Diagostic value of respiratory impedance measurements in elderly subjects

Diagnostic Potential of Oscillometry: A Population-based Approach

Rationale: Respiratory resistance (Rrs) and reactance (Xrs) as measured by oscillometry and their intrabreath changes have emerged as sensitive parameters for detecting early pathological impairments during tidal breathing. Objectives: This study evaluates the prevalence and association of abnormal oscillometry parameters with respiratory symptoms and respiratory diseases in a general adult population. Methods: A total of 7,560 subjects in the Austrian LEAD (Lung, hEart, sociAl, boDy) Study with oscillometry measurements (computed with the Resmon Pro FULL; Restech Srl) were included in this study. The presence of respiratory symptoms and doctor-diagnosed respiratory diseases was assessed using an interview-based questionnaire. Rrs and Xrs at 5 Hz, their inspiratory and expiratory components, the area above the Xrs curve, and the presence of tidal expiratory flow limitation were analyzed. Normality ranges for oscillometry parameters were defined. Measurements and Main Results: The overall prevalence of abnormal oscillometry parameters was 20%. The incidence of abnormal oscillometry increased in the presence of symptoms or diagnoses: 17% (16-18%) versus 27% (25-29%), P < 0.0001. All abnormal oscillometry parameters except Rrs at 5 Hz were significantly associated with respiratory symptoms/diseases. Significant associations were found, even in subjects with normal spirometry, with abnormal oscillometry incidence rates increasing by 6% (4-8%; P < 0.0001) in subjects with symptoms or diagnoses. Conclusions: Abnormal oscillometry parameters are present in one-fifth of this adult population and are significantly associated with respiratory symptoms and disease. Our findings underscore the potential of oscillometry as a tool for detecting and evaluating respiratory impairments, even in individuals with normal spirometry.

Comparison of oscillometry devices using active mechanical test loads

Noninvasiveness, low cooperation demand and the potential for detailed physiological characterisation have promoted the use of oscillometry in the assessment of lung function. However, concerns have been raised about the comparability of measurement outcomes delivered by the different oscillometry devices. The present study compares the performances of oscillometers in the measurement of mechanical test loads with and without simulated breathing.

Six devices (five were commercially available and one was custom made) were tested with mechanical test loads combining resistors (R), gas compliances (C) and a tube inertance (L), to mimic respiratory resistance (R_rs) and reactance (X_rs) spectra encountered in clinical practice. A ventilator was used to simulate breathing at tidal volumes of 300 and 700 mL at frequencies of 30 and 15 min⁻¹, respectively. Measurements were evaluated in terms of R, C, L, resonance frequency (f_res), reactance area (A_X) and resistance change between 5 and 20 or 19 Hz (R_5–20(19)).

Increasing test loads caused progressive deviations in R_rs and X_rs from calculated values at various degrees in the different oscillometers. While mean values of R_rs were recovered acceptably, some devices exhibited serious distortions in the frequency dependences of R_rs and X_rs, leading to large errors in C, L, f_res, A_X and R_5–20(19). The results were largely independent of the simulated breathing.

Simplistic calibration procedures and mouthpiece corrections, in addition to unknown instrumental and signal processing factors, may be responsible for the large differences in oscillometry measures. Rigorous testing and ongoing harmonisation efforts are necessary to better exploit the diagnostic and scientific potential of oscillometry.

The clinical utility of oscillometry is limited by the lack of standardisation of devices. This study tested six oscillometers, and reveals very different performances at higher mechanical impedances observed in children and adults with lung disease.http://bit.ly/317sfjH

Reference values for respiratory impedance measured by the forced oscillation technique in adult men and women

INTRODUCTION

The forced oscillation technique (FOT) is a particularly useful test of the mechanical properties of the respiratory system that has an increasingly important role in lung function laboratories. There is general agreement in the literature that the determination of reference values is of utmost importance in the clinical use of the FOT.

