Peripheral airway involvement in asthma assessed by single-breath SF6 and He washout

Flexible Bronchoscopy Biopsy Tools and Techniques to Optimize Diagnostic Yield: A Contemporary Review

Background:

Flexible bronchoscopy is essential in diagnosing many pathologic conditions, and tools such as lavage and biopsies using brushes, forceps, and needles are paramount to success.

Introduction:

Bronchoscopists worldwide are routinely confronted with questions about such tools regarding the type, size, utility, costs, safety, anticipated yield, and others. Does the underlying suspected condition matter to the choice of instruments used? What is the anticipated outcome for benign versus malignant diseases? These and other questions are raised daily by bronchoscopists.

Methods:

Pubmed was reviewed for research in the English language pertaining to diagnostic bronchoscopy. The literature is conflicting on the benefits of the types of tools available. The success of brush biopsies, forceps, and transbronchial needle aspiration is only partially dependent on the size of the instrument used or its other characteristics. Multiple biopsies are needed, and different approaches may be complementary in some circumstances.

Results:

By understanding the factors that involve in a biopsy, the bronchoscopist is more likely to be successful when a crucial diagnosis is mandatory.

Conclusion:

This review aims to be a reference to bronchoscopists everywhere as they contemplate their approach to flexible diagnostic bronchoscopy.

Priorities for future research into asthma diagnostic tools: A PAN-EU consensus exercise from the European asthma research innovation partnership (EARIP)

The diagnosis of asthma is currently based on clinical history, physical examination and lung function, and to date, there are no accurate objective tests either to confirm the diagnosis or to discriminate between different types of asthma. This consensus exercise reviews the state of the art in asthma diagnosis to identify opportunities for future investment based on the likelihood of their successful development, potential for widespread adoption and their perceived impact on asthma patients. Using a two-stage e-Delphi process and a summarizing workshop, a group of European asthma experts including health professionals, researchers, people with asthma and industry representatives ranked the potential impact of research investment in each technique or tool for asthma diagnosis and monitoring. After a systematic review of the literature, 21 statements were extracted and were subject of the two-stage Delphi process. Eleven statements were scored 3 or more and were further discussed and ranked in a face-to-face workshop. The three most important diagnostic/predictive tools ranked were as follows: "New biological markers of asthma (eg genomics, proteomics and metabolomics) as a tool for diagnosis and/or monitoring," "Prediction of future asthma in preschool children with reasonable accuracy" and "Tools to measure volatile organic compounds (VOCs) in exhaled breath."

Small airways dysfunction: the link between allergic rhinitis and allergic asthma

Abnormal airway reactivity and overproduction of nitric oxide (NO) occurring in small airways have been found in asthma. If the "one airway, one disease" concept is consistent, such dysfunctions should also be detected in the peripheral airways of patients suffering from allergic rhinitis.We investigated whether peripheral airway reactivity and NO overproduction could be documented in distal airways in patients with allergic rhinitis. Exhaled NO fraction (F_eNO) and the slope (S) of phase III of the single-breath washout test (SBWT) of helium (He) and sulfur hexafluoride (SF₆) were measured in 31 patients with allergic asthma, 23 allergic rhinitis patients and 24 controls, before and after sputum induction. SBWT is sensitive to airway calibre change occurring in the lung periphery.The F_eNO decrease was more significant in asthma and rhinitis than in controls (-55.1% and -50.0%, respectively, versus -40.8%) (p=0.007 and p=0.029, respectively). S_SF6 and S_He increased in all groups. Change in S_He (ΔS_He) > ΔS_SF6 was observed in rhinitis (p=0.004) and asthma (p<0.001), whereas ΔS_SF6 = ΔS_He in controls (p=0.431).This study provides evidence of peripheral airway dysfunction in patients with allergic rhinitis quite similar to that described in asthma. Furthermore, a large proportion of the increased NO production reported in allergic rhinitis appears to originate in the peripheral airways.

Small airway function before and after cold dry air challenge in pediatric asthma patients during remission

BACKGROUND

We wanted to compare cold dry air challenge (CACh) induced changes in spirometric parameters with changes in nitrogen multiple breath washout (N₂ MBW) parameters in pediatric asthma patients during clinical remission over the past year (ie, with "inactive asthma"). As N₂ MBW assesses ventilation heterogeneity we expected to gain detailed information about peripheral airways contribution.

METHODS

In subjects with normal spirometry N₂ MBW, spirometry and body plethysmography were performed at baseline, after CACh, and after salbutamol inhalation. An initial measurement of the fraction of exhaled nitric oxide (FeNO) was conducted.

