Respiratory input impedance measurement: Forced oscillation methods

The paper reviews how forced oscillation techniques (FOT) for measuring respiratory input impedance Z_{rs, in} have recently been used in clinical applications. Z_{rs, in} is clinically relevant, as it provides data on both the resistive, R_rs, and nonresistive, X_rs, 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 Z_{rs, in} for routine lung-function assessment, sleep and mechanical ventilation are reviewed. For clinicians, Z_{rs, in} is repeatable and sensitive to airway resistance. It is helpful for assessing unco-operative and severely obstructed patients, for monotoring 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 Z_{rs, in}. Forced oscillation techniques, however, require further standardisation, and Z_{rs, in} is limited by upper-airway shunt artifacts. In conclusion, measurement of Z_{rs, in} using FOT is an important and sophisticated non-invasive lung-function test, showing good potential for future clinical applications.     

Within-Breath Analysis of Respiratory Mechanics in Asthmatic Patients by Forced Oscillation

INTRODUCTION:

The within-breath analysis of respiratory mechanics by the monofrequency Forced Oscillation Technique (mFOT) is of great interest in both physiopathology studies and the diagnosis of respiratory diseases. However, there are limited data on the use of this technique in the analysis of asthma. This study evaluates within-breath mechanics of asthmatic individuals and the contribution of the mFOT in the asthma diagnosis.

METHODS:

Twenty-two healthy and twenty-two asthmatic subjects, including patients with mild (n=8), moderate (n=8), and severe (n=6) obstruction, were studied. Forced Oscillation Technique data were interpreted using the mean respiratory impedance (Zt), the impedance during inspiration (Zi), expiration (Ze), at the beginning of inspiration (Zii), and at expiration (Zie). The peak-to-peak impedance (Zpp) was also calculated by the subtraction of Zii from Zie. Receiver operating characteristic curves were used to determine the sensitivity (Se) and specificity (Sp) of m Forced Oscillation Technique parameters in identifying asthma.

RESULTS:

Respiratory impedance values were significantly higher in asthmatics: Zt (p<0.001), Zi (p<0.001), Ze (p<0.001), Zii (p<0.001), Zie (p<0.001), and Zpp (p<0.003). The best parameters for detecting asthma were Zi, Zii, and Zie (Se=90.9%, Sp=90.9%), followed by Zt and Ze. These results are in close agreement with recently published theories and pathophysiological fundamentals.

CONCLUSIONS:

mFOT permits a non-invasive and detailed analysis in different phases of the respiratory cycle, providing parameters that are adequate for the diagnosis of asthma with high accuracy. These results confirm the high clinical and scientific potential of this methodology in the evaluation of asthmatic patients.     

Effect of generator nonlinearities on the accuracy of respiratory impedance measurements by forced oscillation

Measurements of respiratory impedance by means of the forced oscillation technique (FOT) are usually made using a loudspeaker as the excitation device. Its nonlinear nature can introduce artifacts that coincide with the frequencies applied to excite the respiratory system, limiting the accuracy of the impedance estimation. In this paper, this hypothesis is evaluated in the case of both a traditional estimator and the unbiased estimator proposed byDaróczy andHantos (1982). A simulated study under apnoea conditions in the pressure range 0.5–3.0 cmH₂O peak-to-peak reveals that loudspeaker nonlinearities introduce a characteristic pattern of dispersion in both the resistance and reactance curves that can be significantly decreased (p≃0.03, signtest) by reducing the nonlinearities. A simulation of spontaneous breathing shows the same pattern, and is observed in the case of traditional as well as unbiased estimators. The dispersion is quantified by the mean absolute distance between the theoretical and simulated data and decreases with the reduction of nonlinearities when impedance is estimated with a traditional estimator (from 6.63 to 4.72% in real estimates and from 6.78 to 3.47% in imaginary estimates) as well as with an unbiased estimator (real estimates from 4.84 to 1.57% and 5.61 to 2.06% in imaginary estimates). Studies with normal subjects show the same dispersion pattern, which decreases if the generator nonlinearities are reduced. These results supply substantial evidence that reducing generator nonlinearities can contribute to the production of more reliable mechanical impedance FOT measurements.     