OBJECTIVE

Our aim was to present reference values for whole-breath FOT measurements, establish which anthropometric variables were more predictive of impedance parameters, and provide all the details to adequately adopt these reference equations in individual laboratories.

METHODS

We prospectively evaluated a randomly selected non-smoking sample of the adult Brazilian population (288 subjects, 144 males and 144 females aged 20-86 years). The volunteers were separated by sex and divided into six groups based on decade of age. Sex-specific linear prediction equations were developed by multiple regression analysis using age, body mass and height as explanatory variables.

RESULTS

Age introduced a slight, but significant, reduction of resistance in men (P < .001) and women (P < .001). In general, significantly higher values of resistance were observed in females (P < .0001). Among the anthropometric variables analyzed, height was the best predictor for all parameters studied.

CONCLUSION

This study provides an original frame of reference for the FOT in Brazilian males and females aged 20-86 years. Height was the best predictor of respiratory impedance parameters. Details contributing to an adequate adoption of these reference equations elsewhere by transference and verification are also provided.

Impulse oscillometry system as an alternative diagnostic method for chronic obstructive pulmonary disease

We aimed to compare impulse oscillation system (IOS) and traditional pulmonary function tests (PFTs) for the assessment of the severity of chronic obstructive pulmonary disease (COPD), and to assess the use of IOS parameters to identify patients who were forced expiratory volume in 1 second (FEV1)%pred < 50%.Patients with COPD (n = 215) were enrolled at the Ninth Hospital of Xi'an Affiliated Hospital of Xi'an Jiaotong University between October 2014 and September 2016. All patients were assessed by traditional PFT and IOS. Diagnostic performance of IOS parameters to determine indication for patients of FEV1%pred < 50% was assessed on receiver-operating characteristics (ROC) curve analysis.Out of 215 patients, 18, 83, 78, and 36 patients were classified as grade 1, 2, 3, and 4, respectively, according to the Global Initiative for Chronic Obstructive Lung Disease (GOLD) severity grading. On Spearman correlation analysis, FEV1%pred, MMEF 75%-25%, and residual volume/total lung capacity (RV/TLC) correlated with total respiratory impedance (Z5)%pred, resistance at 5 Hz (R5)-resistance at 20 Hz (R20), R5-R20% R5, R5, R5%pred, frequency response (Fres), reactance area (Ax), and reactance at 5 Hz (X5). On ROC curve analysis, the area under the curve (AUC) of X5 absolute value, Fres, Ax, Z5%pred, R5-R20, and R5-R20% R5 were 0.748, 0.755, 0.760, 0.705, 0.715, and 0.735, respectively, for COPD patients who required inhalational glucocorticoid therapy.IOS parameters showed a good correlation with traditional pulmonary function parameters; reactance parameters showed a stronger correlation than that of the resistance parameters. IOS can be used as an alternative method for pulmonary function assessment in patients with COPD with FEV1%pred < 50% who need inhalational glucocorticoid therapy.

CLINICAL TRIAL REGISTRATION NUMBER

ChiCTR-OCH-14004904.

Accuracy of forced oscillation technique to assess lung function in geriatric COPD population

Introduction

Performing lung function test in geriatric patients has never been an easy task. With well-established evidence indicating impaired small airway function and air trapping in patients with geriatric COPD, utilizing forced oscillation technique (FOT) as a supplementary tool may aid in the assessment of lung function in this population.

Aims

To study the use of FOT in the assessment of airflow limitation and air trapping in geriatric COPD patients.

Study design

A cross-sectional study in a public hospital in Hong Kong. ClinicalTrials.gov ID: NCT01553812.

Methods

Geriatric patients who had spirometry-diagnosed COPD were recruited, with both FOT and plethysmography performed. “Resistance” and “reactance” FOT parameters were compared to plethysmography for the assessment of air trapping and airflow limitation.

