MMEF_25-75 may predict significant BDR and future risk of exacerbations in asthmatic children with normal baseline FEV₁

Abstract

(1) Background: Several recent studies on the clinical value of spirometry indexes demonstrated high sensitivity of FEF_25-75 as a marker of bronchial obstruction in asthmatics with normal baseline spirometry. Our study aims to evaluate the clinical value of maximal mid-expiratory flow in children with asthma. (2) Methods: For two years, 257 children were included - 211 with asthma and 46 healthy controls. Pre- and post-bronchodilator spirometry, atopic status determination and asthma control assessment were performed. (3) Results: The small airway obstruction (SAO) group (FEV₁≥80%, ММEF_25/75<65%) demonstrated significantly lower values for FEV₁, FEV₁/FVC, PEFR, МMMF_25/75 and a significant higher bronchodilator response (BDR, ΔFEV₁% init. ≥12%) compared to normal baseline spirometry group (FEV₁>80%, MMEF_25/75≥65%) (Р<0.0001). In addition, we found a statistically significant difference in FEF_25-75/FVC median between asthmatics and healthy controls (Р<0.0001) regardless of the FEV₁ value. Children with SAO have a 2.338-fold higher risk of poor asthma outcome (OR 95% CI [1.077-5.294]) and a 6.171-fold (OR 95% CI [2.523-15.096]) greater probability of demonstrating positive BDR, compared to children with normal baseline spirometry. MMEF_25/75 was found to be a good predictor for positive BDR with AUC 0.843 (CI 0.781-0.845) and a best cut-off value of 58.1% (77.8% sensitivity and 78.8% specificity). (4) Conclusion: Our results confirmed that a small but substantial group of asthmatic children with normal baseline FEV₁ and low MMEF_25-75 are at higher risk for poor asthma outcomes.

Introduction

Spirometry is the “gold standard” for diagnosing and monitoring patients with bronchial asthma [1-6]. Small airways are peripheral non-cartilaginous bronchioles with an internal diameter of less than 2 mm, starting from the eighth generation of airways to the periphery of lung parenchyma [7]. Nevertheless, they play a significant role in small airways obstruction, especially in asthmatic children.

As early as the 1990’s, it has been demonstrated that patients with asymptomatic asthma showed a more than sevenfold increase in small airway resistance, even though spirometry values (FEV₁, FVC% predicted) and plethysmography resistances were normal [8]. Later, Synek et al. documented the presence of inflammation in both large and small airways [9]. Moreover, persistent uncontrolled inflammation in small airways contributes to poor asthma control and clinical course. A systematic review reported that dysfunction of small airways is associated with poor asthma control, frequent exacerbations, presence of asthma symptoms at night, triggered by allergens and exercise exertion, as well as bronchial hyperreactivity [10]. On the other hand, clinical trials have shown that treatment with small particle inhaled corticosteroids reduces the number of exacerbations and improves clinical asthma control [11,12].

According to the ERS/ATS 2005 Taskforce, spirometry results are considered normal at values for FVC≥80% of predicted (or ≥LLN lower limit of normal), FEV₁≥80% from predicted or above LLN and normal FEV₁/FVC ratio [13-19]. Therefore, according to spirometry performance and interpretation, presence of bronchial obstruction is typically characterized by reduced FEV₁ (<80% predicted or below LLN), decreased FEV₁/FVC ratio (Tiffeneau index) and normal FVC (in cases of severe obstruction, it can be even reduced) [18,19].

One of the limitations of FEV₁ is that it does not adequately reflect the presence of small airway dysfunction because it depends predominantly on FVC in patients with asthma due to increased residual volume [20]. Recent studies have demonstrated the presence of significant small airway dysfunction and obstruction in patients with a normal baseline FEV₁ [18,21,22]. Studies in well-controlled asthma indicate the persistence of small airway obstruction and inflammation, regardless of the normal values of the indices reflecting the large airway calibre. Similarly, clinical trials have shown that treatment with small particle inhaled corticosteroids (ICS) reduces the number of exacerbations and improves clinical asthma control [11,12].