RESULTS

Forty-three (20 female) subjects, mean age 13.7 years (range 6.5-18.6) performed reproducible N₂ MBW measurements. Ten were tested hyperresponsive (23.3%) and 33 normoresponsive (76.7%). Baseline spirometry and body plethysmography as well as FRC (N₂ MBW) were similar in both groups. Scond (0.031 vs 0.022), Sacin (0.057 vs 0.067), and FeNO (92.0 vs 28.5 ppb) were not statistically different between hyperresponsive and nomoresponsive subjects at baseline. Subjects with airway hyperresponsiveness (AHR) showed significant increases in lung clearance index (LCI, P = 0.011) and Scond (P = 0.008) after CACh, and significant decreases after salbutamol (LCI: P = 0.005; Scond: P = 0.005). In contrast, normoresponsive subjects showed no relevant changes after CACh, and only a decrease of Scond after salbutamol (P = 0.007). There were significant correlations between the CACh induced changes in FEV1 and changes in LCI (r = -0.45, P = 0.003), Scond (r = -0.30, P = 0.047), and Sacin (r = -0.47, P = 0.008), respectively.

CONCLUSION

Our study provides evidence of small airway involvement in children and adolescents with inactive asthma and airway hyperresponsiveness.

A simple method to reconstruct the molar mass signal of respiratory gas to assess small airways with a double-tracer gas single-breath washout

For the assessment of small airway diseases, a noninvasive double-tracer gas single-breath washout (DTG-SBW) with sulfur hexafluoride (SF₆) and helium (He) as tracer components has been proposed. It is assumed that small airway diseases may produce typical ventilation inhomogeneities which can be detected within one single tidal breath, when using two tracer components. Characteristic parameters calculated from a relative molar mass (MM) signal of the airflow during the washout expiration phase are analyzed. The DTG-SBW signal is acquired by subtracting a reconstructed MM signal without tracer gas from the signal measured with an ultrasonic sensor during in- and exhalation of the double-tracer gas for one tidal breath. In this paper, a simple method to determine the reconstructed MM signal is presented. Measurements on subjects with and without obstructive lung diseases including the small airways have shown high reliability and reproducibility of this method.

Small airway dysfunction and bronchial asthma control : the state of the art

According to national and international guidelines, achieving and maintaining asthma control is a major goal of disease management. In closely controlled clinical trials, good asthma control can be achieved , with the medical treatments currently available, in the majority of patients , but large population-based studies suggest that a significant proportion of patients in real-life setting experience suboptimal levels of asthma control and report lifestyle limitations with a considerable burden on quality of life. Poor treatment adherence and persistence, failure to use inhalers correctly, heterogeneity of asthma phenotypes and associated co-morbidities are the main contributing factors to poor disease control. Now, it is widely accepted that peripheral airway dysfunction , already present in patients with mild asthma, is a key contributor of worse control. The aim of this paper is to investigate the association between small-airways dysfunction and asthma symptoms/control. We therefore performed a PubMed search using keywords : small airways; asthma (limits applied: Humans, English language) and selected papers with a study population of asthmatic patients, reporting measurement of small-airways parameters and clinical symptoms/control.

Relationship between the baseline alveolar volume-to-total lung capacity ratio and airway responsiveness

BACKGROUND AND OBJECTIVE

Ventilation heterogeneity (VH) has been linked to airway responsiveness (AR) based on various measures of VH involving inert gas washout, forced oscillation and lung imaging. We explore whether VH at baseline, as measured by the simple ratio of single breath alveolar volume to plethysmographically determined total lung capacity (VA/TLC), would correlate with AR as measured by methacholine challenge testing.

METHODS

We analysed data from spirometry, lung volumes, diffusing capacity and methacholine challenge to derive the VA/TLC and the dose-response slope (DRS) of forced expiratory volume in 1 s (DRS-FEV1) during methacholine challenge from 136 patients. We separated out airway closure versus narrowing by examining the DRS for forced vital capacity (DRS-FVC) and the DRS for FEV1/FVC (DRS-FEV1/FVC), respectively. Similarly, we calculated the DRS for sGaw (DRS-sGaw) as another measure of airway narrowing. We performed statistical analysis using Spearman rank correlation and multifactor linear regression using a backward stepwise modelling procedure.

RESULTS

We found that the DRS-FEV1 correlated with baseline VA/TLC (rho = -0.26, P < 0.01), and VA/TLC and FEV1 were independently associated with DRS-FEV1 (R(2)  = 0.14, P = 0.01). In addition, VA/TLC was associated with both airway narrowing and closure in response to methacholine.

CONCLUSIONS

These results confirm that baseline VA/TLC is associated with AR, and reflects both airway closure and airway narrowing following methacholine challenge.

Abnormal small airways function in children with mild asthma

BACKGROUND

Small airways disease is a hallmark in adults with persistent asthma, but little is known about small airways function in children with mild asthma and normal spirometry. We assessed ventilation heterogeneity, a marker of small airways function, with an easy tidal breath single-breath washout (SBW) technique in school-aged children with mild asthma and normal FEV1 and healthy age-matched control subjects.