Results

In total, 158 COPD subjects with a mean age of 71.9±0.7 years and percentage of forced expiratory volume in 1 second of 53.4±1.7 L were recruited. FOT values had a good correlation (r=0.4–0.7) to spirometric data. In general, X values (reactance) were better than R values (resistance), showing a higher correlation with spirometric data in airflow limitation (r=0.07–0.49 vs 0.61–0.67), small airway (r=0.05–0.48 vs 0.56–0.65), and lung volume (r=0.12–0.29 vs 0.43–0.49). In addition, resonance frequency (Fres) and frequency dependence (FDep) could well identify the severe type (percentage of forced expiratory volume in 1 second <50%) of COPD with high sensitivity (0.76, 0.71) and specificity (0.72, 0.64) (area under the curve: 0.8 and 0.77, respectively). Moreover, X values could stratify different severities of air trapping, while R values could not.

Conclusion

FOT may act as a simple and accurate tool in the assessment of severity of airflow limitation, small and central airway function, and air trapping in patients with geriatric COPD who have difficulties performing conventional lung function test. Moreover, reactance parameters were better than resistance parameters in correlation with air trapping.

The application of impulse oscillation system for the evaluation of treatment effects in patients with COPD

There are only a few reports of the use of impulse oscillation system (IOS) for the evaluation of COPD treatment. In this study, we applied IOS and spirometry to evaluate the effectiveness of fluticasone propionate and salmeterol (SFC) combined with tiotropium (TIO) in COPD patients. Following a 4-week run-in period with TIO (18 μg once daily) treatment, COPD patients were randomized to SFC (250/50 μg twice daily; SFC+TIO group, n=25), or TIO alone (TIO group, n=31). Pulmonary functions were recorded by IOS and spirometry before and after the study period. The SFC+TIO group showed significant improvements in inspiratory resistance at 5 Hz and resonant frequency, as well as in FVC and FEV1, after the 12-week treatment (p<0.05). Since there were no significant correlations between improvements in IOS measurements and FVC or FEV1, IOS may provide a physiological point of view that is different from spirometry and seemed to be applicable as an additional assessment tool targeting COPD patients.

Impulse oscillometry in the diagnosis of airway resistance in chronic obstructive pulmonary disease

Spirometry is a standard lung function test for diagnosis and staging of chronic obstructive pulmonary disease (COPD). Impulse oscillometry (IOS) can be complementary to spirometry, especially in patients at advanced age and with physical or mental disorders who cannot be diagnosed through spirometry. The aim of this study was to compare IOS and spirometry in the assessment of airway obstruction in COPD. The study was conducted in 112 stable COPD patients, including 29 females and 83 males of the mean age of 69±11 years. The oscillometric evaluation included total (R5), peripheral (R5-R20), and negative reactance (X5), which were compared with the predicted forced expiratory volume in 1 s (FEV1%pred). The findings show a significantly negative correlation between FEV1%pred and the R5, R5-R20, and X5. COPD patients had increased R5, R5-R20, and X5. The severity of bronchial obstruction found by impulse oscillometry correlated well the spirometric assessment. IOS is a simple to perform test that may be helpful for functional examination of COPD patients.

On the respiratory mechanics measured by forced oscillation technique in patients with systemic sclerosis

Background

Pulmonary complications are the most common cause of death and morbidity in systemic sclerosis (SSc). The forced oscillation technique (FOT) offers a simple and detailed approach to investigate the mechanical properties of the respiratory system. We hypothesized that SSc may introduce changes in the resistive and reactive properties of the respiratory system, and that FOT may help the diagnosis of these abnormalities.