Lung function, evaluated in particular with FEV₁, shows a very poor correlation with the severity of asthma and asthma symptoms, demonstrated in studies in adults and children [23-29]. However, FEV₁ proves to be a good predictor of future risk of asthma exacerbations, and FEV₁/FVC ratio is a more sensitive index in defining the severity of bronchial obstruction [25,30]. Moreover, FEV₁/FVC along with FEF_25-75 (FEF_25-75 - Forced expiratory flow, mid-expiratory phase (at 25-75%) or MMEF_25-75 - Maximal mid-expiratory flow at 25-75%) are the most commonly reported indices in children that are characterized by preserved FEV₁, regardless of asthma severity [25,31]. In addition, some studies on FEV₁, FEV₁/FVC and FEF_25-75 significance define FEF_25-75 as a more sensitive indicator of bronchial obstruction in both children and adults [18,32-35]. However, more studies are needed to confirm their significance in asthmatic children.

FEF_25-75 is a less effort-dependent spirometry index than FEV₁ and is traditionally considered to reflect the calibre of small airways [37,38]. On the other hand, FEF₅₀ and FEF_25-75 correlate with expiratory lung CT scan data for pulmonary hyperinflation [39-42].

Our study aims to determine and evaluate the clinical significance of peripheral obstruction indices FEV_25/75, FEV₅₀, FEV₇₅, MMEF_25/75/FVC, and their post-bronchodilator change in asthmatic children with worsened asthma control but normal FEV₁.

Materials and methods

Study subjects and design

We present a prospective observational study which uses epidemiological, instrumental and immunological methods. For a two-years period (October 2013 to December 2015), we enrolled 257 children divided into two groups - the Asthma group (children with asthma, diagnosed by pediatric pulmonologist), n=211 and Healthy control group, n=46.

Of 211 children with asthma, 77 were girls (36%) and 134 boys (64%), at a mean age of 10.1±3.54 years. In the control group, we enrolled age and sex-matched healthy children without a family history of asthma, a personal history of recurrent bronchial obstruction episodes (not more than two episodes in infancy), and no symptoms of allergic rhinitis enrolled. In addition, asthma patients and control group did not differ statistically by height, weight, and BMI. The asthmatic children were enrolled in the study at their worsening of asthma control (outpatient visits in the Clinic) or asthma symptoms exacerbation (inpatient, 166 children; 78.6%).

Before study enrolment, all parents and children over 12 years old signed written informed consent and child assent, according to the Ethics Committee on Scientific Research requirements at Medical University of Sofia. The study includes epidemiological, instrumental, and laboratory (immunological) methods. All used methods, study design, data acquisition and analysis were performed in compliance with protocols approved by the Ethics Committee on Scientific Research at the Medical University of Sofia (ethical approval No. 5/17.04.2013, scientific project identification code 23D/2013).

Epidemiological methods

We have a detailed history of asthma onset and clinical course of comorbidities and control treatment step for the previous eight weeks for all asthma patients.

The level of asthma control at the last four weeks to prior enrollment was assessed by the validated Bulgarian translation of the Asthma Control Questionnaire (ACQ) for children aged 10-18 years and an interview-based version of the questionnaire for children 6-10 years. For the inpatient group, answers to ACQ refer to the four weeks prior asthma exacerbation for outpatients’ group prior to the enrollment visit. ACQ questionnaire and ACQ-IA were provided for use in this study with the express written permission of Prof. Elizabeth Juniper and QOL TECHNOLOGIES Ltd 2003, who owns the copyright for their use [47]. ACQ6 includes six questions reading: night symptoms, severity of the symptoms, the everyday activity limitations, shortness of breath, wheezing and the use of bronchodilator - all in the last seven days before the interview. All of them were answered by the children. In ACQ7, the attending physician adds on the result from FEV₁ before bronchodilator. Each question (including the FEV₁ one) has a 7-point scale (from 0 - no problem/symptoms to 6 - maximal problems/symptom). Each question adds evenly to the final score which is the average number of all - so basically good controlled asthma without symptoms scores 0, loss of asthma control is indicate by score 6.