METHODS

The primary outcome was the double-tracer gas phase III slope (SDTG), an index of ventilation heterogeneity in acinar airways derived from the tidal double-tracer gas SBW test. The second outcome was the nitrogen phase III slope (SN2), an index of global ventilation heterogeneity derived from the tidal nitrogen SBW test using pure oxygen. Triplicate SBW and spirometry tests were performed in healthy children (n=35) and children with asthma (n=31) at baseline and in children with asthma after bronchodilation.

RESULTS

Acinar (SDTG) but not global (SN2) ventilation heterogeneity was significantly increased in asthma despite normal FEV1. Of the 31 children with asthma, abnormal results were found for SDTG (≤-2 z scores) in 11; forced expiratory flow, midexpiratory phase (FEF25%-75%) in three; and FEV1 in zero. After bronchodilation, SDTG, SN2, FEF25%-75%, and FEV1 significantly changed (mean [95% CI] change from baseline, 36% [15%-56%], 38% [18%-58%], 17% [9-25%], and 6% [3%-9%], respectively).

CONCLUSIONS

Abnormal acinar ventilation heterogeneity in one-third of the children suggests that small airways disease may be present despite rare and mild asthma symptoms and normal spirometry. The easy tidal SBW technique has considerable potential as a clinical and research outcome in children with asthma.

Exhaled nitric oxide: a biomarker integrating both lung function and airway inflammation changes

BACKGROUND

The increased fraction of exhaled nitric oxide (Feno) values observed in asthmatic patients are thought to reflect increased airway inflammation. However, Feno values can be affected by airway caliber reduction, representing a bias when using Feno values to assess asthma control.

OBJECTIVE

We sought to determine the effect of changes in both airway caliber and inflammation on Feno values using the allergen challenge model.

METHODS

FEV1 and Feno values were measured during early airway responses (EARs) and late airway responses after challenge with house dust mite allergens in 15 patients with mild allergic asthma. Helium and sulfur hexafluoride (SF6) phase III expired concentration slopes (SHe and SSF6, respectively) from single-breath washout tests were measured to identify sites of airway constriction.

RESULTS

In EARs, FEV1 and Feno value decreases reached 36.8% and 22%, respectively (P < .001). ΔSHe was greater than ΔSSF6 (+189.4% vs +82.2%, P = .001). In late airway responses FEV1 and Feno value decreases reached 31.7% and 28.7%, respectively (P < .001), with the same ΔSHe and ΔSSF6 pattern (+155.8% vs +76%, P = .001). Eight hours after the EAR, FEV1 was still decreased (P < .001), whereas Feno values had returned to baseline. At 24 hours, FEV1 had returned to baseline, with Feno values increased by 38.7% (P = .04).

CONCLUSION

In patients with mild allergic asthma, airway caliber changes modulate changes in Feno values resulting from airway inflammation. Therefore Feno should no longer be considered solely an inflammation biomarker but rather a biomarker that integrates both airway inflammation and lung function changes. Furthermore, early and late phases resulting from allergen exposure were shown to involve similar lung regions.

Evaluation of the impact of alveolar nitrogen excretion on indices derived from multiple breath nitrogen washout

BACKGROUND

A large body of evidence has now accumulated describing the advantages of multiple breath washout tests over conventional spirometry in cystic fibrosis (CF). Although the majority of studies have used exogenous sulphur hexafluoride (SF6) as the tracer gas this has also led to an increased interest in nitrogen washout tests, despite the differences between these methods. The impact of body nitrogen excreted across the alveoli has previously been ignored.

METHODS

A two-compartment lung model was developed that included ventilation heterogeneity and dead space (DS) effects, but also incorporated experimental data on nitrogen excretion. The model was used to assess the impact of nitrogen excretion on washout progress and accuracy of functional residual capacity (FRC) and lung clearance index (LCI) measurements.

RESULTS

Excreted nitrogen had a small effect on accuracy of FRC (1.8%) in the healthy adult model. The error in LCI calculated with true FRC was greater (6.3%), and excreted nitrogen contributed 21% of the total nitrogen concentration at the end of the washout. Increasing DS and ventilation heterogeneity both caused further increase in measurement error. LCI was increased by 6-13% in a CF child model, and excreted nitrogen increased the end of washout nitrogen concentration by 24-49%.

CONCLUSIONS

Excreted nitrogen appears to have complex but clinically significant effects on washout progress, particularly in the presence of abnormal gas mixing. This may explain much of the previously described differences in washout outcomes between SF6 and nitrogen.

