Respiratory system reactance as an indicator of the intrathoracic airway response to methacholine in children

Quality assessment pathway for respiratory oscillometry

There is strong evidence to support the addition of respiratory oscillometry to standard lung function testing. The key parameters are sensitive in identifying the presence and severity of airways disease [1], and clinically meaningful cut-offs have been established to identify bronchodilator response [2] and bronchial hyperresponsiveness [3–6] independent of spirometry. While clinical uptake is increasing with the availability of commercial devices, oscillometry is yet to be widely adopted as a standard test. This has been in part due to a lack of standardisation in equipment specifications and inconsistent terminology, but also human-related factors such as measurement protocols and objective quality control. The recently published international technical standards [7] have partly addressed most of these issues, and the development of global reference equations is currently in progress. Nevertheless, there remains a strong need to develop standard methods to optimise measurement quality and operator competency.

A flow-chart-driven procedure is presented to facilitate respiratory oscillometry operator competency and measurement quality. A novel feature is a quality grading system, in line with other standards of lung function.https://bit.ly/3G4r0X1

Methacholine-Induced Variations in Airway Volume and the Slope of the Alveolar Capnogram Are Distinctly Associated with Airflow Limitation and Airway Closure

Mechanisms driving alteration of lung function in response to inhalation of a methacholine aerosol are incompletely understood. To explore to what extent large and small airways contribute to airflow limitation and airway closure in this context, volumetric capnography was performed before (n = 93) and after (n = 78) methacholine provocation in subjects with an intermediate clinical probability of asthma. Anatomical dead space (VDaw), reflecting large airway volume, and the slope of the alveolar capnogram (slope3), an index of ventilation heterogeneity linked to small airway dysfunction, were determined. At baseline, VDaw was positively correlated with lung volumes, FEV₁ and peak expiratory flow, while slope3 was not correlated with any lung function index. Variations in VDaw and slope3 following methacholine stimulation were correlated to a small degree (R² = -0.20). Multivariate regression analysis identified independent associations between variation in FEV₁ and variations in both VDaw (Standardized Coefficient-SC = 0.66) and Slope3 (SC = 0.35). By contrast, variation in FVC was strongly associated with variations in VDaw (SC = 0.8) but not Slope3. Thus, alterations in the geometry and/or function of large and small airways were weakly correlated and contributed distinctly to airflow limitation. While both large and small airways contributed to airflow limitation as assessed by FEV1, airway closure as assessed by FVC reduction mostly involved the large airways.

Useful marker of oscillatory lung function in methacholine challenge test-comparison of reactance and resistance with dose-response slope

BACKGROUND

There are few studies focusing on the comparison of resistance (Rrs) and reactance (Xrs) in impulse oscillometry system (IOS) in the bronchial challenge test using dose-response slope (DRS), a quantitative index of bronchial hyperresponsiveness.

MATERIAL AND METHODS

We conducted a case-control study of 144 asthmatic and 218 non-asthmatic children to compare the diagnostic accuracy of two-point linear DRS for FEV1 , Rrs5 , and Xrs5 (DRS_FEV1 , DRS_Rrs5 , and DRS_Xrs5 ) and assessed various diagnostic cut-off points of provocation concentrations (PC) using receiver operating characteristic (ROC) curves.

RESULTS

DRS_FEV1 had a stronger correlation with DRS_Xrs5 (r = 0.739, P < 0.001) than with DRS_Rrs5 (r = 0.652, P < 0.001) and the area under the ROC curves of DRS_Xrs5 (0.737) was similar to that of DRS_FEV1 (0.732) and higher than that of DRS_Rrs5 (0.668). The area under the ROC curves in order of greater value was as follows: absolute change of Xrs5 (Abs_Xrs5 ) (0.759) > percent change of FEV1 (Pch_FEV1 ) (0.735) > Pch_Xrs5 (0.727) > Abs_Rrs5 (0.690) > Pch_Rrs5 (0.630). PC78 _Xrs5 and PC0.17 _Xrs5 of IOS showed considerably good sensitivity and specificity comparable to those of PC20 _FEV1 by spirometry. Additional 18 (13%) children who showed normal spirometric measures were identified as asthmatics with the use of IOS.

CONCLUSION

The utility of the DRS_Xrs5 to differentiate asthmatics from controls was comparable to that of the DRS_FEV1 and better than that of the DRS_Rrs5 . In addition, IOS could detect additional asthmatic patients who did not show positive responses in spirometry.

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.