Asthma severity was determined according to GINA control treatment (Global Initiative for Asthma) step and based on baseline spirometry and extent of asthma symptoms between exacerbations (daytime, nighttime, need for rescue medication, physical activity restriction taken in a 7-degree scale of ACQ) [47,48].

Instrumental methods - investigation of lung function with spirometry

In 175 of 211 enrolled children with asthma and 46 healthy controls we performed a lung function testing (baseline spirometry and reversibility test - BDRT - bronchodilator responsiveness test) according to ATS/ERS 2005 criteria for quality, repeatability, and reproducibility [13,15,16]. In 16 asthmatic children (7.6%), all under the age of 7, the attempt to perform a baseline forced expiratory manoeuvre did not meet quality criteria, another 20 children performed only baseline spirometry without BDRT, and they were excluded from the analysis [12,13,15,16]. The main study groups determination according to recruitment and achievement of technically successful spirometric measurements - baseline and post-BD (BD - bronchodilator) is presented in Figure S1, available in the supplementary file.

All spirometry measurements for the asthmatic group were performed at the Lung function laboratory of the Pediatric Clinic, University Hospital Alexandrovska with Masterscreen Pneumo spirometer ‘98’ (Jager^®, Wuerzburg, Germany). The Lung Function laboratory has a child-friendly environment and a specially trained technician (operator) coached and performed all measurements. Spirometry results were presented as a percentage of the predicted value according to the Zapletal reference equation embedded in the Master screen Pneumo software [46]. Maintained and verified barometer and thermometer were used to calculate the BTPS. All quality control and Standard operating procedures were completed according to the approved locally written protocol and ERS/ATS 2005 recommendations [15,16].

BDRT was performed according to the locally written protocol, following the ERS/ATS 2005 standards and the age of the patients: 15 min after administration of 200 µg (two puffs metered-dose inhaler) Salbutamol (Ventolin) with a spacer or 0.02 ml/kg of the same drug with a nebulizer. In our department, a nebulizer is traditionally used for all inpatients regardless of the severity of asthma exacerbation episode. For outpatients, a metered-dose inhaler was used. Children on bronchodilator therapy were instructed to withhold that medication before baseline testing (at least 4 hours for short-acting β₂-agonist: Salbutamol and 24 hours for long-acting β₂-agonist: formoterol or Salmeterol). BDRT was evaluated by the classical method as a percentage of the change in FEV₁ compared to the baseline measured value and as the absolute change in ml. By ATS/ERS 2005 criteria, BDRT was reported to be significant at ΔFEV₁≥12% (percentage of initial prebronchodilator value, % initial) and/or 200 ml (absolute change (ml) from prebronchodilator value).

Pre- and post-bronchodilator spirometry of children in the control group was performed with a portable Easy One Plus Diagnostic spirometer (ndd Medical Technologies^®) connected to a PC, displaying stimulating animation and plotting curves in real-time in an outpatient setting (GPs practice). Individual disposable mouthpiece (spirettes) was used for each child.

Immunological methods

Atopic status assessment was determined by serological examination of total IgE antibodies by ELISA (EUROIMMUN Total IgE ELISA, Medizinische Labordiagnostica AG). The kit uses indirect sandwich ELISA where the microtiter plate is coated with polyclonal anti-human IgE antibodies. Total IgE concentrations in the samples were measured after photometrical evaluation of the optical density of the enzymatic reactions in the wells at 450/630 nm and via a 4-point calibration curve.

To assess the specific IgE antibodies against aero- and nutritional allergens we used semi-quantitative blot immunoassay - Euroline Allergy Profile Pediatrics, Enzyme Allergo Sorbent Test ((Enzyme Allergo Sorbent Test) EAST) with Euroimmune^® (Medizinische Labordiagnostica, AG, 2014, Germany). The EUROLINE Pediatric (IgE) test kit includes 28 different respiratory and food allergens: gx (grass mix 2 timothy grass, cultivated rye), t3 (birch), w6 (mugwort), d1 (der. Pteronyssinus), d2 (der. Farina), e1 (cat), e2 (dog), e3 (horse), m2 (Cladosporium her.), m3 (Aspergillus fum.), m6 (Alternaria alt.), f1 (egg white), f75 (egg yolk), f2 (cow’s milk), f3 (codfish), f76 (Lactalbumin), f77 (Lactoglobulin), f78 (casein), e204 (bovine serum albumin), f4 (wheat flour), f9 (rice), f14 (soybean), f13 (peanut), f17 (hazelnut), f31 (carrot), f35 (potato), f49 (apple), CCD (CCD marker). The ready test strips were placed on the adhesive foil of the green work protocol prepared beforehand in the EUROLineScan software program, which then calculates the final results of the specific IgE antibodies in patients’ samples by evaluating the intensity of the bands in classes from 0 to 6.