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.     

Simplified oscillation method for assessing nasal obstruction non-invasively and under spontaneous ventilation: A pilot study

The clinical application of the current methods of measuring nasal obstruction has been limited by complicated, invasive and stressful procedures that require the full co-operation of the patient. A pilot study is described where a simple way of evaluating nasal obstruction, based on oscillation methods, was investigated. The technique did not disturb spontaneous breathing and required litle co-operation and comprehension. Significant differences were obtained when clinically classified normal (5.2±1.8 cmH₂Ol⁻¹ s) and patient (10.6±5.9 cmH₂Ol⁻¹ s) groups were evaluated (p<0.01). A significant reduction (p<0.02) was also observed in impedance results before (8.5±1.1 cmH₂Ol⁻¹ s) and after (5.2±1.7 cmH₂Ol⁻¹ s) clinically successful nasal surgery, closely reflecting the clinical conditions of the subjects. This simple forced oscillation technique showed good potential for future clinical applications in the prescreening of nasal patients and the evaluation of therapeutic surgery.     

Sensitivity Analysis of Respiratory Parameter Estimates in the Constant-Phase Model

The constant-phase model is increasingly used to fit low-frequency respiratory input impedance (Zrs), highlighting the need for a better understanding of the use of the model. Of particular interest is the extent to which Zrs would be affected by changes in parameters of the model, and conversely, how reliable are parameters estimated from model fits to the measured Zrs. We performed sensitivity analysis on respiratory data from 6 adult mice, at functional residual capacity (FRC), total lung capacity (TLC), and during bronchoconstriction, obtained using a 1-25 Hz oscillatory signal. The partial derivatives of Zrs with respect to each parameter were first examined. The limits of the 95% confidence intervals, 2-dimensional pairwise and p-dimensional joint confidence regions were then calculated. It was found that airway resistance was better estimated at FRC, as determined by the confidence region limits, whereas tissue damping and elastance were better estimated at TLC. Airway inertance was poorly estimated at this frequency range, as expected. During methacholine-evoked pulmonary constriction, there was an increase in the uncertainty of airway resistance and tissue damping, but this can be compensated for by using the relative (weighted residuals) in preference over the absolute (unweighted residuals) fitting criterion. These results are consistent with experimental observation and physiological understanding.     

Mechanical impedance of the respiratory tract in divers before and after simulated deep dives

Previous studies have inconsistently shown changes in expiratory flows and volumes as well as diffusion capacity of the lungs after single dives and several diving related occupational conditions were considered as possible underlying factors. In this study mechanical impedance of the airways was measured before and after simulated dives to non-invasively determine whether there is evidence for lung function impairment due to hyperbaric exposure. Thirty-three healthy male divers employing air self-contained underwater breathing apparatus were randomly assigned to dry and wet chamber dives in a cross-over design to 600 kPa ambient pressure (total duration 43 min, bottom time 15 min, water temperature 24°C). Immediately before and after diving, oscillometric parameters—e. g. resistance and reactance of the respiratory tract—were measured at defined frequencies (5, 20 Hz). Spirometry was carried out as well (FVC, FEV₁, MEF 25–75). No significant changes between post-exposure values and baseline values were detected by respiratory impedance and spirometry. Diving in accordance to diving regulations and without excessive workload is not a source for acute obstructive lung function changes as the obtained oscillometric data suggested. Moreover this study could not confirm changes in spirometry after simulated diving exposure.     