Cut-points for response to mannitol challenges using the forced oscillation technique

The forced oscillation technique (FOT) can be used to determine airway hyperresponsiveness, but the cut-points for changes in respiratory system conductance (Grs) and reactance (Xrs) that define a positive mannitol challenge are not known. Furthermore, the effects of changes in lung volume on these cut-points or on the repeatability of the test are unknown. In 15 non-asthmatic and 52 asthmatic subjects, response to mannitol challenge was measured by Grs and Xrs, using FOT, and by FEV(1). The FOT variables were adjusted for inspiratory capacity (IC) at each dose. Dose response slope (DRS) was used in receiver operator characteristic (ROC) analysis to compare the ability of adjusted and unadjusted DRSGrs and DRSXrs to detect a positive challenge, defined as PD(15)FEV(1) ≤635 mg mannitol. Mannitol challenges were positive in 32 asthmatic and 2 non-asthmatic subjects. Both DRSGrs and DRSXrs detected positive challenges (p < 0.0001 for both), and this was not altered by adjustment for IC for either DRSGrs (p = 0.21) or DRSXrs (p = 0.90). FOT cut-points for a positive challenge were 27% fall in Grs or 0.93 cm H(2)O/L/s decrease in Xrs at 635 mg. Repeatability of DRSGrs (±2.01 doubling doses) and DRSXrs (±1.95dd) was comparable with DRSFEV(1) (±1.67dd) and was not improved by adjustment for IC. Grs and Xrs, measured by FOT, provide a sensitive, repeatable measure of response to mannitol challenge. Adjusting for lung volume does not alter the ability of these variables to detect a positive response or the repeatability of the measurement.

Comparison of effectiveness in ciclesonide and fluticasone propionate on small airway function in mild asthma

BACKGROUND

Inhaled corticosteroids (ICS) are the mainstay of asthma treatment, but conventional ICS may have limited effectiveness in inflammation and patency of small airways. Ciclesonide is delivered and deposited in the peripheral region of the lung as a small particle corticosteroid. The aim of the study is to compare the effects of ciclesonide with fluticasone propionate on small airway function in asthma.

METHODS

Thirty mild persistent asthma patients treated with 200 microg of fluticasone propionate were randomized to receive either ciclesonide 200 microg once daily or fluticasone propionate 100 microg twice daily for 8 weeks. Small airway function was assessed by impulse oscillometry (IOS) and percentage of eosinophil induced sputum.

RESULTS

We observed that ciclesonide significantly improved IOS measured resistance of small airways (R5-R20; p<0.05), distal reactance (X5; p<0.01), reactance area (AX; p<0.01), and decreased late-phase sputum eosinophil level (p<0.01) compared with fluticasone propionate. There were no significant changes in spirometry indices in either group during the study.

CONCLUSIONS

These findings suggest that ciclesonide improves small airway function and inflammation compared with fluticasone propionate in mild asthma. This study provides evidence that IOS and late-phase induced sputum allows detection of changes in the small airways that can not be detected by spirometry.

Effects of salmeterol and fluticasone propionate combination versus fluticasone propionate on airway function and eosinophilic inflammation in mild asthma

BACKGROUND

Salmeterol and fluticasone propionate combination (SFC) provides better asthma control than fluticasone propionate (FP) alone, however, little is known on the effects of differential treatments on airway function and inflammation in patients with mild asthma.

METHODS

We randomized 27 mild persistent asthma patients treated with the equivalent of 400 microg beclomethasone dipropionate to receive SFC (50/100 microg , 13 patients) or FP (100 microg , 14 patients) twice daily for 8 weeks. We compared the effects of SFC and FP on pulmonary function assessed by spirometry and impulse oscillometry (IOS), eosinophil percentage of induced sputum and serum, and with asthma symptoms and control after each treatment.

RESULTS

We observed that SFC significantly improved forced expiratory volume in one second (p < 0.05), IOS measurements of total resistance R5 (p < 0.01), central resistance R20 (p < 0.05), and distal reactance X5 (p < 0.01) compared with FP. The percentage of eosinophils in sputum, but not in serum, decreased significantly more in the SFC group than in the FP group (p < 0.05). There was also a significant improvement in symptom control in the SFC group (p < 0.05).

CONCLUSIONS

These findings suggest that SFC is more useful than FP in mild asthma cases. The clinical benefit of SFC provides evidence that IOS and induced sputum allows for the detection of changes in airway function and inflammation.

