The Diagnostic Value of Impulse Oscillometry and Plethysmography for the Assessment of Exercise-Induced Bronchoconstriction in Asthmatic Children

Abstract

Exercise-induced bronchoconstriction (EIB) is a key clinical problem for asthmatic children. Exercise challenge tests, used to confirm EIB, are time consuming and require patient cooperation. The aim of this study was to investigate the diagnostic values of fractional exhaled nitric oxide (FeNO), impulse oscillometry (IOS), and plethysmography for prediction of EIB in children with well-controlled asthma. Fifty-nine children with allergic asthma aged 6–18 years were included in the study. FeNO was measured and all patients underwent spirometry, IOS, and plethysmography. An exercise challenge test was performed to assess EIB. EIB was confirmed with an exercise challenge test in 20 (33.9%) patients. Baseline FeNO levels were significantly higher in the group with EIB (P = 0.003). Resistance at 5 Hz (R5) and frequency dependence of resistance (R5–R20) in IOS and total specific airway resistance (SRtot), residual volume (RV), and the ratio of RV to total lung capacity in plethysmography were significantly higher in the group with EIB. In the logistic regression analysis, the higher baseline FeNO, R5–R20, and SRtot values were found to be significantly related to EIB [Odds ratios (OR):1.35 and P = 0.046, OR:1.80, and P = 0.016, and OR:1.10, P = 0.035, respectively]. To differentiate asthmatic children with EIB from those without EIB, the optimal cutoff point for FeNO was 28 ppb [negative predictive value (NPV):86% and positive predictive value (PPV):52%]. An SRtot level lower than 207.6% (NPV:96% and PPV:54%) can be used to exclude EIB. R5–R20 values higher than 15.5% (NPV: 81% and PPV:71%) were associated with EIB in asthmatic children. Baseline SRtot in plethysmography provided the best sensitivity, whereas the baseline R5–R20 in IOS offered the best specificity for EIB. This study suggested that FeNO, IOS, and plethysmography are valuable tools for the assessment of EIB in children with controlled asthma and EIB is strongly correlated to small airway disease markers.

Introduction

Exercise-induced bronchoconstriction (EIB) is a specific sign of active asthma and can be present despite a lack of daily symptoms. The measurement of airway hyperresponsiveness (AHR) is important for the diagnosis and follow-up of childhood asthma.¹ Exercise challenge tests, used to confirm EIB, are widely used for assessment of asthma. However, exercise challenge test methodology is time consuming and complex; therefore, a reliable alternative test would be a useful diagnostic tool for the prediction of EIB.

Airway inflammation is associated with airflow limitation and AHR. The assessment of the association between airway inflammation and EIB is useful for the management of asthma.² Measurement of fractional exhaled nitric oxide (FeNO) is a non-invasive marker of eosinophilic airway inflammation. Previous studies have reported a significant relationship between increased FeNO levels and EIB in asthmatics.^3–5

Impulse oscillometry (IOS) is a lung function test that evaluates airway limitation during tidal breathing and can be easily applied to even young children, as it only requires passive cooperation.⁶ IOS is reported to be a more sensitive technique than spirometry for detecting subtle changes in lung function.^7,8 It can be used for clinical diagnostic testing of patients with airway obstruction and AHR.⁹

Total specific airway resistance (SRtot), measured by plethysmography, provides more sensitive measurements of airway obstruction. It was introduced as an alternative technique to assess lung function in asthmatic children.¹⁰ It is noninvasive, easy to perform and can also be used for detecting AHR.¹¹

The aim of this study was to investigate the diagnostic value of IOS and plethysmography for the prediction of EIB in children with well-controlled asthma.

