between cheek support protocols (by a subject or operator) and groups (healthy or diseased). In conclusion, during impedance measurements using the FOT, application of cheek support either by subjects or the operator is recommended to reduce upper airway artifacts, however, results obtained by two protocols may be different in patients with respiratory diseases. Contribution of the chest wall and position of the arms to the mechanical properties should be carefully considered in physiological studies in which the FOT is attempted.

Clinical investigation of respiratory system admittance in preschool children

INTRODUCTION

The upper airway shunt attenuates measurements of respiratory system impedance (Zrs), with greater impact in young children. Changes in respiratory system admittance, Ars (or Zrs(-1)), are theoretically independent of the shunt. This study compared the ability of Ars, to standard oscillatory outcomes, to determine respiratory disease and differentiate responses to inhaled bronchial challenges in the clinical setting.

METHODS

The forced oscillation technique (FOT) was used to establish reference equations for Ars in healthy preschool children, compare the change in Ars to standard oscillatory outcomes during bronchial challenge with inhaled adenosine-5'-monophosphate (AMP) and to inhaled bronchodilator in healthy children and those with respiratory disease.

RESULTS

Children with respiratory disease had lower baseline Ars than healthy children (P < 0.05). However, there was no improved ability for Ars to differentiate between bronchodilator responses in healthy and disease populations. In contrast, the response to inhaled AMP occurred at a lower concentration, [25 (3.12-400) mg ml(-1); median (10th-90th centile)], as measured by Ars when compared to respiratory system resistance [225 (6.25-400) mg ml(-1); P = 0.016].

CONCLUSION

This study supports the use of Ars during inhaled challenges, but not in response to bronchodilation.

Evaluation of impulse oscillometry during bronchial challenge testing in children

BACKGROUND

The impulse oscillation system (IOS) allows easy measurement of respiratory system impedance (Zrs). The aim of this retrospective study was to evaluate the accuracy of IOS parameters obtained during methacholine challenge by comparison with "the gold standard" forced expiratory volume in the first second (FEV1).

METHODS

Measurements of FEV1 and resistances at 5 and 20 Hz, reactance at 5 Hz, impedance at 5 Hz and resonant frequency were performed in 227 children with suspected asthma, before and during methacholine challenge. Data were analyzed in the overall population and in three subgroups according to the final diagnosis: asthma (n = 72), chronic cough and nonspecific respiratory symptoms (n = 122), allergic rhinitis (n = 33).

RESULTS

All IOS parameters changed significantly during the tests but only changes in X5 were significantly different between responders and nonresponders. Moreover, changes in IOS parameters were not correlated with changes in FEV1 apart from a weak correlation for X5. The receiver operating characteristic (ROC) curve for changes in X5 (to predict a 20% decrease in FEV1 showed a best decision level for a 50% decrease in X5 with a sensitivity of 36% and a specificity of 85%. Results were not different in the asthma group.

CONCLUSION

The accuracy of measurements by IOS during methacholine bronchial challenge in children was not suitable when compared with FEV1 . It could be assumed that spirometry and IOS, while both providing indirect indices of airway patency, are exploring different mechanisms, each with its own methodological potentials and limitations.

Effects of a single dose inhaled corticosteroid on the dynamics of airway obstruction after exercise

RATIONALE

Exercise-induced bronchoconstriction (EIB) is defined as a transient narrowing of the airways induced by exercise. Repetitive measurements of spirometric parameters, such as FEV(1) and expiratory flows, and forced oscillation technique (FOT) measurements can be used to analyze the dynamics of EIB. A single high dose of fluticasone propionate (FP) protects against EIB. The aim of the study was to analyze the effect of FP on the dynamics of exercise-induced airway narrowing as measured with FOT and spirometry.

METHODS

Twelve children performed an exercise challenge on 2 separate days, 4 hr after inhalation of 1 mg FP (pressurized metered dose inhaler) or a placebo. Before and after the exercise flow-volume loops as well as the FOT (frequency range: 4-32 Hz) were measured.

