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Abnormal Small Airways Function in Children With Mild Asthma
CHEST
Original Research ASTHMA
Abnormal Small Airways Function in Children With Mild Asthma Florian Singer, MD, PhD; Chiara Abbas, MD; Sophie Yammine, MD; Carmen Casaulta, MD; Urs Frey, MD, PhD; and Philipp Latzin, MD, PhD
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 5 35) and children with asthma (n 5 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 (ⱕ 22 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. CHEST 2014; 145(3):492–499 Abbreviations: FEF25%-75% 5 forced expiratory flow, midexpiratory phase; Feno 5 fraction of exhaled nitric oxide; He 5 helium; IGW 5 inert gas washout; LCI 5 lung clearance index; N2 5 nitrogen; SBW 5 single-breath washout; SDTG 5 double-tracer gas phase III slope; SF6 5 sulfur hexafluoride; SN2 5 nitrogen phase III slope; sRaw 5 specific airway resistance
airways in the respiratory diffusion-dependent Small lung zone are the major site of pathology in adult
patients with asthma.1 Bronchoconstriction and airways inflammation trigger airway remodeling, which leads to patchy obstruction of small airways.2-4 This heterogeneously impaired structure affects the function of the small airways, especially the evenness of ventilation distribution. Inert gas washout (IGW) tests, such as the single-breath washout (SBW) and multiple-breath washout tests, measure ventilation heterogeneity because spirometry is not sensitive enough.5 Adult patients with persistent asthma have increased ventilation heterogeneity arising within peripheral preacinar (convection-dependent) and acinar (diffusion-dependent) lung zones.6-10 Increased 492
ventilation heterogeneity is a predictor of asthma control7,11,12 and airway hyperresponsiveness and improves after bronchodilation9,13 and the use of corticosteroids.8 In children, mild asthma is the most prevalent asthma phenotype, but the degree of small airways obstruction and its reversibility is not clear in this population. In children with moderate to severe asthma, few studies have shown that ventilation heterogeneity may be elevated compared with control subjects.14-18 Others have concluded that IGW testing is not ideal to assess bronchodilator response19 and is too sophisticated for routine application.20 To overcome the latter drawback, we developed an easy dual-tracer SBW applied during normal tidal breathing. To our knowledge, the technique is the Original Research
first to be based on a validated setup according to American Thoracic Society/European Respiratory Society consensus.21-24 The current study assessed ventilation heterogeneity in children with mild asthma and healthy control subjects to determine whether and in what proportion children with mild asthma present with functional peripheral changes. The primary aim was to compare measures of acinar and global ventilation heterogeneity between children with and without asthma. The secondary aim was to assess the effect of bronchodilation on ventilation heterogeneity and classic measures of airways obstruction in mild asthma. Materials and Methods Study Population We enrolled 70 children aged 6 to 16 years from an asthma outpatient clinic and a healthy volunteer database at the Children’s Hospital Bern. Children with asthma were eligible if they had a history of controlled mild asthma (daytime symptoms ⱕ 2 d/wk), normal FEV1 ⫾ 1.96 z scores, a prescription for low to moderate inhaled corticosteroid doses (ⱕ 200 mg/d fluticasone or equivalent), and no history of increased use of asthma medication during the previous 6 months.25 Healthy children had no history of recurrent wheeze and normal FEV1 values. Respiratory tract infection, antibiotic or oral corticosteroid use in the previous 4 weeks, and premature birth were general exclusion criteria. The study was approved by the Ethics Committee of the Canton of Bern, Switzerland (018/1). Child assent to participate was obtained, and all parents or caregivers gave written informed consent. Study Design The design was a prospective observational study at the Children’s Hospital Bern. Children withheld their asthma medication prior to testing (short-acting b2-receptor agonists for 8 h, inhaled corticosteroids and leukotriene receptor modifiers for 24 h). At baseline, lung function tests were done in the following order: control subjects: (1) spirometry and (2) SBW; children with asthma: (1) fraction of exhaled nitric oxide (Feno), (2) plethysmography, (3) spirometry, and (4) SBW. As repeated tests were applied, all SBW tests followed spirometry. Children with abnormal FEV1 were excluded from the study (n 5 4). Twenty minutes after inhaling Manuscript received April 1, 2013; revision accepted September 4, 2013. Affiliations: From the University Children’s Hospital Zurich (Dr Singer), Zurich; University Children’s Hospital Bern (Drs Singer, Abbas, Yammine, Casaulta, and Latzin), Bern; and University Children’s Hospital Basel (Drs Yammine, Frey, and Latzin), Basel, Switzerland. Drs Singer and Abbas contributed equally to this work. Funding/Support: This work was funded by the Federal Department of Economic Affairs Switzerland, Commission for Technology and Innovation, Innovation Promotion Agency [unrestricted educational grant 14435.1 PFLS-LS] and the Julia Bangerter-Rhyner Foundation. Correspondence to: Philipp Latzin, MD, PhD, Division of Respiratory Medicine, University Children’s Hospital Basel, Spitalstrasse 33, 4005 Basel, Switzerland; e-mail: philipp.latzin@ ukbb.ch © 2014 American College of Chest Physicians. Reproduction of this article is prohibited without written permission from the American College of Chest Physicians. See online for more details. DOI: 10.1378/chest.13-0784 journal.publications.chestnet.org
200 to 400 mg salbutamol (metered-dose inhaler; GlaxoSmithKline plc) through a spacer (Volumatic; GlaxoSmithKline plc), plethysmography, spirometry, and SBW were measured again in the children with asthma. Atopic sensitization was evaluated in children with asthma by skin prick testing of common inhalant allergens (Table 1). Lung Function Assessments Ventilation heterogeneity was assessed by double-tracer gas and nitrogen (N2) tidal SBW tests.21,22 Children breathed through an open bypass system (Exhalyzer D; Eco Medics AG).21,23 Exhaled gas fractions were quantified by the side-stream ultrasonic flow meter (molar mass in g/mol) and an indirect N2% sensor.21-23 Children inhaled the double-tracer gas (or pure oxygen for the N2 SBW) during regular tidal breathing. The double-tracer gas mixture contained 26.3% helium (He), 5% sulfur hexafluoride (SF6), 21% oxygen, and balance N2 from pressurized cylinders (Carbagas). The primary outcome was the slope from the tidal (alveolar) phase III from double-tracer gas and N2 washout curves. According to the current American Thoracic Society/European Respiratory Society consensus,24 tests were applied in triplicate to obtain the averaged slope values. The phase III slopes of each SBW test were computed by linear regression between 65% and 95% of expired volume (Fig 126) and then multiplied with tidal volume.15,24,27 The interobserver agreement for double-tracer gas phase III slope (SDTG) is strong22 (intraclass correlation coefficient, 0.92). SDTG aggregates the phase III slopes from He and SF6, two gases of similar convective but highly differing diffusive properties. SDTG is a specific index of ventilation heterogeneity that estimates distal (diffusion-dependent) gas mixing efficiency near the acinar lung regions.18,22,28,29 The nitrogen phase III slope (SN2) reflects the washout behavior of a single gas only (N2) and, thus, is an unspecific global index of ventilation heterogeneity. SN2 is influenced from inhomogeneous ventilation distribution in proximal (convection-dependent) and distal lung regions.12,22 More details are provided in e-Appendix 1. Feno, a biomarker of eosinophilic airway inflammation, was measured by the single-breath method with an online chemiluminescence analyzer (CLD 77 AM; Eco Medics AG). After inhaling nitric oxide-free air to total lung capacity, children exhaled against an expiratory resistance with a constant flow of 50 mL/s for at least 6 s. Feno ⱖ 35 parts per billion was considered elevated.30,31
Table 1—Population Characteristics Characteristic Children (boys) Age, y Height, cm Weight, kg Current ICS, yes (no) Atopy, yes (no) Feno, ppb Feno ⱖ 35 ppb
Healthy Control Subjects
Children With Asthma
P Value
35 (18) 11.9 ⫾ 0.5 150.1 ⫾ 11.9 42.8 ⫾ 9.9 0 … … …
31 (12) 11.6 ⫾ 0.6 147.9 ⫾ 16.7 44.0 ⫾ 15.6 22 (9) 21 (10) 18.4 (9.0-33.2) 7 (23)
.461 .544 .596 .743 … … … …
Data are presented as mean ⫾ SD, median (interquartile range), or No. (%) unless otherwise indicated. Sex distribution was compared by Fisher exact test and anthropometric data by unpaired t test. ICS use was during 1 mo prior to the study. Atopy in children with asthma was determined by skin prick testing of common inhaled allergens (mixed trees, mixed grasses, cat, dog, house dust mite, and Aspergillus fumigatus). Children with any positive reaction were considered to be atopic sensitized. Feno 5 fraction of exhaled nitric oxide; ICS 5 inhaled corticosteroid; ppb 5 parts per billion. CHEST / 145 / 3 / MARCH 2014
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Figure 1. Single-breath washout measurement. Typical singlebreath washout examples of helium sulfur hexafluoride (doubletracer gas) tests in a healthy boy aged 10 y (light gray line, flattest slope) and one pre-BD and one post-BD test in a boy aged 10 y with mild asthma (dark gray line). To calculate the tidal alveolar phase III slope, the linear regression (black line) is fitted between 65% and 95% of expired volume. Because the molar mass signal is an aggregate signal from exhaled gas fractions, with CO2 playing the major role in the molar mass washout curve (about 28.9-29.5 g/mol),26 CO2 subtraction from the molar mass reveals the corrected molar mass washout curve (about 20.3 to 1.0 g/mol), which reliably estimates the helium sulfur hexafluoride washout behavior.22,23 BD 5 bronchodilation.