Effect of methacholine on peripheral lung mechanics and ventilation heterogeneity in asthma

The forced oscillation technique (FOT) and multiple-breath nitrogen washout (MBNW) are noninvasive tests that are potentially sensitive to peripheral airways, with MBNW indexes being especially sensitive to heterogeneous changes in ventilation. The objective was to study methacholine-induced changes in the lung periphery of asthmatic patients and determine how changes in FOT variables of respiratory system reactance (Xrs) and resistance (Rrs) and frequency dependence of resistance (Rrs5-Rrs19) can be linked to changes in ventilation heterogeneity. The contributions of air trapping and airway closure, as extreme forms of heterogeneity, were also investigated. Xrs5, Rrs5, Rrs19, Rrs5-Rrs19, and inspiratory capacity (IC) were calculated from the FOT. Ventilation heterogeneity in acinar and conducting airways, and trapped gas (percent volume of trapped gas at functional residual capacity/vital capacity), were calculated from the MBNW. Measurements were repeated following methacholine. Methacholine-induced airway closure (percent change in forced vital capacity) and hyperinflation (change in IC) were also recorded. In 40 mild to moderate asthmatic patients, increase in Xrs5 after methacholine was predicted by increases in ventilation heterogeneity in acinar airways and forced vital capacity ( r2 = 0.37, P < 0.001), but had no correlation with ventilation heterogeneity in conducting airway increase or IC decrease. Increases in Rrs5 and Rrs5-Rrs19 after methacholine were not correlated with increases in ventilation heterogeneity, trapped gas, hyperinflation, or airway closure. Increased reactance in asthmatic patients after methacholine was indicative of heterogeneous changes in the lung periphery and airway closure. By contrast, increases in resistance and frequency dependence of resistance were not related to ventilation heterogeneity or airway closure and were more indicative of changes in central airway caliber than of heterogeneity.

Variability of a closed, rebreathing setup for multiple breath wash-out testing in children

OBJECTIVE

The multiple breath wash-out technique (MBW) that measures lung clearance index (LCI) and functional residual capacity (FRC) may be more sensitive than spirometry for identification of early obstructive airways disease. The open MBW setup using mass spectrometry referenced in previous publications is not readily available in the U.S. Our objective was to assess validity and sensitivity of a commercially available device that uses a closed (rebreathing) setup with photoacoustic spectroscopy for MBW testing.

STUDY DESIGN AND METHODS

Subjects aged 5-21 who were either healthy or had a history of cystic fibrosis were enrolled. Subjects completed MBW (Innocor device; Innovision, Denmark) and spirometry; measures obtained included LCI, FRC, and forced expiratory volume in 1 sec, as well as changes in end-tidal carbon dioxide levels (CO(2) ) and tidal volume during MBW testing.

RESULTS

Seventeen subjects attempted a total of 76 MBW maneuvers; 80% were completed and 60% met criteria for acceptability; most were unacceptable due to errors in the tracer gas curve. Substantial intra-subject variability for LCI and FRC were noted (mean 26% ± 55 and 36% ± 63, respectively). Subjects were also noted to have significant increases in exhaled CO(2) and tidal volume during MBW testing.

CONCLUSIONS

In our initial experience using a commercially available closed setup for MBW testing, we found a significant degree of intra-subject variability leading us to suspend testing. Variability could be due to hypercapnea and instability of tidal breathing secondary to the rebreathing setup. Further studies are needed to better understand the closed system MBW setup.

Physiological measurement of the small airways

Noninvasive physiological measurements are reviewed that have been reported in the literature with the specific aim being to study the small airways in lung disease. This has mostly involved at-the-mouth noninvasive measurement of flow, pressure or inert gas concentration, with the intent of deriving one or more indices that are representative of small airway structure and function. While these measurements have remained relatively low-tech, the effort and sophistication increasingly reside with the interpretation of such indices. When aspiring to derive information at the mouth about structural and mechanical processes occurring several airway generations away in a complex cyclically changing cul-de-sac structure, conceptual or semi-quantitative lung models can be valuable. Two assumptions that are central to small airway structure-function measurement are that of an average airway change at a given peripheral lung generation and of a parallel heterogeneity in airway changes. While these are complementary pieces of information, they can affect certain small airways tests in confounding ways. We critically analyzed the various small airway tests under review, while contending that negative outcomes of these tests are probably a true reflection of the fact that no change occurred in the small airways. Utmost care has been taken to not favor one technique over another, given that most current small airways tests still have room for improvement in terms of rendering their content more specific to the small airways. One way to achieve this could consist of the coupling of signals collected at the mouth to spatial information gathered from imaging in the same patient.

Airway closure on imaging relates to airway hyperresponsiveness and peripheral airway disease in asthma

The regional pattern and extent of airway closure measured by three-dimensional ventilation imaging may relate to airway hyperresponsiveness (AHR) and peripheral airways disease in asthmatic subjects. We hypothesized that asthmatic airways are predisposed to closure during bronchoconstriction in the presence of ventilation heterogeneity and AHR. Fourteen asthmatic subjects (6 women) underwent combined ventilation single photon emission computed tomography/computed tomography scans before and after methacholine challenge. Regional airway closure was determined by complete loss of ventilation following methacholine challenge. Peripheral airway disease was measured by multiple-breath nitrogen washout from which S(cond) (index of peripheral conductive airway abnormality) was derived. Relationships between airway closure and lung function were examined by multiple-linear regression. Forced expiratory volume in 1 s was 87.5 ± 15.8% predicted, and seven subjects had AHR. Methacholine challenge decreased forced expiratory volume in 1 s by 23 ± 5% and increased nonventilated volume from 16 ± 4 to 29 ± 13% of computed tomography lung volume. The increase in airway closure measured by nonventilated volume correlated independently with both S(cond) (partial R(2) = 0.22) and with AHR (partial R(2) = 0.38). The extent of airway closure induced by methacholine inhalation in asthmatic subjects is greater with increasing peripheral airways disease, as measured by ventilation heterogeneity, and with worse AHR.