Methods and Materials

Study population

This study was conducted at the Pediatric Allergy and Immunology Department, Mersin University Hospital, Turkey, between January 2016 and June 2016. Children with allergic asthma aged 6–18 years were enrolled. The patients had intermittent and mild-to-moderate, persistent asthma and were all sensitized to house dust mites. They had well-controlled asthma that was defined as the absence of daytime symptoms, nocturnal symptoms, limitation of activities, and need for rescue treatment in the previous month with normal lung function [forced expiratory volume in 1 s (FEV1) % ≥80 predicted and ratio of FEV1 to forced vital capacity (FVC) (FEV1/FVC) ≥80% by spirometry].¹² Regular medications were recorded for all patients. Children who had suffered from a respiratory tract infection in the previous 2 weeks, asthma attack, and a history of systemic steroid usage in the past month and who were not cooperative during lung function testing were excluded from the study.

The study was approved by the Institutional Ethical Committee, Mersin University, and all patients supplied written, informed consent before taking part in the study.

Study protocol

FeNO levels were first measured, and lung function was then evaluated by using IOS, spirometry, and plethysmography before exercise. AHR was assessed by a spirometric exercise challenge test after basal respiratory functions were evaluated.

FeNO measurement

FeNO measurement was performed with an NIOX MINO^® (Aerocrine AB, Sweden) system according to the American Thoracic Society's (ATS) guidelines before performing spirometry, because forced expiration can decrease FeNO.¹³ The children waited for 1 h in the same room where the measurement took place to prevent exposure to extrinsic factors. Participants were instructed to exhale to residual volume (RV). A mouthpiece was then inserted and they were asked to inhale to total lung capacity (TLC) and then to exhale for 10 s at a constant flow rate of 50 mL/sn.

Pulmonary function tests

Spirometry

Spirometry was performed by using a Master Screen spirometry system (JaegerCO, Germany) according to ATS guidelines.¹⁴ The values of FEV1, FEV1 change in percent predicted (FEV1%chg), FVC, the ratio FEV1/FVC, and maximum mid-expiratory flow [MMEF; also known as forced expiratory flow rate (FEF25–75) measured between expired volumes of 25% and 75% of the vital capacity during a forced expiration] were recorded.¹⁵ Bronchodilator response was evaluated by the percent change in FEV1 after 200 mcg of inhaled salbutamol with a metered dose inhaler. Zapletal reference values were used for spirometric indices.¹⁶

Impulse oscillometry

IOS during tidal breathing was performed in compliance with the European Respiratory Society/ATS (ERS/ATS) guidelines¹⁷ by using the MasterScreen system (JaegerCO, Germany). The IOS parameters obtained at the end of the application were resistances at 5–20 Hz (R5, R20), reactance at 5 Hz (X5), area of reactance (AX), and R5–R20 (resistance at 5 Hz minus resistance at 20 Hz). The difference between the absolute values obtained before and after salbutamol inhalation (Δ) was divided by the absolute values before salbutamol, and the result was multiplied by 100. Commercially available predicted values for IOS parameters were based primarily on height according to normal reference values as recommended by the manufacturer.

Plethysmography

Static lung volumes and airway resistance were assessed by whole-body plethysmography according to ATS guidelines.¹⁸ Total specific airway resistance (SRtot), TLC, RV, and the ratio of RV to TLC were recorded and the results were expressed as a percent predicted for age, sex, and height. The reference equations for plethysmography were based on normal reference values as recommended by the manufacturer.

Exercise challenge

Subjects performed an exercise challenge test. In brief, first a basal FEV1 was obtained. Then, they rode a bicycle ergometer (cycle ergometer, 928G3 Monark Exercise AB, Sweden) for 4–6 min until they reached a heart rate of between 80% and 90% of the maximum predicted (220—age in years) in a temperature and humidity controlled environment according to ATS recommendations.¹⁹ Spirometry was repeated 5, 10, 15, 20, and 30 min post-exercise. EIB was defined as a fall in FEV1 of at least 10%.

Statistical analysis

Descriptive analysis was performed by using median (25–75p) values for variables not normally distributed and means ± standard deviations for normally distributed variables. Independent-sample t-tests or Mann-Whitney U tests were used for a comparison of the 2 groups based on the distribution pattern of the variables. The chi-square test was used for categorical endpoints. Logistic regression analysis was used to evaluate associated factors for EIB. For variables with large measurement units, a regression coefficient was calculated by multiplying by 10 to examine the increase in risk when there was a change of 10 units instead of 1 unit.²⁰ Odds ratios (ORs) with 95% confidence intervals were estimated. Receiver operator characteristic (ROC) curve analysis was performed to determine the cut-off value for FeNO and pulmonary indices to predict EIB. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were analyzed. A P value of less than 0.05 was considered statistically significant.