Plethysmography and spirometry were performed with the Jaeger Masterlab (CareFusion Corporation) according to recommendations.32 Specific airway resistance (sRaw) was expressed as percent predicted, and FEV1 and forced expiratory flow, midexpiratory phase (FEF25%-75%) were expressed as z scores.33 Statistical Analysis We calculated the sample size with the published baseline mean ⫾ SD SDTG of 20.06 ⫾0.09 g/mol from 48 healthy schoolaged children from a previous report.22 We postulated a detectable difference in SDTG between children with and without asthma as large as 0.1 ⫾ 0.09 g/mol. Assuming a significance level of 5% and a power of 90%, 22 control subjects and 22 children with asthma would be needed to complete the study. We aimed to recruit at least 30 children per group to allow for correlation analyses. The z scores for SDTG and SN2 were calculated from the healthy children. Abnormal lung function was defined as ⱕ 22 z scores. Data were compared by Fisher exact and t tests, as appropriate. Bronchodilator response (D) was estimated by the relative difference of prebronchodilator minus postbronchodilator values. P , .05 was considered significant. All analyses were done with Stata statistical software (StataCorp LP). Additional analyses are provided in e-Appendix 1.
Results Sixty-six children (31 with asthma) were studied, and 561 tidal SBW tests were achieved. Twenty-one children with asthma (67%) were atopic, and 22 (71%) received inhaled corticosteroids in the previous month. Healthy control subjects and children with asthma
494
were comparable regarding age, height, weight, and sex (Table 1). Children with mild asthma had significantly increased acinar ventilation heterogeneity (Figs 1-3, Table 2). SDTG was abnormal (ⱕ 22 z scores) in 11 children with asthma (36%) and in one control subject (3%) (Fig 3). In the 11 children with asthma, atopic sensitization, corticosteroid use, Feno, spirometry, and plethysmography were comparable with that of the 20 other children with mild asthma (data not shown). The mean SDTG difference between healthy control subjects and children with asthma was 1.49 (95% CI, 0.85-2.13) z scores. In contrast, global ventilation heterogeneity (SN2) was similar in both groups (Figs 2, 3, Table 2). According to the inclusion criteria, FEV1 was normal in children with asthma and did not differ significantly between groups. The mean FEV1 difference was 0.24 z scores (95% CI, 20.22 to 0.70 z scores). FEF25%-75% was lower in the children with asthma, with a mean difference between groups of 0.90 z scores (95% CI, 0.53-1.04 z scores) (Table 2). FEF25%-75% was abnormal in three children with asthma (10%) and two control subjects (6%). Eosinophilic airways inflammation appeared to be modestly elevated in asthma, with seven children (23%) having an increased Feno of ⱖ 35 parts per billion.30 The differences in SDTG and FEF25%-75% between children with asthma and healthy control subjects remained significant after adjusting for potential confounders (e-Table 1). Compared with other lung function indexes, FEF25%-75% showed the closest association with SDTG (R2 5 0.28) (e-Fig 1, e-Table 2), and similarly, FEV1 showed the closest association with SN2 (R2 5 0.28). Receiver operating characteristic analysis showed that SDTG provided the best diagnostic performance for mild asthma (e-Fig 2). Ventilation heterogeneity significantly changed after bronchodilator (Fig 3, Table 2). Acinar ventilation heterogeneity (SDTG), which was significantly increased in the children with asthma prior to bronchodilation, returned to normal after bronchodilation (Table 2) (mean postbronchodilation SDTG in children with asthma, 20.54 ⫾ 1.54 z scores; mean SDTG in control subjects, 20.01 ⫾ 1.07 z scores; P 5 .243). After bronchodilation, the average DSDTG was 35.7% (95% CI, 15.1%-56.3%) in the children with asthma. Global ventilation heterogeneity also significantly changed in the children with asthma after bronchodilation. DSN2 was 38.2% (95% CI, 17.9%-58.4%). sRaw similarly decreased (DsRaw, 33.0; 95% CI, 23.8%-42.2%). Spirometry indexes significantly increased but at a smaller effect size compared with the slope indexes and sRaw (DFEV1, 6.0% [95% CI, 3.3%-8.7%]; DFEF25%-75%, 17.0% [95% CI, 9.2%-24.9%]). After bronchodilation tidal volume, mean and peak tidal expiratory flow
Original Research
Figure 2. Comparison of ventilation heterogeneity in control subjects and children with mild asthma. Shown are the phase III slopes from the single-breath washout tests using a helium sulfur hexafluoride (double-tracer) gas mixture or pure oxygen for nitrogen single-breath washout. In children with asthma (gray lines), phase III slopes are shown pre- and post-BD. Group means are shown as black lines. See Figure 1 legend for expansion of abbreviation.
increased (e-Table 3). This change in breathing pattern was not associated with the bronchodilator effect on SDTG and SN2. Additional results are provided in e-Appendix 1.
Discussion Ventilation heterogeneity is increased in children with mild asthma despite rare symptoms and normal FEV1. More than one-third of children with asthma had abnormal SDTG findings, suggesting impaired small airways function in the acinar lung region. Measures of global ventilation heterogeneity (SN2) and airways obstruction (FEF25%-75%) are normal in the majority of children with mild asthma. sRaw is elevated on average but is only weakly associated with ventilation heterogeneity. Administration of a bronchodilator in children with asthma significantly alters acinar and global ventilation heterogeneity indexes by 36% and 38%, respectively. The classic measures of more proximal airways obstruction (FEV1 and FEF25%-75%) improve after bronchodilator inhalation, but their average bronchodilator effect size is less than that of SDTG and SN2. These simple tidal SBW tests may be used to characterize small airways function. The double-tracer gas SBW test has potential as journal.publications.chestnet.org
a clinical and research outcome in children with mild asthma. The current study has two major strengths. First, the data show for the first time to our knowledge that in children with stringently defined mild asthma, significantly increased ventilation heterogeneity may be present. The population studied is more relevant to most pediatricians than that with more severe disease. Second, the SBW technique relies on an easy tidal breathing protocol and an available device validated as currently recommended.23,24 Compared with multiplebreath washout or spirometry, this tidal SBW has greater success rates in children.22 Practicable SBW techniques are likely to become a standard procedure in children.34 Abnormal small airways function is a previously underestimated subclinical pathology in a considerable proportion of children with mild asthma. In this adequately powered study, 36% of the children with mild asthma had clear evidence (ie, SDTG ⱕ 22 z scores) of impaired gas mixing presumably near the acinar lung region.22,29 To specifically assess physiology in mild asthma, we stringently defined the inclusion criteria. Not surprisingly, classic measures of airways obstruction and global measures of ventilation heterogeneity (SN2) failed to clearly discriminate between health and asthma. FEV1, FEF25%-75%, sRaw, and Feno were CHEST / 145 / 3 / MARCH 2014
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Figure 3. Lung function in control subjects and children with mild asthma. Shown is the phase III slope from the single-breath washout test using a helium sulfur hexafluoride (double-tracer) gas mixture and FEF25-75. In children with asthma (gray lines), phase III slope and FEF25-75 values are shown preand post-BD. Group means are shown as black lines; the dashed lines reflect limits of normal lung function (⫾ 2 z scores). FEF25-75 5 forced expiratory flow, midexpiratory phase. See Figure 1 legend for expansion of other abbreviation.