基于强迫振荡技术的呼吸道阻力及其可变性测量

目的利用强迫振荡技术(forced oscillation technique,FOT)测量成年哮喘病人和健康人的呼吸道阻力,从而验证该技术对哮喘病诊断的意义。方法采用手持式FOT仪器分别测量5个哮喘病人和5个健康人的气道阻力,并对其随时间变化的差异进行统计分析。结果基于FOT测量的气道阻力总是随时间呈周期性变化;哮喘病人和健康人受试者的呼吸道阻力分别为(294.98±86.24)、(151.9±39.20)Pa.s/L,两者存在明显差异。结论 FOT测试所得的哮喘病人的呼吸道阻力和呼吸道阻力可变性均明显高于正常人,这与临床常规诊断的结果一致,从而为临床检测呼吸系统动力学参数提供了一种新方法,为准确诊断哮喘病等肺功能异常提供了重要依据。     

Servo-Controlled Pneumatic Pressure Oscillator for Respiratory Impedance Measurements and High-Frequency Ventilation

The ability to provide forced oscillatory excitation of the respiratory system can be useful in mechanical impedance measurements as well as high frequency ventilation (HFV). Experimental systems currently used for generating forced oscillations are limited in their ability to provide high amplitude flows or maintain the respiratory system at a constant mean pressure during excitation. This paper presents the design and implementation of a pneumatic pressure oscillator based on a proportional solenoid valve. The device is capable of providing forced oscillatory excitations to the respiratory system over a bandwidth suitable for mechanical impedance measurements and HVF. It delivers high amplitude flows (> 1.4 l/s) and utilizes a servo-control mechanism to maintain a load at a fixed mean pressure during simultaneous oscillation. Under open-loop conditions, the device exhibited a static hysteresis of approximately 7%, while its dynamic magnitude and phase responses were flat out to 10 Hz. Broad-band measurement of total harmonic distortion was approximately 19%. Under closed-loop conditions, the oscillator was able to maintain a mechanical test load at both positive and negative mean pressures during oscillatory excitations from 0.1 to 10.0 Hz. Impedance of the test load agreed closely with theoretical predictions. We conclude that this servo-controlled oscillator can be a useful tool for respiratory impedance measurements as well as HFV.     

Seroprevalence of Respiratory Syncytial Virus IgG among Healthy Young Adults in Basic Training for the Republic of Korea Air Force

This investigation enrolled 570 healthy young males gathered from all over the country for military service at the Republic of Korea Air Force boot camp. It confirmed RSV IgG seroprevalence by utilizing the enzyme immunoassay method just prior to undergoing basic training. The mean age of this study was 20.25±1.34 yr old. The results of their immunoassay seroprofiles showed that 561 men (98.4%) were positive, 2 (0.4%) were negative and 7 (1.2%) were equivocal belonging to the grey zone. It was confirmed that RSV is a common respiratory virus and RSV infection was encountered by almost all people before reaching adulthood in Korea. Nine basic trainees belonging to the RSV IgG negative and equivocal grey zone categories were prospectively observed for any particular vulnerability to respiratory infection during the training period of two months. However, these nine men completed their basic training without developing any specific respiratory illness.

Graphical Abstract

相似文献   

Respiratory impedance spectral estimation for digitally created random noise

Measurement of respiratory input mechanical impedance (Z_rs) is noninvasive, requires minimal subject cooperation, and contains information related to mechanical lung function. A common approach to measure Z_rs is to apply random noise pressure signals at the airway opening, measure the resulting flow variations, and then estimate Z_rs using Fast-Fourier Transform (FFT) techniques. The goal of this study was to quantify how several signal processing issues affect the quality of a Z_rs spectral estimate when the input pressure sequence is created digitally. Random noise driven pressure and flow time domain data were simulated for three models, which permitted predictions of Z_rs characteristics previously reported from 0–4, 4–32, and 4–200 Hz. Then, the quality of the Z_rs estimate was evaluated as a function of the number of runs ensemble averaged, windowing, flow signal-to-noise ratio (SNR), and pressure spectral magnitude shape |P(jω)|. For a |P(jω)| with uniform power distribution and a SNR<100, the 0–4 Hz and 4–200 Hz Z_rs estimates for 10 runs were poor (minimum coherence γ²<0.75) particularly where Z_rs is high. When the SNR>200 and 10 runs were averaged, the minimum γ² >0.95. However, when |P(jω)| was matched to |Z_rs|, γ² > 0.91 even for 5 runs and a SNR of 20. For data created digitally with equally spaced spectral content, the rectangular window was superior to the Hanning. Finally, coherence alone may not be a reliable measure of Z_rs quality because coherence is only an estimate itself. We conclude that an accurate estimate of Z_rs is best obtained by matching |P(jω)| to |Z_in| (subject and speaker) and using rectangular windowing.     