Lung function measurement with multiple-breath-helium washout system

Multiple-breath-washout (MBW) measurements are regarded as a sensitive technique which can reflect the ventilation inhomogeneity of respiratory airways. Typically nitrogen is used as the tracer gas and is washed out by pure oxygen in multiple-breath-nitrogen washout (MBNW) tests. In this study, instead of using nitrogen, (4)He is used as the tracer gas with smaller gas density which may be able to reach deeper into our lungs in a given time and the helium washout results may be more sensitive to the ventilation inhomogeneity in small airways. A multiple-breath-helium-washout (MBHW) system developed for the lung function study is also presented. Quartz tuning forks with a resonance frequency of 32,768Hz have been used for detecting the change of the respiratory gas density. The resonance frequency of the quartz tuning fork decreases linearly with increasing density of the surrounding gas. Knowing the CO2 concentration from the infrared carbon dioxide detector, the helium concentration can be determined. Results from 14 volunteers (3 mild asthmatics, 4 tobacco smokers, 1 with asthma history, 1 with COPD history, 5 normal) have shown that mild asthmatics have higher ventilation inhomogeneity in either conducting or acinar airways (or both). A feature has been found in washout curve of single breaths from 4 tobacco smokers with different length of smoking history which may indicate the early stage of respiratory ventilation inhomogeneity in acinar airways.

Lung damage and airway remodelling in severe asthma

Severe asthma is a heterogeneous disease with substantial unmet clinical need. Airway damage and remodelling is a consequence of complex host-environment interactions and is considered to be the cardinal feature leading onto the development and persistence of airflow obstruction. In this review, we shall bring together recent insights into the causes of airway damage and remodelling that propose key roles for pathogens and mechanical damage in addition to allergens, underlying genetic susceptibility, inflammatory and structural cell interactions, and impaired resolution of damage. We shall consider the consequences of airway remodelling in terms of airway geometry, mechanics and clinical expression of disease. Understanding the causes and consequences of airway damage and remodelling will shed light upon the structure-function relationships required to begin to unravel the complexity of severe asthma and will enable us to target current and novel therapies as we begin to move towards realizing personalized medicine.

Lung clearance index: clinical and research applications in children

Multiple breath washout (MBW) has been demonstrated to be sensitive for assessing ventilation inhomogeneity (VI). VI is supposed to reflect changes in peripheral airways which are not apparent using spirometry. The lung clearance index (LCI) is the most robust parameter to quantify VI, and is largely independent of age; therefore, it potentially qualifies as a surrogate outcome parameter for clinical and research purposes, particularly during childhood. This review summarizes the current evidence regarding the clinical value of measuring LCI in children. Feasibility, reproducibility and diagnostic accuracy have been demonstrated; available data confirm that LCI is superior to spirometry in detecting small air way disease. However, there is little information regarding the value in the individual patient, and sparse longitudinal data looking at its prognostic value. Currently, only in patients with Cystic Fibrosis, it appears likely that knowledge of LCI will be useful for routine clinical management.

Prediction of BOS by the single-breath nitrogen test in double lung transplant recipients

BACKGROUND

The present study analyses the ability of the alveolar slope of the single-breath nitrogen washout test (N2-slope) to diagnose and predict the development of the bronchiolitis obliterans syndrome (BOS).

METHODS

We present a retrospective analysis of 61 consecutive bilateral lung or heart-lung transplant recipients who were followed at regular control visits during a three year follow-up. The operating characteristics of the N2-slope to diagnose BOS and potential BOS (BOS 0-p) and to predict BOS were determined based on cut off values of 95% specificity.

RESULTS

The sensitivity of the N2-slope to identify BOS was 96%, and BOS 0-p 100%. The predictive ability to predict BOS with a N2-slope > 478% of the predicted normal was 56%, and if combined with a coincident FEV1 < 90% of the basal value, the predictive ability was 75%.

CONCLUSIONS

The predictive ability of either the N2-slope or of FEV1 to diagnose BOS is limited but the combination of the two appears useful. Follow-up protocols of bilateral lung and heart-lung transplant recipients should consider including tests sensitive to obstruction of the peripheral airways.