Methodology/Principal Findings

We tested these hypotheses in controls (n = 30) and patients with abnormalities classified using spirometry (n = 52) and pulmonary volumes (n = 29). Resistive data were interpreted with the zero-intercept resistance (Ri) and the slope of the resistance (S) as a function of frequency. Reactance changes were evaluated by the mean reactance between 4 and 32 Hz (Xm) and the dynamic compliance (Crs,dyn). The mechanical load was evaluated using the absolute value of the impedance in 4 Hz (Z4Hz). A compartmental model was used to obtain central (R) and peripheral (Rp) resistances, and alveolar compliance (C). The clinical usefulness was evaluated by investigating the area under the receiver operating characteristic curve (AUC). The presence of expiratory flow limitation (EFL) was also evaluated. For the groups classified using spirometry, SSc resulted in increased values in Ri, R, Rp and Z4Hz (p<0.003) and reductions in Crs,dyn, C and Xm (p<0.004). Z4Hz, C and Crs,dyn exhibited a high diagnostic accuracy (AUC>0.90). In groups classified by pulmonary volume, SSc resulted in reductions in S, Xm, C and Crs,dyn (p<0.01). Xm, C and Crs,dyn exhibited adequate diagnostic accuracy (AUC>0.80). It was also observed that EFL is not common in patients with SSc.

Conclusions/Significance

This study provides evidence that the respiratory resistance and reactance are changed in SSc. This analysis provides a useful description that is of particular significance for understanding respiratory pathophysiology and to ease the diagnosis of respiratory abnormalities in these patients.

Comparison of changes in lung function measured by plethymography and IOS after bronchoprovocation

AIM

Lung function tests are essential for the diagnosis and management of bronchial asthma. Impulse oscillation (IOS) system is an alternative way to measure lung mechanics for some patients. We investigated the relative sensitivities of IOS, body plethysmography and spirometry in detecting allergen- and methacholine-induced bronchoconstriction.

METHOD

Twenty-two subjects had single allergen inhalation and 8 subjects had 3 methacholine challenges. The tests were stopped when FEV1 fell by 20%. Lung function was measured using IOS (R5, R20, R5-R20, X5, AX, fres), plethysmography (sRaw, sGaw, FRC, lung volumes) and spirometry (FEV1, FVC, PEF, FEF50%) during inhalation challenges, and expressed as percent change from pre-challenge baseline.

RESULTS

All subjects were non-smoking adults with mild allergic asthma. Following allergen challenges, the most sensitive IOS index was R5-R20 and the most sensitive plethysmography and spirometry measurements were sRaw, sGaw and FEF50%. Following methacholine challenge the most sensitive IOS index was AX, the most sensitive plethysmography measurement was sRaw. Overall, IOS (R5-R20, AX, X5Hz) proved to be more sensitive than plethysmography and spirometry measurements following allergen-induced and methacholine-induced bronchoconstriction.

CONCLUSION

Our result shows that IOS is more sensitive than other lung function tests following allergen and methacholine challenge. In addition, IOS can act as an alternative measurement technique of airway resistance and obstruction in patients where manoeuvres involved in plethysmography and spirometry prove difficult to perform.

Inspiratory conductance: an alternative index of the relationship between inspiratory effort and airflow with potential for clinical use

INTRODUCTION AND OBJECTIVES

There is a need for a measure of airway status that is easier for patients to use. If airflow is briefly occluded at the onset of inspiration, the maximum rate of pressure fall, dP/dtmax can be measured. After the occlusion is released, the maximum rate of change of inspiratory airflow (dF/dtmax) can be measured and expressed as a ratio of dP/dtmax to generate an index of inspiratory conductance (IC). We explored the characteristics of IC as a preliminary step towards developing it as an easy-to-use alternative.

METHODS

To measure IC, we constructed an apparatus consisting of a pneumotachograph, a low-resistance spring-loaded valve and a pressure transducer to measure airflow and pressure drop at the start of inspiration. We studied 30 healthy adults to define the values of the index, its stability and its response to external inspiratory resistive loads.