Correlations between forced oscillation technique parameters and pulmonary densitovolumetry values in patients with acromegaly

The aims of this study were to evaluate the forced oscillation technique (FOT) andpulmonary densitovolumetry in acromegalic patients and to examine the correlationsbetween these findings. In this cross-sectional study, 29 non-smoking acromegalicpatients and 17 paired controls were subjected to the FOT and quantification of lungvolume using multidetector computed tomography (Q-MDCT). Compared with the controls,the acromegalic patients had a higher value for resonance frequency [15.3 (10.9-19.7)vs 11.4 (9.05-17.6) Hz, P=0.023] and a lower value for meanreactance [0.32 (0.21-0.64) vs 0.49 (0.34-0.96) cmH₂O/L/s², P=0.005]. In inspiratory Q-MDCT, the acromegalicpatients had higher percentages of total lung volume (TLV) for nonaerated and poorlyaerated areas [0.42% (0.30-0.51%) vs 0.25% (0.20-0.32%), P=0.039 and3.25% (2.48-3.46%) vs 1.70% (1.45-2.15%), P=0.001, respectively].Furthermore, the acromegalic patients had higher values for total lung mass in bothinspiratory and expiratory Q-MDCT [821 (635-923) vs 696 (599-769) g,P=0.021 and 844 (650-945) vs 637 (536-736) g, P=0.009,respectively]. In inspiratory Q-MDCT, TLV showed significant correlations with allFOT parameters. The TLV of hyperaerated areas showed significant correlations withintercept resistance (r_s=−0.602, P<0.001) and mean resistance(r_s=−0.580, P<0.001). These data showed that acromegalic patientshave increased amounts of lung tissue as well as nonaerated and poorly aerated areas.Functionally, there was a loss of homogeneity of the respiratory system. Moreover,there were correlations between the structural and functional findings of therespiratory system, consistent with the pathophysiology of the disease.     

新型阻抗式呼吸信号分析仪的实验和临床评价

目的 对前期研发的阻抗式呼吸信号分析仪进行改良,提高振荡发生器的稳定性和线性.方法 重点改进振荡发生器,包括采用直线音圈电机作为振荡动力源、磁栅尺伺服控制电机运行以及优化振荡泵的结构等.模拟机械通气的工作环境,向振荡发生器施加不同的压力负荷,测量音圈电机线圈的位移并与预设值比较,评价振荡发生器工作的稳定性.采用5 Hz不同振幅的振荡信号测量呼吸力学模型的吸气阻抗,同时采用吸气末闭合法测量模型的吸气阻力,比较两者的差异,评价振荡发生器的线性.选择机械通气的慢性阻塞性肺疾病(COPD)及肺部影像正常的重型颅脑损伤患者各1例,采用5 Hz振荡信号分别测定患者的呼吸阻抗(Rrs)和电抗(Xrs).结果 新设计的振荡发生器在压力负荷为15 ～ 40 cmH2O(1cmH2O=0.098 kPa)范围内,持续输出稳定的正弦振荡气流,振荡频率为0.01～ 16Hz、振幅为0.5～2 cmH2O范围内可调.振荡频率为5 Hz,振幅分别为0.5、1、1.5和2 cmH2O的信号分别测定的模型吸气阻抗均值为(16.56±1.17) cmH2O·s-1 ·L-1,吸气末闭合法测定的模型吸气阻力均值为(19.33±0.67) cmH2O·s-1 ·L-1,两者存在线性相关(r=0.51,P＜0.05).临床试验测试显示,COPD患者Rrs和Xrs均高于肺部影像正常者,均差ΔRrs和ΔXrs分别为(8.9± 6.96) cmH2O· s-1·L-1和(12.31±8.46)cmH2O· s-1· L-1,以ΔXrs增加为显著.结论 新型阻抗式呼吸信号分析仪受机械通气的干扰小、振荡发生器振动具有线性、工作状态稳定,其测量结果能够反映COPD患者的肺部力学特征,区分不同部位的气道阻塞.     