Non-invasive biomarkers and pulmonary function in smokers

Limited information exists regarding measurement, reproducibility and interrelationships of non-invasive biomarkers in smokers. We compared exhaled breath condensate (EBC) leukotriene B₄ (LTB₄) and 8-isoprostane, exhaled nitric oxide, induced sputum, spirometry, plethysmography, impulse oscillometry and methacholine reactivity in 18 smokers and 10 non-smokers. We assessed the relationships between these measurements and within-subject reproducibility of EBC biomarkers in smokers. Compared to non-smokers, smokers had significantly lower MMEF % predicted (mean 64.1 vs 77.7, p = 0.003), FEV₁/FVC (mean 76.2 vs 79.8 p = 0.05), specific conductance (geometric mean 1.2 vs 1.6, p = 0.02), higher resonant frequency (mean 15.5 vs 9.9, p = 0.01) and higher EBC 8-isoprostane (geometric mean 49.9 vs 8.9 pg/ml p = 0.001). Median EBC pH values were similar, but a subgroup of smokers had airway acidification (pH < 7.2) not observed in non-smokers. Smokers had predominant sputum neutrophilia (mean 68.5%). Repeated EBC measurements showed no significant differences between group means, but Bland Altman analysis showed large individual variability. EBC 8-isoprostane correlated with EBC LTB₄ (r = 0.78, p = 0.0001). Sputum supernatant IL-8 correlated with total neutrophil count per gram of sputum (r = 0.52, p = 0.04) and with EBC pH (r = −0.59, p = 0.02). In conclusion, smokers had evidence of small airway dysfunction, increased airway resistance, reduced lung compliance, airway neutrophilia and oxidative stress.

Exercise-induced changes in respiratory impedance in young wheezy children and nonatopic controls

Exercise-induced bronchoconstriction (EIB) is a specific sign of active asthma, but its assessment in young children may be difficult with lung function techniques requiring active cooperation. The aim of the study was to assess the normal pattern of exercise-induced responses of respiratory impedance by using impulse oscillometry (IOS), and to investigate how these responses discriminate wheezy children from control subjects. IOS measurements were performed in a consecutive sample of wheezy children aged 3-7 years (n = 130) and in an aged matched control group of nonatopic children without respiratory symptoms (n = 79) before and after a free running test. After exercise, wheezy children showed significantly larger responses in respiratory resistance (Rrs5), reactance (Xrs5), and the resonance frequency (Fr) than the control subjects. In the control group, the upper 95% confidence limit of the maximal change was 32.5% for Rrs5, 85.7% for Xrs5, and 53.1% for Fr. By using analysis of receiver operating characteristics, the change in Rrs5 distinguished the wheezy children from the control subjects more effectively than change in Xrs5 or Fr. In wheezy children, the response was significantly effected by the outdoor temperature and exercise intensity in terms of maximum heart rate. In conclusion, an increase of 35% in Rrs5 after a free running test can be regarded as an abnormal response. Wheezy children show an enhanced airway response, which is clearly distinguishable from the control subjects. IOS is a feasible method to detect EIB in young children.

The use of plethysmography and oscillometry to compare long-acting bronchodilators in patients with COPD

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

Forced expiratory volume in 1 s (FEV(1)) is the standard measurement used to measure drug effects in chronic obstructive pulmonary disease (COPD) clinical trials. Having previously shown that specific airway conductance (sGaw) measured using body plethysmography and impulse oscillometry (IOS) are more sensitive than FEV(1) for assessing short-acting bronchodilator effects in patients with COPD, we conducted the first randomized, placebo-controlled study to compare long-acting bronchodilators in COPD patients using these techniques.

WHAT THIS STUDY ADDS

sGaw and IOS sensitively differentiated between the effects of tiotropium and salmeterol when FEV(1) measurements were similar. sGaw and IOS measurements are better than FEV(1) for sensitively assessing bronchodilator pharmacology and differentiating between treatments in COPD clinical trials.

AIMS

Assessment of bronchodilator pharmacology in chronic obstructive pulmonary disease (COPD) may be improved by using more sensitive methods than spirometry, such as impulse oscillometry (IOS) and body plethysmography. We sought to compare salmeterol (S) and tiotropium (Tio) using these methods.

