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First published online on May 15, 2007
Chest, doi:10.1378/chest.06-2943
doi:10.1378/chest.06-2943
(Chest. 2007; 132:106-111)
© 2007 American College of Chest Physicians
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Comparison of Percentage Fall in FVC at the Provocative Concentration of Methacholine Causing a 20% Fall in FEV1 Between Patients With Asymptomatic Bronchial Hyperresponsiveness and Mild Asthma*

Young Yoo, MD; Ji Tae Choung, MD; Jinho Yu, MD; Do Kyun Kim, MD; Sun Hee Choi, MD and Young Yull Koh, MD

* From the Department of Pediatrics (Drs. Yoo and Choung), Korea University Anam Hospital, Seoul; Department of Pediatrics (Dr. Yu), Dongguk University International Hospital, Goyang, Gyeonggi; and Department of Pediatrics (Drs. Kim, Choi, and Koh), Seoul National University Hospital, Seoul, Korea.

Correspondence to: Young Yull Koh, MD, Department of Pediatrics, Seoul National University Hospital, 28 Yongon-dong, Chongno-gu, Seoul 110–744, Korea; e-mail: kohyy{at}plaza.snu.ac.kr

Abstract

Background: A significant proportion of individuals who have no symptoms of asthma or other respiratory diseases show bronchial hyperresponsiveness (BHR). BHR is usually assessed by measuring the provocative concentration of methacholine causing a 20% fall in FEV1 (PC20). The percentage fall in FVC at the PC20 ({Delta}FVC) has been suggested to reflect maximal airway response and to be a more useful index of disease severity in asthma than PC20. The aim of this study was to investigate whether asymptomatic BHR would differ from symptomatic BHR with regard to {Delta}FVC.

Methods: Methacholine bronchial challenge tests were conducted in children with no past or current symptoms of asthma, allergic rhinitis, or other respiratory diseases, who were identified among siblings of children with asthma. Forty-three children with asymptomatic BHR (PC20 < 16 mg/mL) were recruited, and 43 children with mild asthma who were matched for age, sex, and PC20 were selected (mild asthma group). The {Delta}FVC on methacholine concentration-response curves was retrospectively analyzed in the two groups.

Results: There were no differences in the frequency of atopy, blood eosinophil counts, serum IgE levels, and spirometric values between the asymptomatic BHR and mild asthma groups. Mean (± SD) {Delta}FVC was significantly (p = 0.005) lower in the asymptomatic BHR group (14.5 ± 3.6%) than in the mild asthma group (16.9 ± 4.3%).

Conclusions: Our results suggest that children with asymptomatic BHR have a lower level of maximal airway response than mild asthmatics with a similar degree of BHR. This may be a possible explanation for the lack of symptoms in subjects with asymptomatic BHR.

Key Words: asymptomatic bronchial hyperresponsiveness • maximal airway response • percentage fall in FVC at the provocative concentration of methacholine causing a 20% fall in FEV1 • provocative concentration of methacholine causing a 20% fall in FEV1

Bronchial hyperresponsiveness (BHR), an exaggerated airway narrowing in response to nonspecific stimuli, is a common characteristic of asthma.1 However, the relation between asthma and BHR is incomplete, so that subjects with BHR do not necessarily have asthma and vice versa.23 In fact, BHR is present in a significant proportion of individuals who have no history of asthma or of other respiratory diseases, and no current symptoms (asymptomatic BHR).4 The underlying mechanisms of asymptomatic BHR are still unclear, and more studies on the factors associated with asymptomatic status on a background of BHR are needed in order to better understand why some subjects with BHR have no symptoms.

BHR is usually defined as an increased sensitivity of the airways to inhaled histamine or methacholine.5 The sensitivity of the airways to these agents is commonly expressed as a provocative concentration of methacholine causing a 20% fall in FEV1 (PC20). However, this does not assess an absent or elevated maximal response plateau on the bronchoconstrictor dose-response curve, which may be the most important pathophysiologic abnormality in asthma6 because it puts asthmatics at risk for serious illness.7

The measurement of maximal airway response is neither safe nor easy to perform because of the risks inherent in producing an excessive fall in FEV1. The percentage fall in FVC at the PC20 ({Delta}FVC) has been proposed to be an indirect index of gas trapping and therefore an expression of the risk of an absent or elevated maximal response plateau.8 It has been shown that {Delta}FVC is significantly correlated with the maximal airway response in patients with mild asthma,9 suggesting a potential role of the {Delta}FVC as a surrogate marker for maximal airway response.