either not or weakly correlated to acinar and global ventilation heterogeneity. Interestingly, SDTG was associated the strongest with FEF25%-75%, a measure of more peripheral obstruction than FEV1. SN2 was associated the strongest with FEV1. These associations support the concept of SDTG being an index of acinar ventilation heterogeneity and SN2 being an index of global ventilation heterogeneity, which is also influenced by proximal airways obstruction. The most relevant finding is that gas mixing efficiency is impaired in the acinar lung region in children
with mild asthma. Several studies reported increased ventilation heterogeneity in adults with asthma of various degrees of severity.6-10,12,13,35 Data in mild asthma are scarce. Most pediatric studies were done in patients with moderate to severe asthma or with frequent wheeze.14-17,19,36,37 Similar to the present data, these studies showed that measures of regional ventilation heterogeneity (ie, conductive ventilation heterogeneity, acinar ventilation heterogeneity) discriminated between patients with asthma and control subjects better than measures of global ventilation heterogeneity
Table 2—Lung Function Values Compared at Baseline and Before and After Bronchodilation Prebronchodilation Value SDTG, g/mol SDTG, z score SN2, N2% SN2, z score FEV1, z score FEF25%-75%, z score sRaw, % predicted
Postbronchodilationa
Prebronchodilation vs Postbronchodilationb
Control (n 5 35)
Asthma (n 5 31)
P Value
Asthma
P Value
Mean (95% CI)
P Value
20.06 ⫾ 0.05 20.01 ⫾ 1.07 3.51 ⫾ 1.73 0.01 ⫾ 1.02 20.36 ⫾ 0.83 20.26 ⫾ 0.93 …
20.14 ⫾ 0.07 21.47 ⫾ 1.49 2.96 ⫾ 1.60 20.32 ⫾ 0.93 20.60 ⫾ 0.80 21.10 ⫾ 0.72 216.2 ⫾ 59.3
, .001 , .001 .292 .292 .298 .001 …
20.09 ⫾ 0.08 20.54 ⫾ 1.54 1.83 ⫾ 1.24 20.98 ⫾ 0.73 20.12 ⫾ 0.79 20.51 ⫾ 0.87 144.9 ⫾ 36.9
.243 .243 , .001 , .001 .620 .458 …
0.05 (0.02-0.08) 0.93 (0.36-1.50) 1.13 (0.53-1.73) 0.66 (0.31-1.02) 0.48 (0.30-0.66) 0.59 (0.34-0.84) 71.3 (51.3-91.3)
.002 .002 .001 .001 , .001 , .001 , .001
Data are presented as mean ⫾ SD unless otherwise indicated. Data were compared by unpaired or paired t tests as appropriate. FEF25%-75% 5 forced expiratory flow, midexpiratory phase; SDTG 5 double-tracer gas phase III slope; SN2 5 nitrogen phase III slope; sRaw 5 specific airway resistance. aControl subjects vs children with asthma (postbronchodilation). bChildren with asthma only. 496
Original Research
(lung clearance index [LCI]), contrasting with findings from patients with cystic fibrosis lung disease wherein LCI usually is already elevated early in life.38,39 Because the LCI is a more global index of ventilation heterogeneity, like the SN2, it may not be able to differentiate patients with asthma from control subjects, but more sensitive tests like conductive and acinar ventilation heterogeneity can. In cystic fibrosis, the disease may have already caused such gross ventilation heterogeneity that the LCI can differentiate between the disease and healthy states. The overlap in LCI between patients with asthma and control subjects is strikingly similar to the SN2 distribution in the present study. In addition, evidence of acinar involvement was detected in adults with mild to moderate asthma.9,17 Others reported that conductive ventilation heterogeneity may be more frequently abnormal than acinar ventilation heterogeneity in preschoolaged children with severe wheezing.14,19,37 The differences among studies may relate to different IGW techniques and populations studied. Nevertheless, specific indexes of regional ventilation heterogeneity may be used in the future to improve diagnosis and follow-up of patients with asthma. Increased acinar ventilation heterogeneity may improve with intervention in asthma. In six of the 11 children with abnormal prebronchodilator SDTG, the SDTG returned to normal after bronchodilator inhalation (Fig 3). Interestingly, in the subgroup of children without current inhaled corticosteroid use (n 5 9), bronchodilator response in SDTG was greater than in children with inhaled corticosteroid use (e-Appendix 1), possibly indicating remodeling effects even in mild asthma. Despite only a few subjects in this subgroup, this finding is important and in line with a previous study in which distal airways function remained abnormal in children with stable asthma, despite of maximal bronchodilation and moderately high daily corticosteroid use.40 In light of the good SDTG repeatability (e-Appendix 1) and the significant but small improvement of FEV1 and FEF25%-75% (D 5 6%-17%), the bronchodilator effect on DSDTG (D 5 36%) could be interpreted as improved small airways function. In a recent study, we assessed SDTG and SN2 repeatability between tidal SBW tests performed 15 min apart in 25 school-aged children with clinically stable cystic fibrosis.41 The coefficient of repeatability of SDTG and SN2 was 0.065 g/mol and 4.1% N2, respectively. Thirteen of 31 children with mild asthma (42%) had a physiologically significant response to the bronchodilator (DSDTG . 0.065 g/mol). Despite normal SN2 at baseline; SN2 also decreased after bronchodilation in the present study. The caliber changes of convective airways possibly generated DSN2 and increased spirometry indexes. It is surprising that postbronchodilator SN2 in children with journal.publications.chestnet.org
asthma was lower than that found in healthy control subjects. However, DSN2 values did not exceed the coefficient of repeatability (4.1% N2). Compared with SDTG and DSDTG, SN2 and DSN2 may be less sensitive and specific in mild asthma. Verbanck and colleagues7-9 similarly showed that increased acinar ventilation heterogeneity is partially reversible. Comparable to this, change in LCI upon bronchodilation was unsystematic in both young children with frequent wheezing and control subjects.14,19 Similarly, Macleod et al15 did not find systematic bronchodilator effects in LCI and conductive ventilation heterogeneity in older children. Reversibility of global ventilation heterogeneity indexes warrants cautious interpretation. Available physiologic measures of small airways function may help to improve asthma phenotyping and treatment.33,42,43 By quantifying the SF6 and He washout behavior (phase III slopes), previous studies elegantly demonstrated that direct and indirect bronchoprovocation affects gas mixing efficiency at various levels of the airway tree.18,36,44 This effect apparently depends on the distribution of receptors triggered by the bronchoprovocation agent. Similarly, the site of particle deposition could be studied.8 The current SBW application reliably reflects the washout curves of SF6 and He; is obtainable; and, thus, may enhance the transition from research settings to clinical routine.23 A limitation of this and many other studies is that IGW indexes were not related to airways pathology or alveolar nitric oxide concentration. Few studies have shown an association between ventilation heterogeneity and peripheral airways histology or abnormal chest CT scan in smokers or in cystic fibrosis lung disease.28,45 However, others did not find a structurefunction relation in preschool-aged children with wheezing.37 Thus, we can only speculate on a possible structure-function relation between IGW indexes and small airways. Because of ethical considerations, we did not administer bronchodilators in healthy children, which would have allowed an estimation of normal bronchodilator effects on SDTG and SN2. Interpretation of treatment response assessed by IGW is complex and requires further study. A technical limitation of the current double-tracer gas is the requirement of SF6, which is restricted in some countries because of the greenhouse effect. However, the current SBW technique does not depend on SF6. Other inert gases, such as the noble gas xenon, with a similar diffusion coefficient as SF6 could be used.46 It is time to look at distal airways to improve asthma phenotyping. Despite few symptoms and normal FEV1, acinar ventilation heterogeneity (SDTG) is increased on average and is abnormal (ⱕ 22 z scores) in one-third of children with asthma. Administration of a bronchodilator significantly changes ventilation heterogeneity CHEST / 145 / 3 / MARCH 2014