RESULTS

Mean IC was 2.49 (SD 0.96) Ls(-1) kPa(-1) , with no significant difference between men and women. The index was stable with a mean long-term variation of 11.25% around the normalised mean, compared with 2.1% for forced expiratory volume in 1 s (FEV1) and within-test variation < 5%. In response to external resistive loading, IC fell in a non-linear but consistent manner from 3.96 (0.32) to 0.96 (0.10) Ls(-1) kPa(-1) across an added resistance range of 0-70 kPasL(-1) .

CONCLUSION

We showed that IC was measurable with a minimal need for subject participation. It was stable and reproducible in normal individuals and responded to added inspiratory resistive loads in a way that suggested it could be of clinical utility.

Low frequency oscillometry parameters in COPD patients are less variable during inspiration than during expiration

Impulse oscillometry (IOS) is a forced oscillation technique that enables pulmonary functional studies to be performed without requiring strenuous maneuvers. IOS assesses different components of respiratory impedance. The aim of this study was to compare the inspiratory and expiratory IOS parameters in COPD patients. IOS and spirometry were performed in 15 COPD patients and 23 healthy subjects. Thereafter, COPD patients were treated with tiotropium and their pulmonary function was re-evaluated. In COPD patients, the variations in the IOS parameters were significantly larger during expiration than during inspiration. The improvement in R5-20 (the difference between the respiratory resistance at 5 and 20 Hz, which reflects the distal lung resistance) after tiotropium treatment was statistically detected only during inspiration (p=0.008), not during expiration (p=0.139). In conclusion, the expiratory IOS parameters varied more than the inspiratory parameters, particularly in COPD patients-possibly because of flow-limitation during expiration. Thus, the evaluation of IOS parameters may be more accurate during inspiration in COPD patients.

Influence of the ageing process on the resistive and reactive properties of the respiratory system

INTRODUCTION

In an increasingly old society, the study of the respiratory system changes and new techniques dedicated to older patients are of interest in physiologic studies as well as in the diagnosis of respiratory diseases.

OBJECTIVES

(1) To investigate the impact of ageing on the resistive and reactive properties of the respiratory system, and (2) to compare the easiness of accomplishment of spirometry and forced oscillation for assessing lung function.

METHODS

We conducted a cross-sectional study in which forced oscillation was used to investigate respiratory system resistive and reactive properties, while spirometry was used as a reference test to evaluate 80 normal subjects aged between 20 and 86 years. A questionnaire was used to evaluate the easiness of accomplishment of spirometry and forced oscillation.

RESULTS

There was a significant increase in the respiratory system resonance frequency (p<0.003) and a reduction in the mean reactance (p<0.004) with increasing age. Respiratory system resistance and dynamic compliance were not related to the ageing process. The easiness of accomplishment of forced oscillation measurements was greater than that of spirometry. This result was particularly relevant in subjects over 70 years old (p<0.05).

CONCLUSIONS

Respiratory system resistance and dynamic compliance are not modified with ageing. On the other hand, respiratory system homogeneity decreases during the ageing process. Forced oscillation is easy to perform and provides information complementary to spirometry. This technique may be a promising alternative and/or complement to other conventional exams used to evaluate older people who are unable to adequately perform spirometric tests.

Impulse oscillometry for estimation of airway obstruction and bronchodilation in adults with mild obstructive asthma

BACKGROUND

Clinical validity of impulse oscillometry (IOS) for the evaluation of airway obstruction and bronchodilation is a controversial issue in adults with asthma.

METHODS

This study enrolled 195 outpatients from October 1998 to October 2004. We performed IOS in 158 asthmatic adults, including 70 asthmatic adults with a forced expiratory volume in 1 second (FEV,) reversibility (group 1), 88 asthmatic adults with hyperresponsiveness to methacholine or sputum eosinophilia (group 2) who did not meet the FEV, criteria, and 37 nonasthmatic adults (group 3).