To our knowledge, there has been no study on the maximal airway response of asymptomatic BHR. The aim of the present study was to compare the {Delta}FVC between subjects with asymptomatic BHR and mild asthmatics with a similar degree of BHR. This would provide indirect information as to whether the level of maximal airway response differs between the two groups. It is conceivable that if the {Delta}FVC is lower in subjects with asymptomatic BHR than in mild asthmatics, it would help to explain their asymptomatic status.

Methods and Materials

We performed a retrospective analysis of methacholine challenge test data from children with asymptomatic BHR and from children with mild asthma who were matched for sex, age, and PC20 levels. Methacholine challenge tests were conducted with the same technique, and followed written policies and procedures.

A group of subjects with asymptomatic BHR, aged 6 to 16 years, were identified among siblings of children with asthma. These subjects were asked to undergo methacholine bronchial challenge and skin-prick tests as part of a study on the prevalence of BHR and atopy in families of asthmatic children. BHR was defined as PC20 < 16 mg/mL and atopy as at least one positive response (a mean wheal diameter ≥ 3 mm in the absence of any reaction in the negative control) to skin-prick tests with a battery of 13 common airborne allergens. The subjects with BHR were chosen regardless of whether they had atopy, and the International Study of Asthma and Allergies in Childhood10 questionnaire concerning asthma and allergic rhinoconjunctivitis was administered to each subject. This questioning was performed by trained nurses. For children aged < 12 years, the answers were obtained from their mothers and children aged ≥ 12 years answered the questions themselves. Subjects were considered to have asymptomatic BHR if they had shown no symptoms of the above diseases, as defined by negative responses to the International Study of Asthma and Allergies in Childhood questionnaire. Other chronic respiratory diseases, such as cystic fibrosis, bronchopulmonary dysplasia, and bronchiolitis obliterans, were ruled out on the basis of clinical, radiologic, and laboratory findings. The consecutive 43 subjects (20 boys and 23 girls) who fulfilled the inclusion criteria were enrolled (mean age, 12.1 ± 2.3 years).

Forty-three children with mild asthma were enrolled for the comparison group. At the time of diagnosis, methacholine bronchial challenge and skin-prick tests were performed. The clinical severity of asthma was assessed according to the National Asthma Education and Prevention Program.11 The subjects whose clinical severity could not be accurately assessed were excluded from the present study. The patients had suffered from episodic wheezing and/or dyspnea with symptom-free intervals. They needed intermittent use of ß2-agonists and/or continuous use of low-dose inhaled corticosteroids to control asthma symptoms. The subjects with a history of major exacerbations requiring systemic corticosteroids, near-fatal asthma, or respiratory diseases apart from asthma were excluded from the present study. Candidates (412 mild asthmatics) were screened and were selected by matching sex, age (within 6 months), and PC20 levels to the subjects with asymptomatic BHR. There was no significant difference in terms of asthma duration, FEV1, FVC, and FEV1/FVC between the selected (n = 43) and nonselected (n = 369) groups. At clinic visits, each subject was routinely evaluated by a battery of tests including blood eosinophil counts, total serum IgE, and spirometry.

Methacholine inhalation tests were performed using a modification of the method described by Chai et al.12 At the time of testing, all subjects had been free of acute respiratory tract infection or asthma exacerbation for a period of 4 weeks. Asthmatic children were asked to discontinue the use of inhaled ß2-agonists (for 24 h), oral theophylline (for 48 h), and inhaled corticosteroids (for 7 days) before testing. Spirometric measurements (FEV1 and FVC) were made using a computerized spirometer (Microspiro-HI 298; Chest; Tokyo, Japan), in accordance with the recommendations of the American Thoracic Society (ATS).13 The time course of the preceding inspiration was standardized (ie, a rapid maximal inspiration without end-inspiratory pause), and the FVC maneuver was continued until a plateau in the volume-time curve display was obvious or exhalation exceeded 6 s. The subjects were trained for spirometry in a reproducible way (ie, a coefficient of variation of FEV1 < 5% in three consecutive flow-volume curves), and they were required to have a FEV1 of at least 80% of the predicted value.14 Methacholine (Sigma Diagnostics; St. Louis, MO) solution was prepared at different concentrations (0.075, 0.15, 0.3, 0.625, 1.25, 2.5, 5, 10, and 25 mg/mL) in buffered saline solution (pH 7.4). A Rosenthal-French dosimeter (Laboratory for Applied Immunology; Baltimore, MD), triggered by a solenoid valve set to remain open for 0.6 s, was used to generate an aerosol from a DeVilbiss 646 nebulizer (DeVilbiss Health Care; Somerset, PA), with air pressurized at 20 lb per square inch. Each subject inhaled five inspiratory capacity breaths of buffered saline solution and increasing concentrations of methacholine at 5-min intervals. This gave an output of 0.009 ± 0.0014 mL (mean ± SD) per inhalation. FEV1 and FVC were measured 90 s after inhalation at each concentration level, and the largest value of triplicate FEV1 or FVC was used for the analysis. The procedure was terminated when the FEV1 decreased by > 20% of its post-saline solution value or when the highest methacholine concentration (25 mg/mL) was reached. The percentage fall in FEV1 from the post-saline solution value was plotted against the log concentration of the inhaled methacholine. PC20 was calculated by interpolating between two adjacent data points. The {Delta}FVC relative to baseline FVC after saline solution inhalation was also calculated using a log-linear interpolation.