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and airways obstruction in mild asthma. IGW testing may shed more light on the dynamic complexity of childhood asthma. The new double-tracer gas SBW is based on tidal breathing and has potential as a clinical and research outcome in children with mild asthma. Acknowledgments Author contributions: Dr Latzin had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Dr Singer: contributed to the study concept and design; data acquisition, analysis, and interpretation; and drafting, critical revision for important intellectual content, and final approval of the manuscript. Dr Abbas: contributed to the study concept and design; data acquisition, analysis, and interpretation; and drafting, critical revision for important intellectual content, and final approval of the manuscript. Dr Yammine: contributed to the data analysis and interpretation and critical revision for important intellectual content and final approval of the manuscript. Dr Casaulta: contributed to the study concept and design; data acquisition, analysis, and interpretation; and critical revision for important intellectual content and final approval of the manuscript. Dr Frey: contributed to the study concept and design, data analysis and interpretation, and critical revision for important intellectual content and final approval of the manuscript. Dr Latzin: contributed to the study concept and design; data acquisition, analysis, and interpretation; and drafting, critical revision for important intellectual content, and final approval of the manuscript. Financial/nonfinancial disclosures: The authors have reported to CHEST that no potential conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article. Role of sponsors: The sponsors had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Other contributions: The authors thank the children and their families for participating in the study. The authors also thank Markus Roos and Ruedi Isler for skillful technical assistance and Tom Riedel, MD; Nicolas Regamey, MD; Elena Proietti, MD; Anne Schmidt, MD; Gisela Wirz; Sandra Lüscher; Monika Graf; and Sharon Schmid for valuable work and support. This work was performed at the Asthma Outpatient Clinic of the University Children’s Hospital Bern, Bern, Switzerland. Additional information: The e-Appendix, e-Figures, and e-Tables can be found in the “Supplemental Materials” area of the online article.
References 1. Contoli M, Bousquet J, Fabbri LM, et al. The small airways and distal lung compartment in asthma and COPD: a time for reappraisal. Allergy. 2010;65(2):141-151. 2. Grainge CL, Lau LC, Ward JA, et al. Effect of bronchoconstriction on airway remodeling in asthma. N Engl J Med. 2011;364(21):2006-2015. 3. King GG, Carroll JD, Müller NL, et al. Heterogeneity of narrowing in normal and asthmatic airways measured by HRCT. Eur Respir J. 2004;24(2):211-218. 4. Hogg JC, Macklem PT, Thurlbeck WM. Site and nature of airway obstruction in chronic obstructive lung disease. N Engl J Med. 1968;278(25):1355-1360. 5. Cosio M, Ghezzo H, Hogg JC, et al. The relations between structural changes in small airways and pulmonary-function tests. N Engl J Med. 1978;298(23):1277-1281. 6. Verbanck S, Schuermans D, Vincken W. Inflammation and airway function in the lung periphery of patients with stable asthma. J Allergy Clin Immunol. 2010;125(3):611-616. 498
7. Downie SR, Salome CM, Verbanck S, Thompson B, Berend N, King GG. Ventilation heterogeneity is a major determinant of airway hyperresponsiveness in asthma, independent of airway inflammation. Thorax. 2007;62(8):684-689. 8. Verbanck S, Schuermans D, Paiva M, Vincken W. The functional benefit of anti-inflammatory aerosols in the lung periphery. J Allergy Clin Immunol. 2006;118(2):340-346. 9. Verbanck S, Schuermans D, Noppen M, Van Muylem A, Paiva M, Vincken W. Evidence of acinar airway involvement in asthma. Am J Respir Crit Care Med. 1999;159(5): 1545-1550. 10. van Veen IH, Sterk PJ, Schot R, Gauw SA, Rabe KF, Bel EH. Alveolar nitric oxide versus measures of peripheral airway dysfunction in severe asthma. Eur Respir J. 2006;27(5): 951-956. 11. Farah CS, King GG, Brown NJ, Peters MJ, Berend N, Salome CM. Ventilation heterogeneity predicts asthma control in adults following inhaled corticosteroid dose titration. J Allergy Clin Immunol. 2012;130(1):61-68. 12. Farah CS, King GG, Brown NJ, et al. The role of the small airways in the clinical expression of asthma in adults. J Allergy Clin Immunol. 2012;129(2):381-387. 13. Verbanck S, Paiva M, Schuermans D, Hanon S, Vincken W, Van Muylem A. Relationships between the lung clearance index and conductive and acinar ventilation heterogeneity. J Appl Physiol. 2012;112(5):782-790. 14. Sonnappa S, Bastardo CM, Wade A, et al. Symptom-pattern phenotype and pulmonary function in preschool wheezers. J Allergy Clin Immunol. 2010;126(3):519-526. 15. Macleod KA, Horsley AR, Bell NJ, Greening AP, Innes JA, Cunningham S. Ventilation heterogeneity in children with well controlled asthma with normal spirometry indicates residual airways disease. Thorax. 2009;64(1):33-37. 16. Gustafsson PM. Peripheral airway involvement in CF and asthma compared by inert gas washout. Pediatr Pulmonol. 2007;42(2):168-176. 17. Keen C, Olin AC, Wennergren G, Gustafsson P. Small airway function, exhaled NO and airway hyper-responsiveness in paediatric asthma. Respir Med. 2011;105(10):1476-1484. 18. Ljungberg HK, Gustafsson PM. Peripheral airway function in childhood asthma, assessed by single-breath He and SF6 washout. Pediatr Pulmonol. 2003;36(4):339-347. 19. Sonnappa S, Bastardo CM, Wade A, Bush A, Stocks J, Aurora P. Repeatability and bronchodilator reversibility of lung function in young children. Eur Respir J. 2013;42(1):116-124. 20. Coates AL. Classical respiratory physiology-gone the way of the dinosaurs? Do we need a Jurassic park? Pediatr Pulmonol. 2000;30(1):1-2. 21. Singer F, Houltz B, Latzin P, Robinson P, Gustafsson P. A realistic validation study of a new nitrogen multiple-breath washout system. PLoS ONE. 2012;7(4):e36083. 22. Singer F, Stern G, Thamrin C, et al. A new double-tracer gas single-breath washout to assess early cystic fibrosis lung disease. Eur Respir J. 2013;41(2):339-345. 23. Singer F, Stern G, Thamrin C, et al. Tidal volume single breath washout of two tracer gases—a practical and promising lung function test. PLoS ONE. 2011;6(3):e17588. 24. Robinson PD, Latzin P, Verbanck S, et al. Consensus statement for inert gas washout measurement using multiple- and single- breath tests. Eur Respir J. 2013;41(3):507-522. 25. Global Initiative for Asthma. GINA report, global strategy for asthma management and prevention. Updated 2012. Global Initiative for Asthma website. www.ginasthma.org. Accessed January 8, 2013. 26. Thamrin C, Latzin P, Sauteur L, Riedel T, Hall GL, Frey U. Deadspace estimation from CO2 versus molar mass measurements in infants. Pediatr Pulmonol. 2007;42(10):920-927. Original Research