RESULTS

Baseline respiratory resistance at 5 Hz (R5), respiratory resistance at 10 Hz, frequency dependency of resistance (R5 to 2)), and resonance frequency were discriminative between asthmatic patients and nonasthmatic patients. The IOS parameters were decreased after bronchodilation in both asthmatic groups compared with the nonasthmatic group. Among these patients, R5 and R5 to 21 were the most discriminative parameters for evaluation of bronchodilation. Approximately one third of the patients with positive methacholine challenge test results or sputum eosinophilia manifested bronchodilation evaluated by these IOS parameters. Overall sensitivities of these parameters were comparable to FEV, for diagnosis of bronchodilation in 158 asthmatic adults. Logistic regression analysis showed that R5 to 20 was the most reliable parameter for prediction of R5 reversibility for all asthmatic adults.

CONCLUSIONS

IOS may complement the estimation of obstruction and bronchodilation for asthmatic adults. Its discriminative power for airway obstruction and sensitivities for bronchodilation were comparable to FEV,.

Inspiratory transductance: a potential new measure of the relationship between inspiratory force and the resistance to pulmonary airflow

Most existing lung function tests require a substantial degree of interactive co-operation between patient and clinician. This reduces the clinical usefulness of such tests in patients with cognitive impairment (mainly elderly people), a reduced level of consciousness (for example in patients with a head injury or a severe metabolic disturbance) and children too young to perform the tests. There is a need for measures of airways status that are easier to apply. If the airflow is briefly occluded during inspiration the pressure fall in the airway is proportional to the force of contraction of inspiratory muscles and, because it occurs before flow begins, it is not mechanically altered by airflow resistance. The maximum rate of pressure fall is referred to as dP/dt(max) with the units kPas(-1). After the brief occlusion, the air column accelerates at a rate that is dependent on both dP/dt(max) and the resistance to airflow. The maximum rate of change of airflow is referred to as dV/dt(max), with the units Ls(-1)s(-1). This paper posits an hypothesis that the ratio dV/dt(max)/dP/dt(max) provides an index with the units Ls(-1)kPa(-1) that is a measure of the transformation of inspiratory effort into airflow, and can be referred to as inspiratory transductance. This index is predicted to rise when airflow resistance falls, such as would occur when an attack of asthma responds to bronchodilator drugs. The index is expected to continue to retain directional integrity even when inspiratory muscles become fatigued. Measuring the index would require little co-operation from a conscious patient and none from an unconscious patient.

Detection of changes in respiratory mechanics due to increasing degrees of airway obstruction in asthma by the forced oscillation technique

Forced expiratory airflows and volumes are often used to assess the airway obstruction in asthmatics. However, forced maneuvers may change bronchial tone and modify airway patency. The aim of this study was to determine whether the Forced Oscillation Technique (FOT), which does not require forced manoeuvres, may be useful to describe the changes in respiratory mechanics in progressive asthma. This study involved 25 healthy and 84 asthmatics, including patients with normal spirometric exam (NE), mild moderate and severe obstruction. Resistive data were interpreted using the respiratory system resistance extrapolated at 0 Hz (R0), the mean respiratory resistance (Rm), and the resistance/frequency slope (S). Reactance data were interpreted by its mean values (Xm), the dynamic compliance (Crs,dyn), and resonant frequency (fr). Receiver operating characteristics curves were used to determine the sensitivity (Se) and specificity (Sp) of FOT parameters in identifying asthma. There were not statistically significant differences between the control and NE groups. Comparing the control and mild groups, significant increases of R0 (P<0.0007), Rm (P<0.003), and S (P<0.003) were observed. In reactive parameters, a significant reduction in Crs,dyn (P<0.04) was observed, while Xm and fr presented significant increases (P<0.0007 and P<0.006, respectively). Comparison between mild and moderate groups showed non-significant modifications in all of the parameters, except for Xm (P<0.02). In the late stages (moderate to severe obstruction), all of the resistive parameters, as well as the reactive ones Xm (P<0.007) and Crs,dyn (P<0.03), presented statistically significant modifications. Among the studied parameters, the effects of airway obstruction in asthma seem to be well described by R0, Rm, S and Xm, which were in close agreement with physiological fundamentals. The best parameters for detecting asthma were R0 (Se=81%, Sp=76%), S (Se=78%, Sp=72%) and Xm (Se=81%, Sp=80%). In conclusion, the results of this study suggest that the FOT can be proposed as an alternative method for the assessment of the respiratory mechanics in asthmatic patients, representing a promising solution to the problem of effort dependence.