Parents gave written informed consent for their children to participate in the study. The study protocol was approved by the Institutional Review Board.

Statistical Analysis
The primary study outcome was {Delta}FVC, and a sample size calculation was based on previous data reported by Gibbons et al.8 A minimum of 37 subjects per group was required to detect a difference of 3.6% between the two groups with 80% power and 5% statistical significance. The values of FEV1 and FVC were expressed as percentages of predicted based on data from our local population.14 The PC20 and serum total IgE values were logarithmically transformed before analysis and were expressed as geometric means and range of 1 SD. Other values were presented as mean ± SD. The variables or frequencies were compared between the two groups using the Student two-tailed t test or {chi}2 test. Correlations between {Delta}FVC and PC20 were examined using the Pearson correlation test. Multiple regression analysis was performed to assess the contributions of blood eosinophils, IgE levels, atopy, and spirometric values to {Delta}FVC. A value of p < 0.05 was accepted to be significant.

Results

The clinical characteristics of the two study groups are shown in Table 1 . There were no differences between the two groups in terms of age, sex ratio, blood eosinophil counts, spirometric values (FEV1, FVC, or FEV1/FVC), PC20, or percentage fall in FEV1 at the last concentration. Neither were there differences in the frequency of atopy, total serum IgE, or the pattern of positive skin test responses. The month in which the methacholine challenge tests were performed was distributed randomly throughout the year in the asymptomatic BHR group and in the mild asthma group.


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  		Table 1.. Clinical Characteristics of the Two Study Groups

 
In each study group, a multiple regression analysis was performed considering {Delta}FVC as the dependent variable, while blood eosinophils, IgE levels, atopy, and spirometric values were treated as independent variables. Analysis indicated that none of the above variables contributed significantly to {Delta}FVC in each group.

The {Delta}FVC levels of the two study groups are shown in Figure 1 . The mean {Delta}FVC in the asymptomatic BHR group was 14.5 ± 3.6%, which was significantly lower than that in the mild asthma group (16.9 ± 4.3%, p = 0.005).


Figure 1
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  		Figure 1.. {Delta}FVC values of the two study groups. BHR = PC20 < 16 mg/mL. Horizontal bars represent mean ± SD. {circ}Subjects with asymptomatic BHR. •Subjects with mild asthma.

 
Scatter plots of {Delta}FVC against PC20 are shown in Figure 2 . No significant correlation was found between {Delta}FVC and PC20 both in the asymptomatic BHR group (r = – 0.072, p = 0.647) and in the mild asthma group (r = – 0.137, p = 0.380).


Figure 2
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  		Figure 2.. Scatter plots of {Delta}FVC against PC20 in the asymptomatic BHR group and in the mild asthma group.

 
Discussion

The present study has shown that children with asymptomatic BHR have a significantly lower {Delta}FVC than mild asthmatics matched for sex, age, and PC20. This result suggests a different profile of maximal airway response in the two study groups, despite a similar degree of BHR.

Our subjects with asymptomatic BHR were derived from siblings of asthmatic subjects, which was based on a previous report15 that showed a high prevalence (41.3%) of asymptomatic BHR in families of subjects with asthma compared with families without asthma (26.7%). In our investigation, BHR was present in 55.7% (88 of 158 children) of siblings of asthmatic children, of whom 48.9% (43 of 88 children) were asymptomatic. This relatively low prevalence (27.2%, 43 of 158 children) of asymptomatic BHR may be explained in part by the exclusion of allergic rhinitics as well as asthmatics from our asymptomatic BHR group in the light of the fact that subjects with allergic rhinitis and no evidence of asthma frequently exhibit BHR.16 It may be possible that some subjects were labeled as having asymptomatic BHR in the common situation where symptoms associated with mild forms of asthma were unrecognized. This did not seem to be the case in our subjects with asymptomatic BHR because they had not previously experienced the symptoms that developed during the methacholine challenges. It is also conceivable that BHR may be asymptomatic because the PC20 cutoff is too high with regard to the provocation method and that it does not reflect an abnormal airway status. PC20 of 16 mg/mL, which was used to define BHR in the present study, appears to be high, but it is considered clinically appropriate because "borderline" BHR (PC20, 4 to 16 mg/mL), as defined by the ATS guidelines,17 was present in a considerable proportion (37.6%, 155 of 412 subjects) of all our patients with mild asthma.