Results

A total of 59 children aged 6–18 years participated in the study. The median age was 11.0 (9.0–14.0). Thirty-nine (66.1%) of the patients were male. Forty (67.8%) of the patients were using inhaled corticosteroids (ICS). EIB was confirmed with exercise challenge in 20 (33.9%) patients. There was no significant difference between patients with and without EIB in terms of age, gender, body mass index (BMI), ICS usage, serum eosinophil count, and serum IgE levels. However, FeNO levels were significantly higher in the group with the positive exercise challenge test (P = 0.003) (Table 1 and Fig. 1a). The clinical and demographic characteristics of the patients are shown in Table 1.

FIG. 1.

(a) FeNO levels (median, 95% CI) in patients with positive and negative exercise challenge test. (b) R5–R20% predicted levels (median, 95% CI) in patients with positive and negative exercise challenge test. (c) SRtot% predicted levels (median, 95% CI) in patients with positive and negative exercise challenge test. FeNO, fractional exhaled nitric oxide; CI, confidence intervals.

Table 1.

Clinical and Demographic Characteristics and Atopy Markers of Patients with Positive and Negative Exercise Challenge

Variable	Patients with positive exercise challenge n = 20	Patients with negative exercise challenge n = 39	P
Age (years)	12.0 (9.0–15.0)	11.0 (9.0–14.0)	0.479
Male gender	14 (70.0%)	25 (64.1%)	0.651
Height	150.0 (133.5–164.0)	146.0 (128.0–161.0)	0.391
BMI (kg/m²)	20.7 (16.6–23.4)	17.4 (16.0–21.4)	0.118
ICS usage	13 (65.0%)	27 (69.2%)	0.742
Serum eosinophil count ( × 10³/μL)	400.0 (125.0–695.0)	400.0 (200.0–700.0)	0.646
Total IgE (IU/mL)	295.5 (124.7–1670.0)	444.0 (209.0–915.0)	0.532
FeNO (ppb)	37.5 (28.2–63.7)	25.0 (15.0–33.0)	0.003

BMI, body mass index; FeNO, fractional exhaled nitric oxide; ICS, inhaled corticosteroids.

P values in bold are statistically significant.

Comparison of pulmonary function tests for patients with negative and positive exercise challenge test

The values of FEV1% predicted (P = 0.006), FEV1/FVC% predicted (P < 0.001), and MMEF% predicted (P < 0.001) were significantly lower in the group with positive exercise challenge than those with negative exercise challenge (Table 2). FEV1%chg predicted (P = 0.010), resistance at 5 Hz (R5% predicted) (P = 0.003), resistance at 5 Hz minus resistance at 20 Hz (R5–R20% predicted) (P < 0.001) (Fig. 1b), total specific airway resistance (SRtot% predicted) (P < 0.001) (Fig. 1c), RV% (predicted) (P = 0.044), and the ratio of RV/TLC% (predicted) (P = 0.049) levels were significantly higher in children with EIB than those without EIB (Table 2). No significant difference was found between the groups in terms of FVC% predicted (P = 0.017), ΔR5% predicted (P = 0.603), resistance at 20 Hz (R20% predicted) (P = 0.321), reactance at 5 Hz (X5) (P = 0.575), area of reactance (AX) (P = 0.206), ΔAX (P = 0.987), and TLC% (predicted) (P = 0.493) levels.

Table 2.