27. Aurora P, Kozlowska W, Stocks J. Gas mixing efficiency from birth to adulthood measured by multiple-breath washout. Respir Physiol Neurobiol. 2005;148(1-2):125-139. 28. Van Muylem A, De Vuyst P, Yernault JC, Paiva M. Inert gas single-breath washout and structural alteration of respiratory bronchioles. Am Rev Respir Dis. 1992;146(5 pt 1): 1167-1172. 29. Van Muylem A, Paiva M, Baran D, Yernault JC. Structural change of the acinus during growth assessed by singlebreath tracer gas washouts. Pediatr Pulmonol. 1996;22(4): 230-235. 30. Dweik RA, Boggs PB, Erzurum SC, et al; American Thoracic Society Committee on Interpretation of Exhaled Nitric Oxide Levels (FENO) for Clinical Applications. An official ATS clinical practice guideline: interpretation of exhaled nitric oxide levels (FENO) for clinical applications. Am J Respir Crit Care Med. 2011;184(5):602-615. 31. See KC, Christiani DC. Normal values and thresholds for the clinical interpretation of exhaled nitric oxide levels in the US general population: results from National Health and Nutrition Examination Survey 2007-2010. Chest. 2013;143(1): 107-116. 32. Beydon N, Davis SD, Lombardi E, et al; American Thoracic Society/European Respiratory Society Working Group on Infant and Young Children Pulmonary Function Testing. An official American Thoracic Society/European Respiratory Society statement: pulmonary function testing in preschool children. Am J Respir Crit Care Med. 2007;175(12):1304-1345. 33. Stanojevic S, Wade A, Cole TJ, et al; Asthma UK Spirometry Collaborative Group. Spirometry centile charts for young Caucasian children: the Asthma UK Collaborative Initiative. Am J Respir Crit Care Med. 2009;180(6):547-552. 34. Perez T. Is it really time to look at distal airways to improve asthma phenotyping and treatment? Eur Respir J. 2011;38(6): 1252-1254. 35. Brown NJ, Thorpe CW, Thompson B, et al. A comparison of two methods for measuring airway distensibility: nitrogen washout and the forced oscillation technique. Physiol Meas. 2004;25(4):1067-1075.
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36. Gustafsson PM, Ljungberg HK, Kjellman B. Peripheral airway involvement in asthma assessed by single-breath SF6 and He washout. Eur Respir J. 2003;21(6):1033-1039. 37. Sonnappa S, Bastardo CM, Saglani S, Bush A, Aurora P. Relationship between past airway pathology and current lung function in preschool wheezers. Eur Respir J. 2011;38(6): 1431-1436. 38. Aurora P, Stanojevic S, Wade A, et al; London Cystic Fibrosis Collaboration. Lung clearance index at 4 years predicts subsequent lung function in children with cystic fibrosis. Am J Respir Crit Care Med. 2011;183(6):752-758. 39. Kieninger E, Singer F, Fuchs O, et al. Long-term course of lung clearance index between infancy and school-age in cystic fibrosis subjects. J Cyst Fibros. 2011;10(6):487-490. 40. Merkus PJ, van Pelt W, van Houwelingen JC, et al. Inhaled corticosteroids and growth of airway function in asthmatic children. Eur Respir J. 2004;23(6):861-868. 41. Abbas C, Singer F, Yammine S, Casaulta C, Latzin P. Treatment response of airway clearance assessed by single-breath washout in children with cystic fibrosis. J Cyst Fibros. 2013; 12(6):567-574. 42. Siddiqui S, Usmani OS. Small airways, big challenge: measuring the unseen? Nat Med. 2012;18(11):1619-1621. 43. Shi Y, Aledia AS, Galant SP, George SC. Peripheral airway impairment measured by oscillometry predicts loss of asthma control in children. J Allergy Clin Immunol. 2013;131(3): 718-723. 44. Michils A, Elkrim Y, Haccuria A, Van Muylem A. Adenosine 59-monophosphate challenge elicits a more peripheral airway response than methacholine challenge. J Appl Physiol. 2011; 110(5):1241-1247. 45. Hall GL, Logie KM, Parsons F, et al; AREST CF. Air trapping on chest CT is associated with worse ventilation distribution in infants with cystic fibrosis diagnosed following newborn screening. PLoS ONE. 2011;6(8):e23932. 46. Kirby M, Svenningsen S, Kanhere N, et al. Pulmonary ventilation visualized using hyperpolarized helium-3 and xenon129 magnetic resonance imaging: differences in COPD and relationship to emphysema. J Appl Physiol. 2013;114(6):707-715.
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Original Research ASTHMA
Abnormal Small Airways Function in Children With Mild Asthma Florian Singer, MD, PhD; Chiara Abbas, MD; Sophie Yammine, MD; Carmen Casaulta, MD; Urs Frey, MD, PhD; and Philipp Latzin, MD, PhD
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 5 35) and children with asthma (n 5 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 (ⱕ 22 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. CHEST 2014; 145(3):492–499 Abbreviations: FEF25%-75% 5 forced expiratory flow, midexpiratory phase; Feno 5 fraction of exhaled nitric oxide; He 5 helium; IGW 5 inert gas washout; LCI 5 lung clearance index; N2 5 nitrogen; SBW 5 single-breath washout; SDTG 5 double-tracer gas phase III slope; SF6 5 sulfur hexafluoride; SN2 5 nitrogen phase III slope; sRaw 5 specific airway resistance
airways in the respiratory diffusion-dependent Small lung zone are the major site of pathology in adult
patients with asthma.1 Bronchoconstriction and airways inflammation trigger airway remodeling, which leads to patchy obstruction of small airways.2-4 This heterogeneously impaired structure affects the function of the small airways, especially the evenness of ventilation distribution. Inert gas washout (IGW) tests, such as the single-breath washout (SBW) and multiple-breath washout tests, measure ventilation heterogeneity because spirometry is not sensitive enough.5 Adult patients with persistent asthma have increased ventilation heterogeneity arising within peripheral preacinar (convection-dependent) and acinar (diffusion-dependent) lung zones.6-10 Increased 492
ventilation heterogeneity is a predictor of asthma control7,11,12 and airway hyperresponsiveness and improves after bronchodilation9,13 and the use of corticosteroids.8 In children, mild asthma is the most prevalent asthma phenotype, but the degree of small airways obstruction and its reversibility is not clear in this population. In children with moderate to severe asthma, few studies have shown that ventilation heterogeneity may be elevated compared with control subjects.14-18 Others have concluded that IGW testing is not ideal to assess bronchodilator response19 and is too sophisticated for routine application.20 To overcome the latter drawback, we developed an easy dual-tracer SBW applied during normal tidal breathing. To our knowledge, the technique is the Original Research
first to be based on a validated setup according to American Thoracic Society/European Respiratory Society consensus.21-24 The current study assessed ventilation heterogeneity in children with mild asthma and healthy control subjects to determine whether and in what proportion children with mild asthma present with functional peripheral changes. The primary aim was to compare measures of acinar and global ventilation heterogeneity between children with and without asthma. The secondary aim was to assess the effect of bronchodilation on ventilation heterogeneity and classic measures of airways obstruction in mild asthma. Materials and Methods Study Population We enrolled 70 children aged 6 to 16 years from an asthma outpatient clinic and a healthy volunteer database at the Children’s Hospital Bern. Children with asthma were eligible if they had a history of controlled mild asthma (daytime symptoms ⱕ 2 d/wk), normal FEV1 ⫾ 1.96 z scores, a prescription for low to moderate inhaled corticosteroid doses (ⱕ 200 mg/d fluticasone or equivalent), and no history of increased use of asthma medication during the previous 6 months.25 Healthy children had no history of recurrent wheeze and normal FEV1 values. Respiratory tract infection, antibiotic or oral