Statistical analysis

Statistical analysis of raw data was performed with SPSS^®, IBM 2009, version 19 (2010), and Excel (v. 2010). The graphical images presenting the statistics are mainly done using Excel and SPSS v.19. descriptive statistics, Kolmogorov-Smirnoff and Shapiro-Wilks, T-tests, Mann-Whitney, ANOVA - post-hoc-analysis, or Kruskal-Wallis test, respectively, χ² or Fisher’s Exact test respectively, correlation analysis, and binary logistic regression analysis were used.

ROC curve analysis was also applied where the best cut-off points were chosen those values that are the least distant from the upper left corner of the coordinate system (coordinates 0; 1) or those with the highest sensitivity + specificity (modified Juden index).

The odds ratio (OR) for case-control study was also calculated.

Significance level α=0.05 was chosen, i.e., for p<α, the null hypothesis was rejected.

Results

Subject’s characteristics

Of 211 children in the Asthma group, 77 were girls (36%) and 134 boys (64%), at a mean age of 10.1±3.54 years- or girls 10.5±3.75 years and boys 9.9±3.41 years. Children were predominantly 7 to 15 years of age, 12.9% of children were preschoolers, and 8% were over 15. The main epidemiological characteristics of asthmatic children are presented in Table 1.

Table 1.

Subject’s characteristics in the Control group and asthma group

Parameter	Control group (N=46)	Asthma group (N=211)	р
Birth sex, male	28 (60.9%)	134 (63.5%)	n.s.
Birth sex, female	18 (39.1)	77 (36.5%)	n.s.
Age, years, mean (SD)	10.26 (2.98)	10.18 (3.54)	n.s.
Hight, cm, mean (SD)	144.75 (16.987)	143.33 (18.660)	n.s.
Weight, kg, mean (SD)	37.75 (13.645)	40.61 (16.661)	n.s.
BMI, mean (SD)	17.34091 (3.90)	18.87008 (3.00)	n.s.

In the healthy control group age and sex-matched healthy children without family history of asthma, without personal history of recurrent bronchial obstruction episodes (not more than two episodes in infancy) and no symptoms of allergic rhinitis were enrolled. The asthma patients and control group did not differ statistically by height, weight, and BMI (Table S1, available in supplementary file).

In the study population, asthmatic children reported an average of 2 exacerbations of asthma in the previous 12 months and an average of one hospitalization and/or need of systemic corticosteroid for more than 3 days. For the previous 12 months, children lost an average of four days of school due to asthma control deterioration. Limitation of physical activity was reported by 28.3%, exercise-induced attacks - 22.3%, and allergen-induced attacks - 19.5% of children (Table 1).

Family history of bronchial asthma was reported in almost half of children - 48.4% (n=103), with 25% (n=53) of children in their immediate family members (mother, father, brothers, or sisters). In 16 children, there was evidence of bronchial asthma in more than one family member.

In 47.9% (n=101) of the children, there was personal history data for atopy (food and/or drug allergy, atopic dermatitis, followed by allergic conjunctivitis, insect allergy, cow’s milk allergy, urticaria with an unspecified causative agent, etc.).

Indicators of baseline spirometry and bronchodilator response differed significantly between children with asthma and healthy controls (Table 2) as well as between children with asthma grouped according to the degree of impaired baseline FEV₁ (>80%, <80%≥70%, <70%≥60% and <60%). A significant difference in BMI was found in the four groups according to baseline FEV₁. Children with the worst baseline spirometry - lowest FEV₁<60% had a higher BMI than children with mild bronchial obstruction (P=0.045).