Comparison of Pulmonary Function Tests (Spirometry, Impulse Oscillometry, and Plethysmography) of Patients with Negative and Positive Exercise Challenge

Variable	Patients with positive exercise challenge n = 20	Patients with negative exercise challenge n = 39	P
Spirometry
FVC% predicted^a	99.1 ± 12.6	103.4 ± 10.6	0.177
FEV1% predicted^a	89.8 ± 17.0	102.1 ± 10.7	0.006
FEV1%chg predicted^a	9.5 (5.5–14.7)	5.9 (2.3–8.3)	0.010
FEV1/FVC%	89.8 ± 8.7	99.0 ± 8.6	<0.001
MMEF%predicted^a	60.1 ± 23.6	82.8 ± 20.8	<0.001
Impulse oscillometry
R5 Hz%predicted	131.8 (112.0–167.6)	108.7 (93.8–125.3)	0.003
ΔR5% predicted	−22.8 (−36.0 to −2.2)	−16.6 (−29.0 to −6.0)	0.603
R20Hz% predicted^a	118.6 ± 36.0	110.8 ± 23.3	0.321
X5	−0.21 (−0.38 to −0.13)	−0.21 (−0.31 to −0.14)	0.575
AX	1.9 (0.9–3.9)	1.4 (0.7–2.6)	0.206
ΔAX	−39.2 (−60.7 to−13.0)	−40.4 (−56.0 to −22.0)	0.987
R5–R20% predicted	19.1 (1.6–34.9)	−0.5 (−11.6 to 7.9)	<0.001
Plethysmography
SRtot% predicted	270.1 (230.7–386.5)	199.7 (172.0–248.2)	<0.001
TLC% predicted^a	102.3 ± 12.0	100.2 ± 10.1	0.493
RV% predicted	97.5 (79.2–148.3)	86.0 (69.4–103.9)	0.044
RV/TLC% predicted	97.9 (77.1–134.9)	86.9 (66.1–104.2)	0.049

The values were presented as mean ± standard deviation. The other values were presented as median (25–75p).

FEV, forced expiratory volume; FVC, forced vital capacity; TLC, total lung capacity; MMEF, maximum mid-expiratory flow.

P values in bold are statistically significant.

Logistic regression and ROC curve of factors that are related to positive exercise challenge test

Higher FeNO, R5–R20% predicted, and SRtot% predicted values were found to be significantly related to EIB in the logistic regression analysis (aOR:1.35, P = 0.046, aOR:1.80, P = 0.016, and aOR:1.10, P = 0.035, respectively) independent of age, sex, BMI, ICS usage, and serum eosinophil count (Table 3). Each 10% increase in FeNO, SRtot, and R5–R20 predicts a 1.35-, 1.10-, and 1.80-fold increase in the risk of EIB, respectively.

Table 3.

Logistic Regression Analysis of Factors That Are Related to Positive Exercise Challenge

Variable	Odds ratio (95% CI)	P
Age (years)	0.93 (0.71–1.21)	0.593
Male gender	0.99 (1.17–5.69)	0.992
BMI (kg/m²)	1.08 (0.86–1.35)	0.488
ICS usage	0.30 (0.05–1.73)	0.181
FeNO (ppb)	1.35 (1.01–1.79)	0.046
Serum eosinophil count(×10³/μL)	0.99 (0.99–1.00)	0.160
FEV1% predicted	0.96 (0.89–1.03)	0.304
MMEF% predicted	1.00 (0.92–1.09)	0.966
R5–R20% predicted	1.80 (1.11–2.91)	0.016
SRtot% predicted	1.10 (1.01–1.22)	0.035

P values in bold are statistically significant.

In the ROC curve analysis, FeNO levels lower than 28 ppb were found to be significantly related to the absence of EIB (AUC: 0.737, P < 0.001, sensitivity: 75%, specificity: 66.7%, NPV: 86%, and PPV:%52%) in children with controlled asthma (Fig. 2a). Twenty-four of the 28 children with FeNO levels lower than 28 ppb had no EIB, indicating an NPV of 86%.

FIG. 2.

(a) The receiver operating characteristic curve for FeNO to predict exercise challenge test result. (b) The receiver operating characteristic curve for SRtot% value to predict exercise challenge test result. (c) The receiver operating characteristic curve for R5–R20% value to predict exercise challenge test result.