corticosteroid use in the previous 4 weeks, and premature birth were general exclusion criteria. The study was approved by the Ethics Committee of the Canton of Bern, Switzerland (018/1). Child assent to participate was obtained, and all parents or caregivers gave written informed consent. Study Design The design was a prospective observational study at the Children’s Hospital Bern. Children withheld their asthma medication prior to testing (short-acting b2-receptor agonists for 8 h, inhaled corticosteroids and leukotriene receptor modifiers for 24 h). At baseline, lung function tests were done in the following order: control subjects: (1) spirometry and (2) SBW; children with asthma: (1) fraction of exhaled nitric oxide (Feno), (2) plethysmography, (3) spirometry, and (4) SBW. As repeated tests were applied, all SBW tests followed spirometry. Children with abnormal FEV1 were excluded from the study (n 5 4). Twenty minutes after inhaling Manuscript received April 1, 2013; revision accepted September 4, 2013. Affiliations: From the University Children’s Hospital Zurich (Dr Singer), Zurich; University Children’s Hospital Bern (Drs Singer, Abbas, Yammine, Casaulta, and Latzin), Bern; and University Children’s Hospital Basel (Drs Yammine, Frey, and Latzin), Basel, Switzerland. Drs Singer and Abbas contributed equally to this work. Funding/Support: This work was funded by the Federal Department of Economic Affairs Switzerland, Commission for Technology and Innovation, Innovation Promotion Agency [unrestricted educational grant 14435.1 PFLS-LS] and the Julia Bangerter-Rhyner Foundation. Correspondence to: Philipp Latzin, MD, PhD, Division of Respiratory Medicine, University Children’s Hospital Basel, Spitalstrasse 33, 4005 Basel, Switzerland; e-mail: philipp.latzin@ ukbb.ch © 2014 American College of Chest Physicians. Reproduction of this article is prohibited without written permission from the American College of Chest Physicians. See online for more details. DOI: 10.1378/chest.13-0784 journal.publications.chestnet.org
200 to 400 mg salbutamol (metered-dose inhaler; GlaxoSmithKline plc) through a spacer (Volumatic; GlaxoSmithKline plc), plethysmography, spirometry, and SBW were measured again in the children with asthma. Atopic sensitization was evaluated in children with asthma by skin prick testing of common inhalant allergens (Table 1). Lung Function Assessments Ventilation heterogeneity was assessed by double-tracer gas and nitrogen (N2) tidal SBW tests.21,22 Children breathed through an open bypass system (Exhalyzer D; Eco Medics AG).21,23 Exhaled gas fractions were quantified by the side-stream ultrasonic flow meter (molar mass in g/mol) and an indirect N2% sensor.21-23 Children inhaled the double-tracer gas (or pure oxygen for the N2 SBW) during regular tidal breathing. The double-tracer gas mixture contained 26.3% helium (He), 5% sulfur hexafluoride (SF6), 21% oxygen, and balance N2 from pressurized cylinders (Carbagas). The primary outcome was the slope from the tidal (alveolar) phase III from double-tracer gas and N2 washout curves. According to the current American Thoracic Society/European Respiratory Society consensus,24 tests were applied in triplicate to obtain the averaged slope values. The phase III slopes of each SBW test were computed by linear regression between 65% and 95% of expired volume (Fig 126) and then multiplied with tidal volume.15,24,27 The interobserver agreement for double-tracer gas phase III slope (SDTG) is strong22 (intraclass correlation coefficient, 0.92). SDTG aggregates the phase III slopes from He and SF6, two gases of similar convective but highly differing diffusive properties. SDTG is a specific index of ventilation heterogeneity that estimates distal (diffusion-dependent) gas mixing efficiency near the acinar lung regions.18,22,28,29 The nitrogen phase III slope (SN2) reflects the washout behavior of a single gas only (N2) and, thus, is an unspecific global index of ventilation heterogeneity. SN2 is influenced from inhomogeneous ventilation distribution in proximal (convection-dependent) and distal lung regions.12,22 More details are provided in e-Appendix 1. Feno, a biomarker of eosinophilic airway inflammation, was measured by the single-breath method with an online chemiluminescence analyzer (CLD 77 AM; Eco Medics AG). After inhaling nitric oxide-free air to total lung capacity, children exhaled against an expiratory resistance with a constant flow of 50 mL/s for at least 6 s. Feno ⱖ 35 parts per billion was considered elevated.30,31
Table 1—Population Characteristics Characteristic Children (boys) Age, y Height, cm Weight, kg Current ICS, yes (no) Atopy, yes (no) Feno, ppb Feno ⱖ 35 ppb
Healthy Control Subjects
Children With Asthma
P Value
35 (18) 11.9 ⫾ 0.5 150.1 ⫾ 11.9 42.8 ⫾ 9.9 0 … … …
31 (12) 11.6 ⫾ 0.6 147.9 ⫾ 16.7 44.0 ⫾ 15.6 22 (9) 21 (10) 18.4 (9.0-33.2) 7 (23)
.461 .544 .596 .743 … … … …
Data are presented as mean ⫾ SD, median (interquartile range), or No. (%) unless otherwise indicated. Sex distribution was compared by Fisher exact test and anthropometric data by unpaired t test. ICS use was during 1 mo prior to the study. Atopy in children with asthma was determined by skin prick testing of common inhaled allergens (mixed trees, mixed grasses, cat, dog, house dust mite, and Aspergillus fumigatus). Children with any positive reaction were considered to be atopic sensitized. Feno 5 fraction of exhaled nitric oxide; ICS 5 inhaled corticosteroid; ppb 5 parts per billion. CHEST / 145 / 3 / MARCH 2014
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Figure 1. Single-breath washout measurement. Typical singlebreath washout examples of helium sulfur hexafluoride (doubletracer gas) tests in a healthy boy aged 10 y (light gray line, flattest slope) and one pre-BD and one post-BD test in a boy aged 10 y with mild asthma (dark gray line). To calculate the tidal alveolar phase III slope, the linear regression (black line) is fitted between 65% and 95% of expired volume. Because the molar mass signal is an aggregate signal from exhaled gas fractions, with CO2 playing the major role in the molar mass washout curve (about 28.9-29.5 g/mol),26 CO2 subtraction from the molar mass reveals the corrected molar mass washout curve (about 20.3 to 1.0 g/mol), which reliably estimates the helium sulfur hexafluoride washout behavior.22,23 BD 5 bronchodilation.
Plethysmography and spirometry were performed with the Jaeger Masterlab (CareFusion Corporation) according to recommendations.32 Specific airway resistance (sRaw) was expressed as percent predicted, and FEV1 and forced expiratory flow, midexpiratory phase (FEF25%-75%) were expressed as z scores.33 Statistical Analysis We calculated the sample size with the published baseline mean ⫾ SD SDTG of 20.06 ⫾0.09 g/mol from 48 healthy schoolaged children from a previous report.22 We postulated a detectable difference in SDTG between children with and without asthma as large as 0.1 ⫾ 0.09 g/mol. Assuming a significance level of 5% and a power of 90%, 22 control subjects and 22 children with asthma would be needed to complete the study. We aimed to recruit at least 30 children per group to allow for correlation analyses. The z scores for SDTG and SN2 were calculated from the healthy children. Abnormal lung function was defined as ⱕ 22 z scores. Data were compared by Fisher exact and t tests, as appropriate. Bronchodilator response (D) was estimated by the relative difference of prebronchodilator minus postbronchodilator values. P , .05 was considered significant. All analyses were done with Stata statistical software (StataCorp LP). Additional analyses are provided in e-Appendix 1.