METHODS

In this double-blind, randomized, four-way crossover study, 32 COPD patients received single doses of Tio (18 microg), S (50 and 100 microg) or placebo. Specific airway conductance (sGaw), forced expiratory volume in 1 s (FEV(1)) and IOS were measured pre- and up to 26 h postdose. Comparisons between treatments were analysed by weighted means (WM) between 0 and 12 (WM 0-12 h) and 12-24 h (WM 12-24 h) postdose. Data are expressed as mean difference (or geometric ratio for nonparametric data) with 95% confidence intervals.

RESULTS

Tio and S100 significantly improved FEV(1), sGaw and IOS parameters up to 26 h and S50 up to 16 h. WM analysis showed no difference between Tio and S100 in FEV(1) for 0-12 h or 12-24 h. Maximum mid-expiratory flow (-0.06; -0.11, -0.01) and R35 (0.02; 0.01, 0.03) demonstrated superiority of S100 compared with Tio for WM 0-12 h sGaw (1.12; 1.02, 1.23), R5 (-0.06; -0.09, -0.02), R15 (-0.03; -0.05, -0.01), and resonant frequency (RF) (-2.30; -3.83, -0.77) showed superiority of Tio compared with S100 for WM 12-24 h. At 26 h, sGaw, R5, R15, X5 and RF also showed superiority of Tio compared with S100.

CONCLUSIONS

sGaw and IOS parameters sensitively differentiated between the effects of Tio and S when FEV(1) measurements were similar. Clinical trials in patients with COPD should use IOS and sGaw to assess comprehensively bronchodilator pharmacology.

Comparison of the effects of salmeterol/fluticasone propionate with fluticasone propionate on airway physiology in adults with mild persistent asthma

Background

This study compared the effect of inhaled fluticasone propionate (FP) with the combination of salmeterol/fluticasone propionate (SFC) on lung function parameters in patients with mild asthma.

Methods

Adult patients with mild persistent asthma (≥ 80% predicted FEV₁) receiving 200–500 μg of BDP or equivalent were randomised to receive either FP 100 μg or SFC 50/100 μg twice daily from a Diskus^® inhaler for four weeks. The primary outcome was the change from baseline in airway resistance (sRaw) at 12 hrs post dose measured by whole body plethysmography. Impulse oscillometry and spirometry were also performed.

Results

A comparison of the geometric mean sRaw at 12 hrs post dose in the SFC group to the FP group gave a ratio of 0.76 (0.66 – 0.89, p < 0.001) at week 2 and 0.81 (0.71 – 0.94, p = 0.006) at week 4. Similarly, significant results in favour of SFC for oscillometry measurements of resistance and reactance were observed. FEV₁ was also significantly superior at week 2 in the SFC group (mean difference 0.16L, 95% CI; 0.03 – 0.28, p = 0.015), but not at week 4 (mean difference 0.17L, 95% CI -0.01 – 0.34, p = 0.060).

Conclusion

SFC is superior to FP in reducing airway resistance in mild asthmatics with near normal FEV₁ values. This study provides evidence that changes in pulmonary function in patients with mild asthma are detected more sensitively by plethysmography compared to spirometry

Trial registration number

NCT00370591.

Assessment of bronchodilator responsiveness in preschool children using forced oscillations

BACKGROUND

The forced oscillation technique (FOT) requires minimal patient cooperation and is feasible in preschool children. Few data exist on respiratory function changes measured using FOT following inhaled bronchodilators (BD) in healthy young children, limiting the clinical applications of BD testing in this age group. A study was undertaken to determine the most appropriate method of quantifying BD responses using FOT in healthy young children and those with common respiratory conditions including cystic fibrosis, neonatal chronic lung disease and asthma and/or current wheeze.

METHODS

A pseudorandom FOT signal (4-48 Hz) was used to examine respiratory resistance and reactance at 6, 8 and 10 Hz; 3-5 acceptable measurements were made before and 15 min after the administration of salbutamol. The post-BD response was expressed in absolute and relative (percentage of baseline) terms.

RESULTS

Significant BD responses were seen in all groups. Absolute changes in BD responses were related to baseline lung function within each group. Relative changes in BD responses were less dependent on baseline lung function and were independent of height in healthy children. Those with neonatal chronic lung disease showed a strong baseline dependence in their responses. The BD response in children with cystic fibrosis, asthma or wheeze (based on both group mean data and number of responders) was not greater than in healthy children.

CONCLUSIONS

The BD response assessed by the FOT in preschool children should be expressed as a relative change to account for the effect of baseline lung function. The limits for a positive BD response of -40% and 65% for respiratory resistance and reactance, respectively, are recommended.