An SRtot level lower than 207.6% was found to be significantly related to the absence of EIB (AUC: 0.828, P < 0.001, sensitivity: 95%, specificity: 61.5%, NPV: 96%, and PPV: 54%) in children with controlled asthma (Fig. 2b). Twenty-three of the 24 children with SRtot levels lower than 207.6% had no EIB, indicating a high sensitivity and NPV of 95% and 96%, respectively.

An R5–R20 level higher than 15.5% was found to be significantly related to the risk of EIB (AUC: 0.786, P < 0.001, sensitivity: 60%, specificity: 89.7%, NPV: 81% and PPV: 71%) in children with controlled asthma (Fig. 2c). Twelve of the 17 children with R5–R20 levels higher than 15.5% had EIB, indicating a specificity and PPV of 89.7% and 71%, respectively.

Discussion

EIB is a common feature of asthma but can also occur without asthma. The assessment of EIB is used for both the diagnosis and follow-up of asthma.^21,22 However, exercise challenge tests take a long time and require special equipment such as a treadmill or bicycle ergometer and a temperature and humidity controlled environment that are not easily available. Thus, a reliable and more practical screening test would be a feasible tool for the prediction of EIB in children.

In this study, EIB was confirmed in 20 (33.9%) patients despite the fact that they had well-controlled asthma and 65% of them were using ICSs. R5% and R5–R20% levels in IOS, SRtot%, RV%, and RV/TLC% levels in plethysmography, and FeNO values were significantly higher in asthmatic children with positive exercise challenge. In the logistic regression analysis, the higher FeNO, R5–R20% predicted, and SRtot% predicted values were found to be significantly related to EIB. SRtot levels lower than 207.6% helped to exclude EIB with a high NPV of 96%. It was also possible to rule out the presence of EIB with FeNO values lower than 28 ppb and R5–R20 levels lower than 15.5% with high reliability (NPV 86% and 81%, respectively). On the other hand, an R5–R20 level higher than 15.5% helped to predict EIB with a specificity of 89.7% and a PPV of 71%.

The exercise challenge test is an indirect bronchial provocation test considered to be correlated better with clinical asthma than direct challenge tests.^21,23 Because of its greater relation to airway inflammation and greater responsiveness to anti-inflammatory treatments, the indirect challenge could be the challenges of choice as a guide for evaluating and monitoring asthma.²¹ Therefore, markers of airway inflammation can be useful for predicting EIB.^2,24

FeNO, a non-invasive marker of eosinophilic airway inflammation, has become a reliable tool for the management of asthma in children.^25,26 It has been demonstrated that airway inflammation and AHR persists in most patients with well-controlled asthma despite the fact that their condition is controlled.^27,28 In this study, although all the patients had controlled asthma, 33.9% had a positive exercise provocation test. In this regard, FeNO is of particular value as a marker to predict AHR in children with well-controlled asthma.^5,26 We found that FeNO levels were significantly higher in asthmatic children with EIB than without, which indicated the contribution of eosinophilic inflammation to EIB. Previous studies have investigated the predictive value of FeNO for diagnosing EIB.^24,29,30 Lex et al. showed that no child in their study with normal pre-exercise FeNO (<25 ppb) had an abnormal exercise challenge result.⁴ In this study, an FeNO level lower than 28 ppb was found to be reliable for ruling out the presence of EIB, with an NPV of 86% among asthmatic children who otherwise had a controlled disease.

Plethysmography is one of the most commonly used methods for measuring airway resistance in asthmatic children.³¹ It was introduced as an alternative technique to assess lung function because it is easy to perform and non-invasive. Also, there is good evidence that body plethysmography has higher sensitivity than spirometry for detecting AHR.^32,33 The comparison of spirometry and plethysmography during bronchial challenges showed that up to 20% of subjects demonstrated a positive response for specific airway resistance (sRaw) without sufficient response in FEV1.³³ SRtot, which is an effort-independent parameter measured by a plethysmograph, is a very sensitive index for diagnosis of peripheral airway obstruction.^31,33,34 We found that SRtot levels were significantly higher in asthmatic children with EIB than without, and this suggested the involvement of peripheral airway obstruction in EIB. In addition, SRtot levels lower than 207.6% helped to exclude EIB with a high sensitivity of 95% and NPV of 96%. In cases of SRtot levels less than 207.6%, exercise challenge tests are unnecessary in children with controlled asthma. Further, RV and RV/TLC, which reflect air trapping, were also higher in the group with EIB. Plethysmographic assessment of lung volumes provides a sensitive measure of air trapping and lung hyperinflation. RV and RV/TLC are important measures of small airway dysfunction and can be raised before the onset of abnormal spirometry in asthma.^35–37