Adenosine 5'-monophosphate challenge elicits a more peripheral airway response than methacholine challenge

Adenosine 5'-monophosphate (AMP) and methacholine are commonly used to assess airway hyperreactivity. However, it is not fully known whether the site of airway constriction primarily involved during challenges with either agent is similar. Using a ventilation distribution test, we investigated whether the constriction induced by each agent involves the lung periphery in a similar fashion. Ventilation distribution was evaluated by the phase III slope (S) of the single-breath washout, using gases with different diffusivities like helium (He) and hexafluorosulfur (SF(6)). A greater postchallenge increase in S(He) reflects alterations at the level of terminal and respiratory bronchioles, while a greater increase in S(SF6) reflects alterations in alveolar ducts, increases to an equal extent reflecting alterations in more proximal airways where gas transport is still convective for both gases. S(SF6) and S(He) were measured in 15 asthma patients before and after airway challenges (20% forced expired volume in 1-s fall) with AMP and methacholine. S(He) increased to a greater extent than S(SF6) after AMP challenge (5.7 vs. 3.7%/l; P = 0.002), with both slopes increasing to an equal extent after methacholine challenge (3.1%/l; P = 0.959). The larger increase in S(He) following AMP challenge suggests distal ventilation impairment up to the level of terminal and respiratory bronchioles. With methacholine, the similar increases in S(He) and S(SF6) suggest a less distal impairment. AMP, therefore, seems to affect more extensively the very peripheral airways, whereas methacholine seems to have an effect on less distal airways.

Tidal volume single breath washout of two tracer gases--a practical and promising lung function test

Background

Small airway disease frequently occurs in chronic lung diseases and may cause ventilation inhomogeneity (VI), which can be assessed by washout tests of inert tracer gas. Using two tracer gases with unequal molar mass (MM) and diffusivity increases specificity for VI in different lung zones. Currently washout tests are underutilised due to the time and effort required for measurements. The aim of this study was to develop and validate a simple technique for a new tidal single breath washout test (SBW) of sulfur hexafluoride (SF₆) and helium (He) using an ultrasonic flowmeter (USFM).

Methods

The tracer gas mixture contained 5% SF₆ and 26.3% He, had similar total MM as air, and was applied for a single tidal breath in 13 healthy adults. The USFM measured MM, which was then plotted against expired volume. USFM and mass spectrometer signals were compared in six subjects performing three SBW. Repeatability and reproducibility of SBW, i.e., area under the MM curve (AUC), were determined in seven subjects performing three SBW 24 hours apart.

Results

USFM reliably measured MM during all SBW tests (n = 60). MM from USFM reflected SF₆ and He washout patterns measured by mass spectrometer. USFM signals were highly associated with mass spectrometer signals, e.g., for MM, linear regression r-squared was 0.98. Intra-subject coefficient of variation of AUC was 6.8%, and coefficient of repeatability was 11.8%.

Conclusion

The USFM accurately measured relative changes in SF₆ and He washout. SBW tests were repeatable and reproducible in healthy adults. We have developed a fast, reliable, and straightforward USFM based SBW method, which provides valid information on SF₆ and He washout patterns during tidal breathing.

Peripheral lung mechanics in asthma: exploring the outer limits

Asthma is a disease characterized by airways hyperresponsiveness (AHR), which is traditionally thought to involve the large, central airways. However, there is increasing evidence of the importance of peripheral airway involvement in asthma as well. Our group has developed particular expertise in measuring peripheral lung mechanics in both humans and mice. This presentation will review data on lung mechanics in subjects with asthma obtained by both classical means and uniquely through the wedged bronchoscope, as well as relevant experiments in mice. Our findings reveal that the lung periphery is hyperresponsive to stimuli in asthmatic subjects, with evidence of airway closure. We also show that the overall impedance of the lung is determined by a combination of peripheral airway narrowing and central airway shunting that occurs in both normal and asthmatic subjects. Experiments in mice have revealed the importance of airway closure in contributing to the phenomenon of AHR. Based on the effects of fibrin on lung mechanics, fibrin may contribute to airway closure via inactivation of surfactant. Another mechanism contributing to AHR is the heterogeneity of airway narrowing. We have explored this in humans by combining the forced oscillation technique with computerized tomography imaging of the lung, and demonstrated that heterogeneity is common to both normal and asthmatic subjects. Further experiments are ongoing and planned in both mice and humans to elucidate the role of fibrin, surfactant and heterogeneous airway narrowing and closure in contributing to AHR in asthma.

Lung volume, breathing pattern and ventilation inhomogeneity in preterm and term infants

Background

Morphological changes in preterm infants with bronchopulmonary dysplasia (BPD) have functional consequences on lung volume, ventilation inhomogeneity and respiratory mechanics. Although some studies have shown lower lung volumes and increased ventilation inhomogeneity in BPD infants, conflicting results exist possibly due to differences in sedation and measurement techniques.