In the present study, subjects with mild asthma were chosen for the comparison group in order to avoid difficulty in interpreting the difference in {Delta}FVC because this is related to asthma severity.18 At the initial evaluation, subjects with asymptomatic BHR had profiles of sex, age, and PC20 that were somewhat different from those in all our patients with mild asthma. Therefore, for an adequate comparison, patients with mild asthma were selected by matching sex, age, and PC20 levels with those of the subjects with asymptomatic BHR.

In the present study, the {Delta}FVC in the asymptomatic BHR group was significantly lower than in the mild asthma group. The increased {Delta}FVC might be a consequence of progressive shortening in expiration during bronchoprovocation testing, leading to incomplete emptying of the lung. However, this seems unlikely to be the reason because compliance with ATS criteria,13 including the occurrence of an expiratory plateau, was checked on all occasions. Although inhaled corticosteroids had been discontinued in the mild asthma group at least 1 week before the challenge, they may have had some residual effects. However, corticosteroids would rather decrease than increase {Delta}FVC19 and therefore cannot account for the higher {Delta}FVC observed in the mild asthma group. It is known that {Delta}FVC is related to some extent to the baseline airway caliber20 and may be affected by age-related lung elasticity.21 However, these factors could not explain the difference in {Delta}FVC in the present study because the two groups were similar in terms of age and baseline FEV1. Previous studies2223 have shown that subjects with BHR are more likely to have positive skin test results than those without BHR regardless of the presence of symptoms. We found no significant difference in the prevalence of atopy between the two groups. Although some of the sensitized allergens in the present study populations were seasonal, we believe that our results are not affected significantly by this factor because the challenge tests were performed randomly throughout the year, and also because the profiles of sensitization were similar between the two groups. Challenges were performed at the same time of day, thus ruling out a possible influence of circadian variation on {Delta}FVC.

{Delta}FVC could be measured safely and conveniently during routine bronchoprovocation testing. Previous studies8,18,24 indicated that {Delta}FVC measurements may give additional information about the state of the airways than that provided by PC20. Unlike the PC20, {Delta}FVC was found to be significantly related to the average number of oral corticosteroid prescriptions per month, which suggests that it may be a more useful index of disease severity of asthma than PC20.818 We have shown that adolescents with symptomatic asthma have a higher level of {Delta}FVC than those with clinical remission, irrespective of PC20 levels in the latter group.24

The present study was based on the hypothesis that {Delta}FVC may reflect gas trapping due to excessive bronchoconstriction.8 This was supported by a study9 showing a significant relationship between {Delta}FVC and maximal airway response in patients with mild asthma. If {Delta}FVC is a surrogate marker for maximal airway response, our findings suggest that the level of maximal airway response in subjects with asymptomatic BHR is lower than that in asthma patients with a similar degree of BHR. There are several possible explanations for the lack of symptoms in subjects with asymptomatic BHR.4 These include intermittent BHR, poor perception or no recognition of respiratory symptoms, minimal variability of airway obstruction, and too little airway inflammation/remodeling. A low degree of maximal airway response may be one of the mechanisms involved in asymptomatic BHR because maximal airway response reflects the potential severity of airway obstruction in each individual,7 and thus is presumed to be the most important pathophysiologic abnormality in asthma.6 It should be mentioned that the difference in {Delta}FVC between the two groups is small, albeit statistically significant, and there is a considerable degree of overlap. This suggests that other factors are also likely to be important for the absence of symptoms in association with BHR. In addition, although interesting on a theoretical basis, measurements of {Delta}FVC would have little practical value in differentiating subjects with asymptomatic BHR from asthmatics.

In conclusion, our results suggest that children with asymptomatic BHR have a lower level of maximal airway response than mild asthmatics with a similar degree of BHR. This may be a possible explanation for the lack of symptoms in subjects with asymptomatic BHR.

Footnotes

Abbreviations: ATS = American Thoracic Society; BHR = bronchial hyperresponsiveness; {Delta}FVC = percentage fall in FVC at the provocative concentration of methacholine causing a 20% fall in FEV1; PC20 = provocative concentration of methacholine causing a 20% fall in FEV1

This study was supported in part by BK 21 Project for Medicine, Dentistry, and Pharmacy, Seoul National University.

The authors have no financial or other potential conflicts of interest to disclose.

Received for publication December 7, 2006. Accepted for publication March 20, 2007.

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