IOS is a simple and non-invasive technique that measures respiratory resistance and reactance at different oscillation frequencies during spontaneous tidal breathing.⁹ R5–R20, a measure of frequency dependence of resistance, reflects resistance in small airways.³⁸ The association of AHR and small airway function is of great importance in asthmatic children.³⁹ In addition, small airway dysfunction is considered a prominent feature in clinical entities such as EIB.⁴⁰ In this regard, the increased sensitivity of IOS is promising for the assessment of AHR as well as for small airway function in asthmatics.^11,39,41 Consistent with these data, we found that R5–R20 levels were significantly higher in asthmatic children with EIB than without, and this suggests the involvement of small airway dysfunction in EIB. In addition, R5–R20 levels lower than 15.5% can help to exclude EIB, with an NPV of 81%. On the other hand, 71% of the children with R5–R20 levels higher than 15.5% had EIB, indicating specificity and PPVs of 89.7% and 71%, respectively.

The major limitation of this study was its small sample size. The strength of this study was its demonstration of the association between FeNO, baseline SRtot and R5–R20, and EIB in children with controlled asthma. These surrogate tests are easy to perform and time efficient; therefore, they are of great utility for the assessment of EIB in asthmatic children. In addition, because the prevalence of EIB in our study population was consistent with previous publications,^19,42–44 PPVs and NPVs of the surrogate tests (FeNO, IOS, and plethysmography) identified in this study can be applied to most children with controlled asthma.

Conclusions and Clinical Implications

In conclusion, baseline SRtot in plethysmography yielded greater sensitivity whereas baseline R5–R20 in IOS offered greater specificity in relation to EIB. In addition, this study suggested that FeNO, IOS, and plethysmography are valuable tools for the assessment of EIB in asthmatic children even when the disease is under control and EIB is strongly correlated to small airway disease markers.

Institution at Which the Work Was Performed

The work was performed at the Pediatric Allergy and Immunology Department of Mersin University, Faculty of Medicine.

Footnotes

Authors' Contribution

T.A. and A.U. recruited the subjects. T.A. wrote the first draft of the article. S.B.B. and S.K. took part in the critical revision of the article. S.K. designed the whole study, completed writing, and supervised the project. All authors agreed on the final version of the article.

Author Disclosure Statement

No competing financial interests exist.

References

Weiler

, Brannan

, Randolph

, Hallstrand

, Parsons

, Silvers

, et al. Exercise-induced bronchoconstriction update-2016. J Allergy Clin Immunol, 2016; 138:1292–1295.e36.

Yoshikawa

, Shoji

, Fujii

, Kanazawa

, Kudoh

, Hirata

, et al. Severity of exercise-induced bronchoconstriction is related to airway eosinophilic inflammation in patients with asthma. Eur Respir J, 1998; 12:879–884.

Nishio

, Odajima

, Motomura

, Nakao

, Nishima

. Exhaled nitric oxide and exercise-induced bronchospasm assessed by FEV1, FEF25–FEF75% in childhood asthma. J Asthma, 2007; 44:475–478.

Lex

, Dymek

, Heying

, Kovacevic

, Kramm

, Schuster

. Value of surrogate tests to predict exercise-induced bronchoconstriction in atopic childhood asthma. Pediatr Pulmonol, 2007; 42:225–230.

Scollo

, Zanconato

, Ongaro

, Zaramella

, Zacchello

, Baraldi

. Exhaled nitric oxide and exercise-induced bronchoconstriction in asthmatic children. Am J Respir Crit Care Med, 2000; 161:1047–1050.