RESULTS

The FEV(1) , and FEF(50) fell significantly after exercise within groups; the peak fall in FEV(1) after FP was significantly smaller than after placebo (respectively, 19.3 ± 14.6% and 29.2 ± 14.8%, P = 0.03, 95% CI: 0.9-18.8%). The fall in FEV(1) and FEF(50) peaked 3 min after exercise and showed a subsequent partial recovery. The fall in the FEV(1) /FVC ratio showed a later peak fall (12 min after exercise). The resistance increased while the reactance decreased significantly after exercise. FP significantly decreased the maximal increase in Rrs(6) when compared to the placebo (respectively 176.5 ± 59.1% and 201.0 ± 63.8%, P = 0.05, 95% CI: 0.5-48.7%). The maximal decrease in Xrs(6) was not significantly affected by FP (P = 0.06).

CONCLUSION

Repetitive spirometric and FOT measurements after exercise show a rapid narrowing and steady recovery of the patency of the conducting airways, and indicate a delayed and prolonged recovery of the smaller airways. A single high dose of inhaled FP seems to employ its effect mainly in the conducting airways.

The site of airway obstruction among patients of emphysema: role of impulse oscillometry

Objectives:

To detect the site of airway obstruction in patients of emphysema by impulse oscillometry (IOS) and to compare its observatios with flow volume loop (FV loop) studies.

Methods:

Twenty-five patients of emphysema were subjected to both impulse oscillometry (IOS) and conventional spirometry. The resulting impedance spectra by IOS and FV loop were utilized to identify the site of airway obstruction in each of the patients. Both methods applied were also compared for sensitivity to identify airway and specify the site of obstruction.

Results:

Four patients were excluded from the final analysis as their impedance spectra showed significant upper airway influence, which would have made the localization of the site of airway obstruction by IOS invalid. Both IOS and FV loop could detect airway obstruction in all 21 patients. FV loop localized the site of obstruction as combined central and peripheral airways in 15 patients (71.4%) and as peripheral in 6 (28.6%).The IOS however, revealed the presence of the predominant site of obstruction peripheral in all the 21 patients, and both central and peripheral obstruction in 5 patients (23.8%).

Conclusions:

IOS had proven to be advantageous over FV loop studies as it could identify central and peripheral airway obstruction separately and established the predominant site of obstruction.

Fractional order model parameters for the respiratory input impedance in healthy and in asthmatic children

This paper provides an evaluation of a fractional order model for the respiratory input impedance, using two groups of subjects, respectively healthy and asthmatic children. The purpose is to verify if the model is able to deliver statistically meaningful parameter values in order to classify the two groups. The data are gathered with the non-invasive lung function test of forced oscillations technique, by means of a multisine signal within the 4-48Hz frequency range. Based on our previous work, a fractional order model for this range of frequencies is obtained. Additional parameters are proposed to evaluate the two groups. The results indicate that the model was unable to detect significant changes between the asthmatic children with normal spirometry results (as result of medication) and the healthy children. Due to medication intake during the hours prior to the exam, bronchial challenge did not modify substantially the respiratory parameters. Our findings correspond to similar studies reported in the specialized literature. Combined model parameters, such as the tissue damping and the tissue elastance were significantly different in the two groups (p<0.01). Two extra indexes are introduced: the quality factor and the power factor, providing significantly different results between the two groups (p≪0.01). We conclude that the model can be used in the respective frequency range to characterize the two groups efficiently.

Airway response to exercise by forced oscillations in asthmatic children

Forced expiratory volume in 1 s (FEV1) detection of exercise-induced bronchoconstriction (EIB) to identify asthma has good specificity but rather low sensitivity. The aim was to test whether sensitivity may be improved by measuring respiratory resistance (Rrs) by the forced oscillation technique (FOT). Forty-seven asthmatic and 50 control children (5-12 y) were studied before and after running 6 min on a treadmill. Rrs in inspiration (Rrsi) and expiration (Rrse), FEV1 and Rrsi response to a deep inhalation (DI) were measured before and after exercise. In asthmatics versus controls, exercise induced significantly larger increases in Rrsi (p < 0.001) and larger decreases in FEV1 (p = 0.004). Asthmatics but not controls showed more bronchodilation by DI after exercise (p = 0.02). At specificity >0.90, sensitivity was 0.53 with 25% increase Rrsi and 0.45 with 27% increase Rrse or 5% decrease FEV1. It is concluded that the FOT improves sensitivity of exercise challenge, and the Rrsi response to DI may prove useful in identifying the mechanism of airway obstruction.