Methodology/Principal Findings

We studied 127 infants with BPD, 58 preterm infants without BPD and 239 healthy term-born infants, at a matched post-conceptional age of 44 weeks during quiet natural sleep according to ATS/ERS standards. Lung function parameters measured were functional residual capacity (FRC) and ventilation inhomogeneity by multiple breath washout as well as tidal breathing parameters. Preterm infants with BPD had only marginally lower FRC (21.4 mL/kg) than preterm infants without BPD (23.4 mL/kg) and term-born infants (22.6 mL/kg), though there was no trend with disease severity. They also showed higher respiratory rates and lower ratios of time to peak expiratory flow and expiratory time (t_PTEF/t_E) than healthy preterm and term controls. These changes were related to disease severity. No differences were found for ventilation inhomogeneity.

Conclusions

Our results suggest that preterm infants with BPD have a high capacity to maintain functional lung volume during natural sleep. The alterations in breathing pattern with disease severity may reflect presence of adaptive mechanisms to cope with the disease process.

Effect of heterogeneous ventilation and nitric oxide production on exhaled nitric oxide profiles

Elevated exhaled nitric oxide (NO) in the breath of asthmatic subjects is thought to be a noninvasive marker of lung inflammation. Asthma is also characterized by heterogeneous bronchoconstriction and inflammation, which impact the spatial distribution of ventilation in the lungs. Since exhaled NO arises from both airway and alveolar regions, and its level in exhaled breath depends strongly on flow, spatial heterogeneity in flow patterns and NO production may significantly affect the exhaled NO signal. To investigate the effect of these factors on exhaled NO profiles, we developed a multicompartment mathematical model of NO exchange using a trumpet-shaped central airway segment that bifurcates into two similarly shaped peripheral airway segments, each of which empties into an alveolar compartment. Heterogeneity in flow alone has only a minimal impact on the exhaled NO profile. In contrast, placing 70% of the total airway NO production in the central compartment or the distal poorly ventilated compartment can significantly increase (35%) or decrease (-10%) the plateau concentration, respectively. Reduced ventilation of the peripheral and acinar regions of the lungs with concomitant elevated NO production delays the rise of NO during exhalation, resulting in a positive phase III slope and reduced plateau concentration (-11%). These features compare favorably with experimentally observed profiles in exercise-induced asthma and cannot be simulated with single-path models. We conclude that variability in ventilation and NO production in asthmatic subjects impacts the shape of the exhaled NO profile and thus impacts the physiological interpretation.

Association between Gm allotypes and asthma severity from childhood to young middle age

Immunoglobulin constant heavy G chain (IGHG) gene polymorphisms are associated with atopy and can be determined by the serum Gm allotypes. We studied whether certain polymorphisms are related to asthma severity and to the extent or intensity of allergic sensitization in asthmatic subjects followed from childhood to young middle age. Fifty-five subjects (28 males) with childhood asthma were all followed-up prospectively on six occasions from a mean age of 9 to 35 years in a study including asthma severity scoring, spirometry, skin prick, and specific serum IgE antibody testing. At the last visit, extended lung function tests and a cold air challenge were performed, and IGHG gene polymorphisms were identified by the alternative serum IgG subclass allotypes, employing ELISA and double immunodiffusion. The 19 subjects with the homozygous IGHG*bf/*bf genotype (originating from the IGHG3*b and the IGHG1*f alleles, which are in strong linkage disequilibrium), showed significantly higher asthma scores, lower airway function, and greater bronchodilator responses from childhood to adulthood, and in middle age greater airway hyperresponsiveness, compared to the subjects with the IGHG*bf/*ga or IGHG*ga/*ga genotypes. Among the subjects sensitized to animal danders, those with the IGHG*bf/*bf genotype showed the highest specific IgE levels. In conclusion, IGHG gene polymorphisms were associated with the severity and outcome of childhood asthma, and with the intensity of allergic sensitization.

Ventilation heterogeneity is a major determinant of airway hyperresponsiveness in asthma, independent of airway inflammation

BACKGROUND

Airway hyperresponsiveness is the ability of airways to narrow excessively in response to inhaled stimuli and is a key feature of asthma. Airway inflammation and ventilation heterogeneity have been separately shown to be associated with airway hyperresponsiveness. A study was undertaken to establish whether ventilation heterogeneity is associated with airway hyperresponsiveness independently of airway inflammation in subjects with asthma and to determine the effect of inhaled corticosteroids on this relationship.

METHODS

Airway inflammation was measured in 40 subjects with asthma by exhaled nitric oxide, ventilation heterogeneity by multiple breath nitrogen washout and airway hyperresponsiveness by methacholine challenge. In 18 of these subjects with uncontrolled symptoms, measurements were repeated after 3 months of treatment with inhaled beclomethasone dipropionate.

RESULTS

At baseline, airway hyperresponsiveness was independently predicted by airway inflammation (partial r2 = 0.20, p<0.001) and ventilation heterogeneity (partial r2 = 0.39, p<0.001). Inhaled corticosteroid treatment decreased airway inflammation (p = 0.002), ventilation heterogeneity (p = 0.009) and airway hyperresponsiveness (p<0.001). After treatment, ventilation heterogeneity was the sole predictor of airway hyperresponsiveness (r2 = 0.64, p<0.001).