Biofluid Mechanics of the Pulmonary System

Presents an overview of leading areas of discovery in bio-fluid mechanics related to the pulmonary system, with particular reference to the airways. Areas briefly reviewed include airway gas dynamics, impedance studies, collapsible-tube studies, and airway liquid studies. Emphasis is placed on promising further directions, such as analysis of interacting fluid-mechanical or fluid-structure phenomena, multi-scale modeling across widely varying length and time scales, and integration of advanced simulations into respiratory investigation and pulmonary medicine.

Aging of the respiratory system: impact on pulmonary function tests and adaptation to exertion

Normal aging of the respiratory system is associated with a decrease in static elastic recoil of the lung, in respiratory muscle performance, and in compliance of the chest wall and respiratory system, resulting in increased work of breathing compared with younger subjects and a diminished respiratory reserve in cases of acute illness, such as heart failure, infection, or airway obstruction. In spite of these changes, the respiratory system remains capable of maintaining adequate gas exchange at rest and during exertion during the entire lifespan, with only a slight decrease in Pa(O2) and no significant change in Pa(CO2).

Changes in respiratory mechanics with increasing degrees of airway obstruction in COPD: detection by forced oscillation technique

The Forced Oscillation Technique (FOT) is a method for non-invasively assessing respiratory mechanics during spontaneous breathing, demanding little cooperation. The aim of this study was to test the ability of FOT to describe the changes in respiratory mechanics in progressive COPD. The study was performed in a control group formed by 21 healthy subjects and 79 outpatients with COPD, which were classified by spirometry, according to the degree of airway obstruction, in mild, moderate and severe groups. Resistive impedance data were submitted to linear regression analysis over the 4-16 Hz frequency range, which yielded the total respiratory system resistance extrapolated at 0 Hz (R0), the respiratory system conductance (Grs), mean respiratory resistance (Rm), and the resistance/frequency slope (S). Reactance data were interpreted using the mean values (Xm) over the 4-32 Hz frequency range, the dynamic compliance (Crs,dyn), the dynamic elastance (E(rs,dyn)), and the resonant frequency (fr) data. Considering the control and mild groups, the increase of airway obstruction resulted in a significant increase of R0 (P<0.008), Rm (P<0.001), and a significant reduction in Grs (P<0.002). Reactive parameters, Crs, dyn and Ers,dyn also presented significant modifications. The subsequent increase (mild to moderate) showed a significant raise of R(0) (P<0.007), S (P<0.001), and a reduction in Grs (P<0.015), while significant increases in Xrs (P<0.001), and Ers,dyn (P<0.02), and also a reduction in Crs, dyn (P<0.02) were also observed. In contrast to earlier stages, in the late stage of the airway obstruction increase (moderate to severe obstruction), resistive parameters did not present statistically significant modifications, while significant modifications were observed in Xrs (P<0.02), Crs, dyn (P<0.003) and Ers,dyn (P<0.003). The results of this study demonstrated that the FOT is useful for detecting the respiratory mechanics modifications in COPD patients. The initial phases of airway obstruction in COPD can be described mainly by resistive parameters, while in more advanced phases, reactive parameters seem to be more useful. Since the FOT has the advantage of being a simple method, such a technique may give a significant clinical contribution, representing an alternative and/or complement to the evaluation of respiratory mechanics by means of forced expiration.