Results Sixty-six children (31 with asthma) were studied, and 561 tidal SBW tests were achieved. Twenty-one children with asthma (67%) were atopic, and 22 (71%) received inhaled corticosteroids in the previous month. Healthy control subjects and children with asthma
494
were comparable regarding age, height, weight, and sex (Table 1). Children with mild asthma had significantly increased acinar ventilation heterogeneity (Figs 1-3, Table 2). SDTG was abnormal (ⱕ 22 z scores) in 11 children with asthma (36%) and in one control subject (3%) (Fig 3). In the 11 children with asthma, atopic sensitization, corticosteroid use, Feno, spirometry, and plethysmography were comparable with that of the 20 other children with mild asthma (data not shown). The mean SDTG difference between healthy control subjects and children with asthma was 1.49 (95% CI, 0.85-2.13) z scores. In contrast, global ventilation heterogeneity (SN2) was similar in both groups (Figs 2, 3, Table 2). According to the inclusion criteria, FEV1 was normal in children with asthma and did not differ significantly between groups. The mean FEV1 difference was 0.24 z scores (95% CI, 20.22 to 0.70 z scores). FEF25%-75% was lower in the children with asthma, with a mean difference between groups of 0.90 z scores (95% CI, 0.53-1.04 z scores) (Table 2). FEF25%-75% was abnormal in three children with asthma (10%) and two control subjects (6%). Eosinophilic airways inflammation appeared to be modestly elevated in asthma, with seven children (23%) having an increased Feno of ⱖ 35 parts per billion.30 The differences in SDTG and FEF25%-75% between children with asthma and healthy control subjects remained significant after adjusting for potential confounders (e-Table 1). Compared with other lung function indexes, FEF25%-75% showed the closest association with SDTG (R2 5 0.28) (e-Fig 1, e-Table 2), and similarly, FEV1 showed the closest association with SN2 (R2 5 0.28). Receiver operating characteristic analysis showed that SDTG provided the best diagnostic performance for mild asthma (e-Fig 2). Ventilation heterogeneity significantly changed after bronchodilator (Fig 3, Table 2). Acinar ventilation heterogeneity (SDTG), which was significantly increased in the children with asthma prior to bronchodilation, returned to normal after bronchodilation (Table 2) (mean postbronchodilation SDTG in children with asthma, 20.54 ⫾ 1.54 z scores; mean SDTG in control subjects, 20.01 ⫾ 1.07 z scores; P 5 .243). After bronchodilation, the average DSDTG was 35.7% (95% CI, 15.1%-56.3%) in the children with asthma. Global ventilation heterogeneity also significantly changed in the children with asthma after bronchodilation. DSN2 was 38.2% (95% CI, 17.9%-58.4%). sRaw similarly decreased (DsRaw, 33.0; 95% CI, 23.8%-42.2%). Spirometry indexes significantly increased but at a smaller effect size compared with the slope indexes and sRaw (DFEV1, 6.0% [95% CI, 3.3%-8.7%]; DFEF25%-75%, 17.0% [95% CI, 9.2%-24.9%]). After bronchodilation tidal volume, mean and peak tidal expiratory flow
Original Research
Figure 2. Comparison of ventilation heterogeneity in control subjects and children with mild asthma. Shown are the phase III slopes from the single-breath washout tests using a helium sulfur hexafluoride (double-tracer) gas mixture or pure oxygen for nitrogen single-breath washout. In children with asthma (gray lines), phase III slopes are shown pre- and post-BD. Group means are shown as black lines. See Figure 1 legend for expansion of abbreviation.
increased (e-Table 3). This change in breathing pattern was not associated with the bronchodilator effect on SDTG and SN2. Additional results are provided in e-Appendix 1.
Discussion Ventilation heterogeneity is increased in children with mild asthma despite rare symptoms and normal FEV1. More than one-third of children with asthma had abnormal SDTG findings, suggesting impaired small airways function in the acinar lung region. Measures of global ventilation heterogeneity (SN2) and airways obstruction (FEF25%-75%) are normal in the majority of children with mild asthma. sRaw is elevated on average but is only weakly associated with ventilation heterogeneity. Administration of a bronchodilator in children with asthma significantly alters acinar and global ventilation heterogeneity indexes by 36% and 38%, respectively. The classic measures of more proximal airways obstruction (FEV1 and FEF25%-75%) improve after bronchodilator inhalation, but their average bronchodilator effect size is less than that of SDTG and SN2. These simple tidal SBW tests may be used to characterize small airways function. The double-tracer gas SBW test has potential as journal.publications.chestnet.org
a clinical and research outcome in children with mild asthma. The current study has two major strengths. First, the data show for the first time to our knowledge that in children with stringently defined mild asthma, significantly increased ventilation heterogeneity may be present. The population studied is more relevant to most pediatricians than that with more severe disease. Second, the SBW technique relies on an easy tidal breathing protocol and an available device validated as currently recommended.23,24 Compared with multiplebreath washout or spirometry, this tidal SBW has greater success rates in children.22 Practicable SBW techniques are likely to become a standard procedure in children.34 Abnormal small airways function is a previously underestimated subclinical pathology in a considerable proportion of children with mild asthma. In this adequately powered study, 36% of the children with mild asthma had clear evidence (ie, SDTG ⱕ 22 z scores) of impaired gas mixing presumably near the acinar lung region.22,29 To specifically assess physiology in mild asthma, we stringently defined the inclusion criteria. Not surprisingly, classic measures of airways obstruction and global measures of ventilation heterogeneity (SN2) failed to clearly discriminate between health and asthma. FEV1, FEF25%-75%, sRaw, and Feno were CHEST / 145 / 3 / MARCH 2014
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Figure 3. Lung function in control subjects and children with mild asthma. Shown is the phase III slope from the single-breath washout test using a helium sulfur hexafluoride (double-tracer) gas mixture and FEF25-75. In children with asthma (gray lines), phase III slope and FEF25-75 values are shown preand post-BD. Group means are shown as black lines; the dashed lines reflect limits of normal lung function (⫾ 2 z scores). FEF25-75 5 forced expiratory flow, midexpiratory phase. See Figure 1 legend for expansion of other abbreviation.