Total inspiratory and expiratory impedance in patients with severe chronic obstructive pulmonary disease

OBJECTIVES:

Several studies have confirmed the high potential of the forced oscillation technique for the assessment of respiratory modifications related to chronic obstructive pulmonary disease. However, most of these studies did not employ within-breath analyses of the respiratory system. The aim of this study is to analyze respiratory impedance alterations in different phases of the respiratory cycle of chronic obstructive pulmonary disease patients and to evaluate their clinical use.

METHODS:

39 individuals were evaluated, including 20 controls and 19 individuals with chronic obstructive pulmonary disease who experienced severe airway obstruction. We evaluated the mean respiratory impedance (Zm) as well as values for inspiration (Zi) and expiration cycles (Ze), at the beginning of inspiration (Zbi) and expiration (Zbe). The peak-to-peak impedance (Zpp), and the impedance change (ΔZrs) were also analyzed. The clinical usefulness was evaluated by investigating the sensibility, specificity and the area under the receiver operating characteristic curve.

RESULTS:

The respiratory impedance increased in individuals with chronic obstructive pulmonary disease in all of the studied parameters (Zm, Zi, Ze, Zbi, Zbe, ΔZrs and Zpp). These changes were inversely associated with spirometric parameters. Higher impedances were observed in the expiratory phase of individuals with chronic obstructive pulmonary disease. All of the studied parameters, except for ΔZrs (area under the receiver operating characteristic <0.8), exhibited high accuracy for clinical use (area under the receiver operating characteristic >0.90; Sensibility ≥ 0.85; Sp ≥ 0.85).

CONCLUSIONS:

The respiratory alterations in severe chronic obstructive pulmonary disease may be identified by the increase in respiratory system impedance, which is more evident in the expiratory phase. These results confirm the potential of within-breath analysis of respiratory impedance for the assessment of respiratory modifications related to chronic obstructive pulmonary disease.     

无创通气下应用强迫振荡技术检测呼吸阻力

目的 构建强迫振荡测量系统,在无创正压通气(NPPV)条件下检测人呼吸系统阻抗(Rrs)和电抗(Xrs),用于评估肺阻力(RL)和弹件阻力(EL).方法 基于现有的强迫振荡技术,构建包括振荡发生器和信号分析软件在内的新强迫振荡测量系统,采用呼吸系统一阶线性力学模型(R-I-E模型)验证新系统测量的准确性和可靠性.8例健康正常人为研究对象,分别经鼻罩给予不同水平的双水平气道止压(BiPAP)和持续气道正压(CPAP)通气,探讨在不同通气模式和压力水平下利用Rrs无创评价R,的可行性.结果 (1)振荡压力和振荡流量的相干函数系数值r2=0.98,强迫振荡测量可靠.5 Hz强迫振荡测定的模型阻抗与模型的粘性阻力接近,两者平均相差(1.26±0.44)cm H2O·s·L-1,一敛性范嗣在(0.36～2.14)cm H2O·s·L-1.(2)8例在不同通气模式及压力条件下测定的Rrs值大小接近,与RL平均相差(0.16±1.58)cm H2O·s·L-1.RL的预计方程式为RL=1.40+0.77Rrs,[决定系数(r2)=0.43,P<0.01].(3)强迫振荡弹性阻力EFOT(EFOT=-2πfXrs)显著大于实际值EL,(P<0.01).两者存在低水平的相关(r=0.40,P<0.01).结论 新强迫振荡系统测量准确、可靠.Rrs近似地反映了RL的大小,RL预测值可用于优化NPPV的压力支持水平,使呼吸机提供的压力支持能有效地克服肺阻力.     