CONCLUSIONS

Baseline ventilation heterogeneity is a strong predictor of airway hyperresponsiveness, independent of airway inflammation in subjects with asthma. Its persistent relationship with airway hyperresponsiveness following anti-inflammatory treatment suggests that it is an important independent determinant of airway hyperresponsiveness. Normalisation of ventilation heterogeneity is therefore a potential goal of treatment that may lead to improved long-term outcomes.

Identifying airways responsible for heterogeneous ventilation and mechanical dysfunction in asthma: an image functional modeling approach

We present an image functional modeling approach, which synthesizes imaging and mechanical data with anatomically explicit computational models. This approach is utilized to identify the relative importance of small and large airways in the simultaneous deterioration of mechanical function and ventilation in asthma. Positron emission tomographic (PET) images provide the spatial distribution and relative extent of ventilation defects in asthmatic subjects postbronchoconstriction. We also measured lung resistance and elastance from 0.15 to 8 Hz. The first step in image functional modeling involves mapping ventilation three-dimensional images to the computational model and identifying the largest sized airways of the model that, if selectively constricted, could precisely match the size and anatomic location of ventilation defects imaged by PET. In data from six asthmatic subjects, these airways had diameters <2.39 mm and mostly <0.44 mm. After isolating and effectively closing airways in the model associated with these ventilation defects, we imposed constriction with various means and standard deviations to the remaining airways to match the measured lung resistance and elastance from the same subject. Our results show that matching both the degree of mechanical impairment and the size and location of the PET ventilation defects requires either constriction of airways <2.4 mm alone, or a simultaneous constriction of small and large airways, but not just large airways alone. Also, whereas larger airway constriction may contribute to mechanical dysfunction during asthma, degradation in ventilation function requires heterogeneous distribution of near closures confined to small airways.

Noninvasive assessment of airway alterations in smokers: the small airways revisited

It has been shown that structural changes in small airways of smokers with average smoking histories greater than 35 pack-years could be reflected in the single-breath washout test. The more sophisticated multiple breath washout test (MBW) has the potential to anatomically locate the affected small airways in acinar and conductive lung zones through increased phase III slope indices S(acin) and S(cond), respectively. Pulmonary function, S(acin), and S(cond) were obtained in 63 normal never-smokers and in 169 smokers classified according to smoking history (< 10 pack-years; 10-20 pack-years; 20-30 pack-years; > 30 pack-years). Compared with never-smokers, significant changes in S(acin) (p = 0.02), S(cond) (p < 0.001), and diffusing capacity (DL(CO); p < 0.001) were detected from greater than 10 pack-years onwards. Spirometric abnormality was significant only from greater than 20 pack-years onwards. In smokers with greater than 30 pack-years and DL(CO) less than 60% predicted, the presence of emphysema resulted in disproportionally larger S(acin) than S(cond) increases. We conclude that S(cond) and S(acin) can noninvasively detect airway changes from as early as 10 pack-years onwards, locating the earliest manifestations of smoking-induced small airways alterations around the acinar entrance. In these early stages, the associated DL(CO) decrease may be a reflection of ventilation heterogeneity rather than true parenchymal destruction. In more advanced stages of smoking-induced lung disease, differential patterns of S(acin) and S(cond) are characteristic of the presence of parenchymal destruction in addition to peripheral airways alterations.

Peripheral airway function in childhood asthma, assessed by single-breath He and SF6 washout

To assess whether the peripheral airways are involved in pediatric asthma, 10 asthmatic children (aged 8-15 years), hyperresponsive to dry-air hyperventilation challenge (DACh), performed spirometry and a vital capacity He/SF(6) single-breath washout test at rest, after DACh, and after beta(2)-therapy. The normalized phase III slopes (Sn(III)) of the expired He and SF(6) concentrations served as measures of overall ventilation inhomogeneity, and the (SF(6) - He) Sn(III) difference served to indicate where along the peripheral airways obstruction occurs. While a greater increase in the He vs. SF(6) slope indicates that obstruction has occurred in the vicinity of the acinar entrance, the reverse suggests obstruction deeper in the intraacinar airways. The mean (SD) fall in FEV(1) after DACh was 35 (14)%. Both He and SF(6) Sn(III) increased significantly (P < 0.05) after the challenge, and were restituted after beta(2)-therapy (P < 0.05). After DACh, Sn(III) increased more for He than for SF(6), resulting in a negative (SF(6) - He) Sn(III) difference (P < 0.01), which was restituted after beta(2)-therapy (P < 0.05). Even though there was no correlation between baseline FEV(1) and the magnitude of the subsequent fall in this parameter after DACh (r(2) = 0.04; n.s.), a strong correlation was found between the (SF(6) - He) Sn(III) difference at rest and its change after DACh (r(2) = 0.81; P < 0.001). We conclude that airways close to the acinar entrance participate in the airway response to DACh in asthmatic children. The magnitude of this peripheral airway response is related to the severity of resting peripheral airway dysfunction.