Table 2.

Indicators of baseline spirometry and bronchodilator response in control and asthma group

Parameter (median)	Control group (N=46)	Asthma group (N=195)	р
FVC	99.8%	91.8%	<0.0001
FEV1	98.5%	85.4%	<0.0001
FEV1/FVC	91.5%	92.1%	<0.0001
PEFR	91.0%	81.7%	<0.0001
MMEF25/75	99.5%	52.3%	<0.0001
D FEV1% init.*	3.0%	14.50%	<0.0001
D FEV1 abs., l*	0.050	0.216	0.004

^*asthma BDR group (n=175).

Asthma control as determined by ACQ6 or GINA score does not differ statistically between boys and girls (Table 3).

Table 3.

Distribution of patients (total, girls, boys) according to ACQ6 and GINA asthma control

Asthma control	total N (%)	boys N (%)	girls N (%)	р
ACQ6 score				n.s.
Under 0.75	39 (35.8%)	27 (38.6%)	12 (30.8%)
0.75-1.5	16 (14.7%)	6 (14.3%)	10 (15.4%)
Above 1.5	54 (49.5%)	33 (47.1%)	21 (53.8%)
GINA score				n.s.
No one	29 (26.6%)	11(28.2%)	18 (25.7%)
1-2	19 (23.9%)	7 (17.9%)	26 (27.1%)
3-4	33 (49.5%)	21 (53.8%)	54 (47.1%)

Concomitant allergic rhinitis, diagnosed by specialist (seasonal/year-round), is present in 54.9% (n=116) of children (59% of boys and 48% of girls), more often in school-age children (P=0.003).

One hundred twenty-four children (59%) were CS naïve. They had not received systemic control treatment in the previous eight weeks. Of these, 45 (36%) was children with newly diagnosed bronchial asthma - they were treated with short-acting beta-agonist (Salbutamol) with/without systemic CS in more severe wheezing episodes prior the asthma diagnosis. The remaining 78 (64%) of the CS naïve children are on ICS when needed regimen, or were enrolled in the study during the control treatment break for the summer period. Of the 87 children (41%) with systemic control treatment, 21 (24%) were on monotherapy with leukotriene receptor antagonists (LTRA), 42 (48%) were on ICS (Budesonide or Fluticasone propionate), 10 (12%) were on a combination of ICS with LTRA, 11 (13%) on combined inhaler (ICS with LABA - Budesonide/Formoterol or Fluticasone Propionate/Salmeterol) and three children (3%) on combined inhaler plus LTRA. There was no statistically significant difference in the control treatment between girls and boys.

We found a significant discrepancy in the classification of asthmatic children by severity of the asthma using the three methods (baseline FEV₁ (spirometry), control treatment step and clinical control (ACQ)). The severity of asthma, defined by the control treatment and based on symptoms, was significantly lower than that based on spirometry results (Table S2, available in supplementary file).

Baseline spirometry indices

Baseline spirometry indices were conventionally divided into two groups - those reflecting calibre/function predominantly of large airways, namely FEV₁ and PEFR, and those reflecting calibre/function predominantly of small airways, respectively - MMEF_25/75 and MMEF₇₅. We found that FEV₁/FVC ratio correlated better with the indices reflecting the small airways (P<0.001) at Spearman’s rho =0.746 for MMEF_25/75; 0.680 for MMEF₇₅ and 0.848 for MMEF_25/75/FVC, while for PEFR the correlation coefficient was 0.438 and for FEV₁ 0.571, respectively.

We also found that indexes FEV₁ and PEFR reflect large airway calibre in contrast with the MMEF_25/75, MMEF₇₅, MMEF₅₀, FEV₁/FVC which predominantly reflect small airways. The mean BDR (% change) of the FEV₁ is 17.78% (≥13,9%), PEFR - 16.06% (≥17,69), FVC - 8.1% (≥12.9%) and for the MMEF_25/75 - 57.65 (≥76,5).

According to baseline spirometry, we divided asthmatic children into two main groups: children with normal lung function (“normal” FEV₁≥80%) and impaired lung function (“abnormal” FEV₁<80%).