Identification of bronchodilator responsiveness by forced oscillation admittance in children

The upper airway wall motion may be responsible for significant error when measuring respiratory resistance (Rrs) with the forced oscillation technique (FOT), particularly in young children with airway obstruction. Assessing the response to methacholine from the change in respiratory admittance (Ars, the reciprocal of respiratory impedance, Zrs) avoids the artifact. The aim of the study was to explore the possibility of assessing the response to a bronchodilator from the change in Ars. FOT measurements were performed at 12 Hz in 36 young children before and after salbutamol. Transrespiratory pressure was varied in two ways: directly at the mouth (standard generator, SG) and around the head (head generator, HG), a variant nearly free of upper airway artifact. Salbutamol induced significantly lower Rrs and Zrs change with SG than HG (respectively, p = 0.0003 and 0.05). The relative change in Ars was not significantly different with SG and HG. Both estimates were significantly correlated (p < 0.0001). The change in Ars may thus be useful to avoid the upper airway artefact when assessing the response to salbutamol using the FOT in young children.

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.

Respiratory input impedance measurement: forced oscillation methods

The paper reviews how forced oscillation techniques (FOT) for measuring respiratory input impedance Zrs,in have recently been used in clinical applications. Zrs,in is clinically relevant, as it provides data on both the resistive, Rrs, and nonresistive, Xrs, components of the respiratory system. Additionally, when excitatory test signals extending into low- (<4 Hz) or high-frequency (>100 Hz) ranges are used, reliable partitioning of lung tissue from airway components is feasible. Adult and paediatric studies examining the use of Zrs,in for routine lung-function assessment, sleep and mechanical ventilation are reviewed. For clinicians, Zrs,in repeatable and sensitive to airway resistance. It is helpful for assessing unco-operative and severely obstructed patients, for monitoring mechanics during artificial ventilation and for tracking airway closure during sleep studies. For paediatricians, longitudinal studies of the growth and development of the respiratory system can also be made using Zrs,in. Forced oscillation techniques, however, require further standardisation, and Zrs,in is limited by upper-airway shunt artifacts. In conclusion, measurement of Zrs,in using FOT is an important and sophisticated non-invasive lung-function test, showing good potential for future clinical applications.

Clinical applications of forced oscillation techniques (FOT) in patients with bronchial asthma

BACKGROUND

Forced oscillation technique (FOT) is a method to characterize the mechanical properties of the respiratory system over a wide range of frequencies. Its most important advantage is to require minimal cooperation from the subject. This study was performed to evaluate the usefulness of the FOT applications in patients with bronchial asthma by estimating the associations between asthma severity and FOT parameters, and the relationships between FOT and spirometry parameters.

METHODS

216 patients with asthma were enrolled in this study. Patients were classified into 3 different groups according to their symptoms and pulmonary functions. Respiratory impedance, resistance (at 5 Hz, 20 Hz, 35 Hz) and resonant frequency were measured by FOT. FEV1, FVC and MMEF were measured with conventional spirometry.

RESULTS

There were significant differences of resonant frequency, resistance at 5 Hz and 20 Hz, resistance difference at 5 Hz and 20 Hz according to asthma severity (p < 0.05, respectively). Resonant frequency, resistance at 5 Hz, and impedance were significantly correlated with FEV1 (r = -0.55, -0.48, -0.49, p < 0.05, respectively), and with MMEF in patients with normal pulmonary function (r = -0.37, -0.35, -0.34, p < 0.05, respectively). Resistance at 5 Hz had similar reproducibility compared to FEV1 (resistance at 5 Hz, r = 0.78 vs FEV1, r = 0.79).

CONCLUSION

FOT is a useful and alternative method to evaluate the clinical status of bronchial asthma. Further studies will be needed to clarify its value for a wide range of clinical applications.