Deep inhalation prevents the respiratory elastance response to methacholine in rats

The bronchodilator effect of deep inhalation (DI) may be assessed from the time course of respiratory system resistance (Rrs) and reactance (Xrs) measured by the forced oscillation technique at a single frequency. The aim of the study was to assess the effect of DI in the closed chest rat. Under anesthesia and mechanical ventilation, seven Brown Norway rats were given regular DI (BN-di) and six underwent continuous tidal ventilation (BN) throughout an otherwise similar methacholine (Mch) challenge protocol. Rrs and Xrs were monitored at 20 Hz and apparent respiratory system elastance (Ers) was computed from Xrs. After Mch nebulization, there was a significant increase in Rrs and Ers compared with saline. Ers, but not Rrs, decreased after the DI and BN-di were found to have lower Ers than BN. Thus, DI significantly alters Ers and its response to Mch. Computer simulations suggested reversal of increased viscoelasticity and/or inhomogeneous behavior by the DI in that model.

Montelukast improves pulmonary function measured by impulse oscillometry in children with asthma (Mio study)

BACKGROUND

Systemic drugs-like oral montelukast can reach lower airways, whose inflammation plays a crucial role in the evolution of asthma, while inhaled drugs hardly reach them. The impulse oscillometry (IOS) technique is useful to evaluate both central and peripheral airways function.

OBJECTIVE

To measure the effect of oral montelukast on airways resistance evaluated by oscillometry in children with asthma.

METHODS

In an open study, respiratory function in 23 children with mild asthma and a positive bronchodilator response was assessed by spirometry and oscillometry. They took oral montelukast during 4 weeks and were again evaluated. As a control group, 23 similar patients with no preventive treatment underwent the same study.

MEASUREMENTS AND MAIN RESULTS

Children on oral montelukast showed improvements (measured in kPa s L(-1)) in all oscillometry parameters: mean 0.20 (22.4%) in total respiratory impedance Zrs5, 0.18 (21.8%) in total airway resistance Rrs5, 0.09 (17.8%) in central airway resistance Rrs20, and 0.09 (28.8%) in distal capacitive reactance Xrs5; the frequency of resonance Fres improved 2.3 Hz (8.7%) (P<0.05 in all cases). No changes were found in the control group. Expiratory flows showed no changes except for a small (0.23 L s(-1), 7.4%) but significant worsening of FEF25-75 in the control group.

CONCLUSIONS

Montelukast improves central and especially peripheral airways function in the first month of treatment, as evaluated by IOS, a technique based on tidal breathing analysis which is more sensitive than conventional forced spirometry.

A comparison of plethysmography, spirometry and oscillometry for assessing the pulmonary effects of inhaled ipratropium bromide in healthy subjects and patients with asthma

AIMS

We have compared the ability of plethysmography (sGaw), impulse oscillometry (IOS) and spirometry (FEV(1), MMEF) to detect bronchodilation in response to an anticholinergic.

METHODS

IOS (R5, R20, X5, RF), sGaw and spirometry were measured in 12 healthy subjects and 12 asthmatics. Variability was assessed by performing two measurements, 30 min apart and the effect of increasing the number of readings for each sGaw measurement was also studied. Ipratropium bromide (IB) 10, 20, 100 and 200 microg was administered and the sensitivity of the methods compared by determining the lowest dose that caused changes greater than variability.

RESULTS

In healthy subjects significant changes (P < or = 0.05) occurred at 10 microg for FEV(1) (mean [95% CI]; 1.3%[0.3-2.3]), R5 (mean [95% CI]; -7.9%, [-13.2-2.6]) and R20 (mean [95% CI], -6.4%, [-11.4-1.4]). No significant change was detected when the mean of 3 sGaw readings was used, but with 10 readings significant change was observed at 20 microg; (mean increase [95% CI] 15.2%[8.3-22.1]). In asthmatics significant changes (P < or = 0.05) occurred with IB 10 microg for sGaw (mean [95% CI] 25.6%[11.1-40.1]), R5 (mean [95% CI]-11.3%, [-15.5-7.2]), RF (mean [95% CI] 11.7%[6.1-16.3]), FEV(1) (mean [95% CI] 5.1%[2.6-7.7]) and MMEF (mean [95% CI] 12.3%[2.3-22.2]).

CONCLUSION

IOS and spirometry are more sensitive than sGaw in healthy subjects, but the sensitivity of sGaw improved when the number of readings was increased. The most sensitive method for assessing bronchodilation in asthmatics was sGaw.