Children with normal FEV₁≥80% were divided into 4 subgroups, depending on baseline FEV₁/FVC and baseline MMEF_25/75: group A - children with “normal function” - (with normal FEV₁/FVC≥85%, normal MMEF_25/75≥65%), group B - children with “low Tiffeneau” (FEV₁/FVC<85%, MMEF_25/75≥65%), group D - children with “low Tiffeneau and abnormal MMEF_25/75” (FEV₁/FVC<85%, MMEF_25/75<65%), group C - children with isolated “low MMEF_25/75” (FEV₁/FVC≥85%, MMEF_25/75<65%) [17] (Figure S2, available in supplementary file).

The group of children with Small Airways Obstruction (SAO) is defined as the cases with normal baseline FEV₁ (≥80%) and MMEF_25-75 (<65%), regardless of baseline Tiffeneau index (FEV₁/FVC).

In patients with bronchial obstruction (FEV₁<80%), with an increase in obstruction and the FEV₁% predicted reduction, FEV₁/FVC, PEFR and MMEF_25/75 decreased proportionally (Figure 1).

Figure 1.

Box-plot main spirometry indices in different severity of bronchial obstruction (FEV₁ pre(BD) values). A. Decrease in Tiffeneau index (FEV₁/FVC) with increased severity of bronchial obstruction in asthmatic children; B. Decrease in MMEF_25-75 with increased severity of bronchial obstruction in asthmatic children; C. Decrease in PEF with increased severity of bronchial obstruction in asthmatic children.

Small airway obstruction - clinical value of the baseline maximal mid-expiratory flows - MMEF_25/75, MMEF₅₀, MMEF₇₅

When grouping children with normal baseline FEV₁≥80% in three groups (group A, B, D in Figure S2, available in supplementary file), the presence of peripheral obstruction was found to increase the likelihood of exercise attacks 5.08 times (OR 5.079, 95% CI 1.461-17.653). No significant difference (OR; odds ratio; case-control study) was found in testing the other risks (hospitalization, exacerbation, physical activity limitation, allergic contact attacks), as well as the risk domain.

Adding a fourth group “C” - children with an isolated “low MMEF_25/75” and normal Tiffeneau index, we found that hospitalization was a protective factor for “normal” baseline lung function (OR 0.449, 95% CI 0.206-0.978).

Children with peripheral obstruction, a so-called SAO group (FEV₁≥80%, MMEF_25/75<65%), had significantly lower FEV₁, FEV₁/FVC, PEFR, MMME_25/75, and significantly higher BDR (ΔFEV₁% initial and ΔMMEF_75/25% init.) compared to those with normal function (FEV₁>80%, MMEF_25/75≥65%) (P<0.0001). There was no significant difference in morbidity (risk and risk domain) - hospitalizations, exacerbations in the previous year, exercise and allergen induced exacerbations, restriction in physical activity, BMI difference and presence of allergic rhinitis between the two groups (Table S3, available in supplementary file).

We compared the distribution of asthmatic children with reduced MMEF_25-75 (<65%) in the groups of children with normal and those with reduced FEV₁. A significant difference was found between both groups - 55.9% of children with normal FEV₁ and 98.5% of children with reduced FEV₁ had peripheral (small airways) obstruction (MMEF_25-75<65%) (Table S4, available in supplementary file).

Children with small airway obstruction (SAO, FEV₁≥80%, and MMEF_25/75<65%) have been found to possess a 2.338-fold higher risk of development of one of the risk domain elements (OR 95% CI 1.077-5.294). Children in this group had 3.736-fold (OR 95% CI 1.007-13.860) higher risk to demonstrate a decrease in FEV₁/FVC<85% and 3.857-fold (OR 95% CI 1.518-9.801) higher probability of a low PEF<80%. It was also found that this group of children had a 5.9-fold (OR 95% CI 2.487-13.998) greater probability to demonstrate positive BDR for MMEF_25/75 and 6.171-fold (OR 95% CI 2.523-15.096) - for FEV₁ over 12%, compared to children with normal baseline FEV₁≥80%, without small airway obstruction (MMEF_25-75≥65%).