Analysis of the dynamic characteristics of pressure transducers for studying respiratory mechanics at high frequencies

Differential pressure transducers are commonly used to study respiratory mechanics at physiological frequencies as well as during external forcing at high frequencies. In the latter condition, measuring errors could occur if the input impedance of the pressure transducers is not sufficiently large with respect to that of the respiratory system. In this work we analysed the input impedance Z and the transfer function H of two common pressure transducers (Validyne MP-45 and Celesco LCVR) equipped with membranes of different sensitivities and with connecting tubes of different lengths. Z was measured by the tube method and H was measured by comparison with a flat-response pressure transducer. In agreement with the predictions based on a simple lumped-parameters model, we found that Z reached very low values, especially at the frequencies where H had a resonance peak. For instance, for the widespread Validyne MP-45 transducer (200 Pa) with connecting tubes of 16 cm length and 3·8 mm internal diameter a minimum of Z of 8300 Pa s litre⁻¹ at 96 Hz was measured; at that frequency the amplitude of H attained a value of 3·1. Using the above transducer model we simulated the measurement of a rat input impedance up to 128 Hz using Validyne and Celesco transducers. With the Validyne MP-45 (200 Pa), equipped with the same connecting tubes as above, the computed error reached up to 50 per cent for the real part and 140 per cent for the imaginary part.     

Mechanical impedance of the canine diaphragm

A technique which does not require the measurement of strain has been developed for the investigation of the incremental dynamic properties of soft tissue sheets. Radially prestressed and circularly clamped canine diaphragm samples were exposed to small-amplitude pseudorandom pressure variations. From the measurement of these pressure variations and the volume flow caused by the vibration of the membrane the incremental mechanical impedance spectrum was computed in the 0·25–5 Hz frequency range at three different levels of initial stress. The diaphragm tissue was found to be basically elastic. However, the small viscous component showed a sharp negative frequency dependence between 0·25 and 2 Hz. The quasistatic elastances of the samples were in good, agreement with the elastance values derived from the impedance data. The relationship between the elastance and the initial stress was close to linear. It was concluded that the method is applicable to the study of the incremental dynamic properties of planar soft tissue samples.     

Forced oscillation technique in the detection of smoking-induced respiratory alterations: diagnostic accuracy and comparison with spirometry

INTRODUCTION:

Detection of smoking effects is of utmost importance in the prevention of cigarette‐induced chronic airway obstruction. The forced oscillation technique offers a simple and detailed approach to investigate the mechanical properties of the respiratory system. However, there have been no data concerning the use of the forced oscillation technique to evaluate respiratory mechanics in groups with different degrees of tobacco consumption.

OBJECTIVES:

(1) to evaluate the ability of the forced oscillation technique to detect smoking‐induced respiratory alterations, with special emphasis on early alterations; and (2) to compare the diagnostic accuracy of the forced oscillation technique and spirometric parameters.

METHODS:

One hundred and seventy subjects were divided into five groups according to the number of pack–years smoked: four groups of smokers classified as <20, 20–39, 40–59, and >60 pack–years and a control group. The four groups of smokers were compared with the control group using receiver operating characteristic (ROC) curves.

RESULTS:

The early adverse effects of smoking in the group with <20 pack–years were adequately detected by forced oscillation technique parameters. In this group, the comparisons of the ROC curves showed significantly better diagnostic accuracy (p<0.01) for forced oscillation technique parameters. On the other hand, in groups of 20–39, 40–59, and >60 pack–years, the diagnostic performance of the forced oscillation technique was similar to that observed with spirometry.

CONCLUSIONS:

This study revealed that forced oscillation technique parameters were able to detect early smoking‐induced respiratory involvement when pathologic changes are still potentially reversible. These findings support the use of the forced oscillation technique as a versatile clinical diagnostic tool in helping with chronic obstructive lung disease prevention, diagnosis, and treatment.