Bickel

, Popler

, Lesnick

, Eid

. Impulse oscillometry: interpretation and practical applications. Chest, 2014; 146:841–847.

Frantz

, Nihlén

, Dencker

, Engström

, Löfdahl

, Wollmer

. Impulse oscillometry may be of value in detecting early manifestations of COPD. Respir Med, 2012; 106:1116–1123.

Schulze

, Smith

, Fuchs

, Herrmann

, Dressler

, Rose

, et al. Methacholine challenge in young children as evaluated by spirometry and impulse oscillometry. Respir Med, 2012; 106:627–634.

Komarow

, Myles

, Uzzaman

, Metcalfe

. Impulse oscillometry in the evaluation of diseases of the airways in children. Ann Allergy Asthma Immunol, 2011; 106:191–199.

10.

Kaminsky

. What does airway resistance tell us about lung function?. Respir Care, 2012; 57:85–96.

11.

Naji

, Keung

, Kane

, Watson

, Killian

, Gauvreau

. Comparison of changes in lung function measured by plethymography and IOS after bronchoprovocation. Respir Med, 2013; 107:503–510.

12.

From National Heart, Lung, and Blood Institute. Expert Panel Report 3 (EPR-3): guidelines for the diagnosis and management of asthma-full report. 2007. Avaialable at: www.nhlbi.nih.gov/guidelines/asthma/asthgdln.htm Accessed November 1, 2016.

13.

Dweik

, Boggs

, Erzurum

, Irvin

, Leigh

, Lundberg

; 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; 184:602–615.

14.

Miller

, Crapo

, Hankinson

, Brusasco

, Burgos

, Casaburi

, et al. General considerations for lung function testing. Eur Respir J, 2005; 26:153–161.

15.

Pellegrino

, Viegi

, Brusasco

, Crapo

, Burgos

, Casaburi

, et al. Interpretative strategies for lung function tests. Eur Respir J, 2005; 26:948–968.

16.

Zapletal

, Samanek

, Paul

, et al. Lung function in children and adolescents: methods, reference values. Basel: Karger, 1997.

17.

Beydon

, Davis

, Lombardi

, Allen

, Arets

, Aurora

, et al. An official American Thoracic Society/European Respiratory Society statement: pulmonary function testing in preschool children. Am J Respir Crit Care Med, 2007; 175:1304–1345.

18.

Stocks

, Godfrey

, Beardsmore

, Bar-Yishay

, Castile

; ERS/ATS Task Force on Standards for Infant Respiratory Function Testing. European Respiratory Society/American Thoracic Society. Plethysmographic measurements of lung volume and airway resistance. ERS/ATS Task Force on Standards for Infant Respiratory Function Testing. European Respiratory Society/American Thoracic Society. Eur Respir J, 2001; 17:302–312.

19.

Parsons

, Hallstrand

, Mastronarde

, Kaminsky

, Rundell

, Hull

, et al. An official American Thoracic Society clinical practice guideline: exercise-induced bronchoconstriction. Am J Respir Crit Care Med, 2013; 187:1016–1027.

20.

Hosmer

, Lemeshow

. Multiple logistic regression, in applied logistic regression, 2nd Edition. John Wiley & Sons, New York, 2000, p. 375.

21.

Cockcroft

, Davis

. Direct and indirect challenges in the clinical assessment of asthma. Ann Allergy Asthma Immunol, 2009; 103:363–369.

22.

Randolph

. Diagnostic exercise challenge testing. Curr Allergy Asthma Rep, 2011; 11:482–490.

23.

Anderson

. Indirect challenge tests: airway hyperresponsiveness in asthma: its measurement and clinical significance. Chest, 2010; 138:25–30.

24.

Buchvald

, Hermansen

, Nielsen

, Bisgaard

. Exhaled nitric oxide predicts exercise-induced bronchoconstriction in asthmatic school children. Chest, 2005; 128:1964–1967.

25.

Warke

, Fitch

, Brown

, Taylor

, Lyons

, Ennis

, et al. Exhaled nitric oxide correlates with airway eosinophils in childhood asthma. Thorax, 2002; 57:383–387.