either not or weakly correlated to acinar and global ventilation heterogeneity. Interestingly, SDTG was associated the strongest with FEF25%-75%, a measure of more peripheral obstruction than FEV1. SN2 was associated the strongest with FEV1. These associations support the concept of SDTG being an index of acinar ventilation heterogeneity and SN2 being an index of global ventilation heterogeneity, which is also influenced by proximal airways obstruction. The most relevant finding is that gas mixing efficiency is impaired in the acinar lung region in children
with mild asthma. Several studies reported increased ventilation heterogeneity in adults with asthma of various degrees of severity.6-10,12,13,35 Data in mild asthma are scarce. Most pediatric studies were done in patients with moderate to severe asthma or with frequent wheeze.14-17,19,36,37 Similar to the present data, these studies showed that measures of regional ventilation heterogeneity (ie, conductive ventilation heterogeneity, acinar ventilation heterogeneity) discriminated between patients with asthma and control subjects better than measures of global ventilation heterogeneity
Table 2—Lung Function Values Compared at Baseline and Before and After Bronchodilation Prebronchodilation Value SDTG, g/mol SDTG, z score SN2, N2% SN2, z score FEV1, z score FEF25%-75%, z score sRaw, % predicted
Postbronchodilationa
Prebronchodilation vs Postbronchodilationb
Control (n 5 35)
Asthma (n 5 31)
P Value
Asthma
P Value
Mean (95% CI)
P Value
20.06 ⫾ 0.05 20.01 ⫾ 1.07 3.51 ⫾ 1.73 0.01 ⫾ 1.02 20.36 ⫾ 0.83 20.26 ⫾ 0.93 …
20.14 ⫾ 0.07 21.47 ⫾ 1.49 2.96 ⫾ 1.60 20.32 ⫾ 0.93 20.60 ⫾ 0.80 21.10 ⫾ 0.72 216.2 ⫾ 59.3
, .001 , .001 .292 .292 .298 .001 …
20.09 ⫾ 0.08 20.54 ⫾ 1.54 1.83 ⫾ 1.24 20.98 ⫾ 0.73 20.12 ⫾ 0.79 20.51 ⫾ 0.87 144.9 ⫾ 36.9
.243 .243 , .001 , .001 .620 .458 …
0.05 (0.02-0.08) 0.93 (0.36-1.50) 1.13 (0.53-1.73) 0.66 (0.31-1.02) 0.48 (0.30-0.66) 0.59 (0.34-0.84) 71.3 (51.3-91.3)
.002 .002 .001 .001 , .001 , .001 , .001
Data are presented as mean ⫾ SD unless otherwise indicated. Data were compared by unpaired or paired t tests as appropriate. FEF25%-75% 5 forced expiratory flow, midexpiratory phase; SDTG 5 double-tracer gas phase III slope; SN2 5 nitrogen phase III slope; sRaw 5 specific airway resistance. aControl subjects vs children with asthma (postbronchodilation). bChildren with asthma only. 496
Original Research
(lung clearance index [LCI]), contrasting with findings from patients with cystic fibrosis lung disease wherein LCI usually is already elevated early in life.38,39 Because the LCI is a more global index of ventilation heterogeneity, like the SN2, it may not be able to differentiate patients with asthma from control subjects, but more sensitive tests like conductive and acinar ventilation heterogeneity can. In cystic fibrosis, the disease may have already caused such gross ventilation heterogeneity that the LCI can differentiate between the disease and healthy states. The overlap in LCI between patients with asthma and control subjects is strikingly similar to the SN2 distribution in the present study. In addition, evidence of acinar involvement was detected in adults with mild to moderate asthma.9,17 Others reported that conductive ventilation heterogeneity may be more frequently abnormal than acinar ventilation heterogeneity in preschoolaged children with severe wheezing.14,19,37 The differences among studies may relate to different IGW techniques and populations studied. Nevertheless, specific indexes of regional ventilation heterogeneity may be used in the future to improve diagnosis and follow-up of patients with asthma. Increased acinar ventilation heterogeneity may improve with intervention in asthma. In six of the 11 children with abnormal prebronchodilator SDTG, the SDTG returned to normal after bronchodilator inhalation (Fig 3). Interestingly, in the subgroup of children without current inhaled corticosteroid use (n 5 9), bronchodilator response in SDTG was greater than in children with inhaled corticosteroid use (e-Appendix 1), possibly indicating remodeling effects even in mild asthma. Despite only a few subjects in this subgroup, this finding is important and in line with a previous study in which distal airways function remained abnormal in children with stable asthma, despite of maximal bronchodilation and moderately high daily corticosteroid use.40 In light of the good SDTG repeatability (e-Appendix 1) and the significant but small improvement of FEV1 and FEF25%-75% (D 5 6%-17%), the bronchodilator effect on DSDTG (D 5 36%) could be interpreted as improved small airways function. In a recent study, we assessed SDTG and SN2 repeatability between tidal SBW tests performed 15 min apart in 25 school-aged children with clinically stable cystic fibrosis.41 The coefficient of repeatability of SDTG and SN2 was 0.065 g/mol and 4.1% N2, respectively. Thirteen of 31 children with mild asthma (42%) had a physiologically significant response to the bronchodilator (DSDTG . 0.065 g/mol). Despite normal SN2 at baseline; SN2 also decreased after bronchodilation in the present study. The caliber changes of convective airways possibly generated DSN2 and increased spirometry indexes. It is surprising that postbronchodilator SN2 in children with journal.publications.chestnet.org
asthma was lower than that found in healthy control subjects. However, DSN2 values did not exceed the coefficient of repeatability (4.1% N2). Compared with SDTG and DSDTG, SN2 and DSN2 may be less sensitive and specific in mild asthma. Verbanck and colleagues7-9 similarly showed that increased acinar ventilation heterogeneity is partially reversible. Comparable to this, change in LCI upon bronchodilation was unsystematic in both young children with frequent wheezing and control subjects.14,19 Similarly, Macleod et al15 did not find systematic bronchodilator effects in LCI and conductive ventilation heterogeneity in older children. Reversibility of global ventilation heterogeneity indexes warrants cautious interpretation. Available physiologic measures of small airways function may help to improve asthma phenotyping and treatment.33,42,43 By quantifying the SF6 and He washout behavior (phase III slopes), previous studies elegantly demonstrated that direct and indirect bronchoprovocation affects gas mixing efficiency at various levels of the airway tree.18,36,44 This effect apparently depends on the distribution of receptors triggered by the bronchoprovocation agent. Similarly, the site of particle deposition could be studied.8 The current SBW application reliably reflects the washout curves of SF6 and He; is obtainable; and, thus, may enhance the transition from research settings to clinical routine.23 A limitation of this and many other studies is that IGW indexes were not related to airways pathology or alveolar nitric oxide concentration. Few studies have shown an association between ventilation heterogeneity and peripheral airways histology or abnormal chest CT scan in smokers or in cystic fibrosis lung disease.28,45 However, others did not find a structurefunction relation in preschool-aged children with wheezing.37 Thus, we can only speculate on a possible structure-function relation between IGW indexes and small airways. Because of ethical considerations, we did not administer bronchodilators in healthy children, which would have allowed an estimation of normal bronchodilator effects on SDTG and SN2. Interpretation of treatment response assessed by IGW is complex and requires further study. A technical limitation of the current double-tracer gas is the requirement of SF6, which is restricted in some countries because of the greenhouse effect. However, the current SBW technique does not depend on SF6. Other inert gases, such as the noble gas xenon, with a similar diffusion coefficient as SF6 could be used.46 It is time to look at distal airways to improve asthma phenotyping. Despite few symptoms and normal FEV1, acinar ventilation heterogeneity (SDTG) is increased on average and is abnormal (ⱕ 22 z scores) in one-third of children with asthma. Administration of a bronchodilator significantly changes ventilation heterogeneity CHEST / 145 / 3 / MARCH 2014
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and airways obstruction in mild asthma. IGW testing may shed more light on the dynamic complexity of childhood asthma. The new double-tracer gas SBW is based on tidal breathing and has potential as a clinical and research outcome in children with mild asthma. Acknowledgments Author contributions: Dr Latzin had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Dr Singer: contributed to the study concept and design; data acquisition, analysis, and interpretation; and drafting, critical revision for important intellectual content, and final approval of the manuscript. Dr Abbas: contributed to the study concept and design; data acquisition, analysis, and interpretation; and drafting, critical revision for important intellectual content, and final approval of the manuscript. Dr Yammine: contributed to the data analysis and interpretation and critical revision for important intellectual content and final approval of the manuscript. Dr Casaulta: contributed to the study concept and design; data acquisition, analysis, and interpretation; and critical revision for important intellectual content and final approval of the manuscript. Dr Frey: contributed to the study concept and design, data analysis and interpretation, and critical revision for important intellectual content and final approval of the manuscript. Dr Latzin: contributed to the study concept and design; data acquisition, analysis, and interpretation; and drafting, critical revision for important intellectual content, and final approval of the manuscript. Financial/nonfinancial disclosures: The authors have reported to CHEST that no potential conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article. Role of sponsors: The sponsors had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Other contributions: The authors thank the children and their families for participating in the study. The authors also thank Markus Roos and Ruedi Isler for skillful technical assistance and Tom Riedel, MD; Nicolas Regamey, MD; Elena Proietti, MD; Anne Schmidt, MD; Gisela Wirz; Sandra Lüscher; Monika Graf; and Sharon Schmid for valuable work and support. This work was performed at the Asthma Outpatient Clinic of the University Children’s Hospital Bern, Bern, Switzerland. Additional information: The e-Appendix, e-Figures, and e-Tables can be found in the “Supplemental Materials” area of the online article.
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