A comparison of lung function methods for assessing dose-response effects of salbutamol

Pulmonary function methods which are able to detect small pharmacological effects may be useful for assessing the full dose-response curve of bronchodilatators. We compared the ability of impulse oscillometry (R5, R20, X5, RF), plethysmography (sGaw) and spirometry [forced expiratory volume in 1 s (FEV(1)), maximal mid expiratory flow rate (MMEF)] to measure the dose-response effects of salbutamol in 12 healthy subjects, 12 mild asthmatics (mean FEV(1) 96% predicted) and 12 moderate asthmatics (mean FEV(1) 63% predicted). The techniques were performed twice to assess variability. Then salbutamol 10, 20, 100, 200 and 800 microg was administered. The sensitivity of the methods were compared by determining the lowest dose that caused changes greater than variability. In healthy subjects significant changes (p < or = 0.05) were observed only in FEV(1) (4.1%) and MMEF (14.6%) at 100 microg and sGaw (25.6%) and R20 (8.3%) at 200 microg. In mild asthmatics significant changes were observed in sGaw (15.9%) at 10 microg, X5 (23%), RF (20.3%) and MMEF (15.7%) at 20 microg, R5 (13.9%) and R20 (9.4%) at 100 microg and FEV(1) (7.1%) at 200 microg. All measurements except R20 demonstrated significant changes at 10 micro g in moderate asthmatics. The most sensitive test for assessing bronchodilatation is different in healthy subjects and asthmatics, and varies with severity of airflow obstruction.

Diurnal variations of respiratory system resistance and compliance derived from input impedance in asthmatic children

The forced oscillation technique is more and more often used in the evaluation of the results of bronchomotor tests and in assessing respiratory mechanics in children. There is little information available concerning diurnal variability of indices of respiratory mechanics derived from input impedance (Z(rsin)). In 29 asthmatic children, aged 8-13 years we have studied diurnal changes of respiratory system resistance (R(rs)) and compliance (C(rs)) by measuring input impedance with the head generator every hour from 08:00 to 21:00 h. Diurnal coefficient of variation for zero frequency resistance (R(0)), mean resistance in the range 4-32 Hz (R(avg)) and C(rs) were 10.6+/-4.9, 11.2+/-4.5 and 8.9+/-3.3%, respectively. Significant circadian rhythm on R(0) was detected in 11 children, on R(avg) in nine children and on C(rs) in seven children. Only three children had all the three rhythms significant. We conclude, that diurnal variability of input impedance derived indices of respiratory mechanics is rather small, and circadian variability is present in about one third of the examined children. Diurnal variations of respiratory mechanics in children with significant circadian rhythm do not exceed 20%.

Input respiratory impedance measured by head generator in preschool children

The upper airway wall motion represents a serious problem when measuring the input impedance of the respiratory system (Zrs) by the forced oscillation technique, particularly in young children. To minimize this error, it has been proposed to vary transrespiratory pressure around the head rather than directly at the mouth, using the head generator technique (HGT). The aim of this study was to collect normative data in preschool children in whom the technique may prove most useful. Zrs was measured using HGT and 4-32-Hz pseudorandom noise input in 127 healthy children. Age ranged from 2. 8 to 7.4 years and height (H) from 0.89 to 1.29 m. The fast Fourier transforms of pressure and flow allowed us to calculate respiratory system resistance (Rrs(f)) and reactance (Xrs(f)) at each frequency (f). Resonant frequency (fn), respiratory system inertance (Irs), and compliance (Crs) were derived from the Xrs(f) data. The technique was accepted by more than 95% of the children. A coherence function </=0.95 required us to reject Zrs values in the lower frequency spectrum. These missing data did not allow reliable estimation of fn, Irs, and Crs in 44% of children. Regression equations of Zrs on growth parameters were obtained. H was the only significant predictive variable. The decrease of Rrs with growth was best described by a multiplicative model [e.g., ln (Rrs(20)) = 2. 5301 - 2.3837. ln (H)]. The slope of the regression lines of Xrs on H was dependent on f: positive between 4 and 16 Hz, negative at high frequency (e.g., 32 Hz), and nonsignificant in the intermediate range (e.g., 20 Hz). Irs and fn decreased significantly and Crs increased as H increased. The current study provides reference values for input impedance of the respiratory system in preschool children with upper airway artifacts minimized. HGT should be particularly helpful at or around 20 Hz, where the signal to noise ratio is larger than at lower frequencies but where upper airway artifacts may significantly corrupt measurements with the standard generator.