26.

Barreto

, Zambardi

, Villa

. Exhaled nitric oxide and other exhaled biomarkers in bronchial challenge with exercise in asthmatic children: current knowledge. Paediatr Respir Rev, 2015; 16:68–74.

27.

Muñoz

, Sanchez-Vidaurre

, Roca

, Torres

, Morell

, Cruz

. Bronchial inflammation and hyperresponsiveness in well controlled asthma. Clin Exp Allergy, 2012; 42:1321–1328.

28.

Huang

, Zhang

, Wang

. Airway hyper-responsiveness and small airway function in children with well-controlled asthma. Pediatr Res, 2015; 77:819–822.

29.

ElHalawani

, Ly

, Mahon

, Amundson

. Exhaled nitric oxide as a predictor of exercise-induced bronchoconstriction. Chest, 2003; 124:639–643.

30.

Ramser

, Hammer

, Amacher

, Trachsel

. The value of exhaled nitric oxide in predicting bronchial hyperresponsiveness in children. J Asthma, 2008; 45:191–195.

31.

Urbankowski

, Przybyłowski

. Methods of airway resistance assessment. Pneumonol Allergol Pol, 2016; 84:134–141.

32.

Merget

, Nensa

, Heinze

, Taeger

, Bruening

. Spirometry or body plethysmography for the assessment of bronchial hyperresponsiveness?. Adv Exp Med Biol, 2016; 921:1–10.

33.

Criée

, Sorichter

, Smith

, Kardos

, Merget

, Heise

, et al; Working Group for Body Plethysmography of the German Society for Pneumology and Respiratory

Care

. Body plethysmography—its principles and clinical use. Respir Med, 2011; 105:959–971.

34.

Desiraju

, Agrawal

. Impulse oscillometry: the state-of-art for lung function testing. Lung India, 2016; 33:410–416.

35.

Perez

, Chanez

, Dusser

, Devillier

. Small airway impairment in moderate to severe asthmatics without significant proximal airway obstruction. Respir Med, 2013; 107:1667–1674.

36.

Jain

, Abejie

, Bashir

, Tyner

, Vempilly

. Lung volume abnormalities and its correlation to spirometric and demographic variables in adult asthma. J Asthma, 2013; 50:600–605.

37.

Kraft

, Pak

, Martin

, Kaminsky

, Irvin

. Distal lung dysfunction at night in nocturnal asthma. Am J Respir Crit Care Med, 2001; 163:1551–1556.

38.

Brashier

, Salvi

. Measuring lung function using sound waves: role of the forced oscillation technique and impulse oscillometry system. Breathe(Sheff), 2015; 11:57–65.

39.

Alfieri

, Aiello

, Pisi

, Tzani

, Mariani

, Marangio

, et al. Small airway dysfunction is associated to excessive bronchoconstriction in asthmatic patients. Respir Res, 2014; 15:86.

40.

Bjermer

. The role of small airway disease in asthma. Curr Open Pulm Med, 2014; 20:23–30.

41.

Short

, Anderson

, Manoharan

, Lipworth

. Usefulness of impulse oscillometry for the assessment of airway hyperresponsiveness in mild-to-moderate adult asthma. Ann Allergy Asthma Immunol, 2015; 115:17–20.

42.

Randolph

. An update on exercise-induced bronchoconstriction with and without asthma. Curr Allergy Asthma Rep, 2009; 9:433–438.

43.

Grzelewski

, Grzelewska

, Majak

, Stelmach

, Kowalska

, Stelmach

, et al. Fractional exhaled nitric oxide (FeNO) may predict exercise-induced bronchoconstriction (EIB) in schoolchildren with atopic asthma. Nitric Oxide, 2012; 27:82–87.

44.

Weiler

, Bonini

, Coifman

, Craig

, Delgado

, Capão-Filipe

, et al. American Academy of Allergy, Asthma & Immunology Work Group report: exercise-induced asthma. J Allergy Clin Immunol, 2007; 119:1349–1358.