Analyzed with binary logistic regression, the results showed a similar trend. It was confirmed that at MMEF_25-75<65% (peripheral obstruction regardless of baseline FEV₁), the risk of occurrence of a risk domain element was increased 2.27 times (HR 95% CI 1.120-4.603) (Table S5, available in supplementary file).

Among asthmatic patients with “normal” lung function (FEV₁≥80%) we compared asthma control (ACQ6 and ACQ7 score) in the sub-groups of children with and without peripheral obstruction (normal and low MMEF_25-75). The results indicated no difference in asthma control according to the ACQ6/ACQ7 score between the two groups (borderline significance, p=0.051). This may be due to the small sample size (low number of patients with normal or low MMEF_25-75 in the subgroups with normal baseline spirometry, according to pre-FEV₁). There was a tendency for children with peripheral obstruction to have poor asthma control.

We defined the so-called “risk domain” - number of hospitalizations for the previous year (≥1), exercise-induced exacerbation (≥1) for the previous year, history of allergen-induced exacerbation, school absenteeism due to asthma in the last 12 months (≥5), lack of asthma control (ACQ score ≥1.5), atopy (elevated total IgE titer according to the age and/or positive specific IgE), concomitant obesity and/or allergic rhinitis (AR), diagnosed by otorhinolaryngologic, very low FEV₁<60%.

Flow volume loop’s shape - clinical value of the baseline and post-bronchodilator forced expiratory decay (MMEF_25-75/FVC index)

We calculated the pre- and post-bronchodilator MMEF_25-75/FVC index (Forced expiratory decay, informative of the flow volume loop’s shape) in the group of children with asthma and in the healthy controls group. At a ratio approximately equal to 1.0, the shape of the loop was linear (normal), and in the ratio <1 the shape was obstructive (concave). There was a statistically significant difference between the median MMEF_25-75/FVC of healthy children and those with asthma (P<0.0001). In children with asthma, the curve was markedly obstructive regardless of the value of FEV₁ (Table S6, available in supplementary file and Figure 2).

Figure 2.

Box-plot index MMEF_25-75/FVC (Forced expiratory decay) in healthy controls and children with asthma (median, IQR).

Тhere was a significant difference in the pre- and post-bronchodilator shape of the flow-volume loop (MMEF_25-75/FVC value) in the group of children with asthma (P≤0.0001). Even post- bronchodilator, the shape of the curve in children with asthma retains its markedly obstructive character (MMEF_25-75/FVC is below 1.0-0.75, IQR 0.57-0.91) (Table S7, available in supplementary file).

Evaluation of the MMEF_25/75 as a predictive index for positive BDR

ROC curve method was used to evaluate sensitivity and specificity of baseline spirometry predictors for a positive BDR (standard criteria ≥FEV₁% init. ≥12%). MMEF_25/75 was found to be the strongest predictive index with an area under the curve 0.843 AUC (CI 0.781-0.845). In both of the best cut-off values for MMEF_25-75, combining the best sensitivity and specificity (77.8% or 78.8%) for the specific patient population, was determined 58.1%, which is close to that reported in the literature (60%, 65%). A threshold for MMEF_25/75<65% shows a high sensitivity of 82.7% but a lower specificity of 54.5%.

The AUC for FEV₁% pred. was 0.796 (95% CI 0.728-0.863, P<0.001), for FVC% pred. was estimated as 0.650 (95% CI 0.568-0.733, P=0.001), for PEF% pred. - 0.666 (95% CI 0.585-0.748, P<0.001), MMEF_75/25% pred. - 0.843 (95% CI 0.781-0.905, P<0.001), and for FEV₁/FVC% - 0.773 (95% CI 0.701-0.845, P<0.001). The data are presented on Figure 3.

Figure 3.

ROC curve for the predictive value of baseline spirometry to reveal a positive BDRT (ΔFEV₁% init. ≥12%).

Discussion

We confirmed that pulmonary function, in particular presented as FEV₁% pred., showed very poor correlation with asthma severity and symptoms, which has been demonstrated in several studies in both adults and children [24-26,27,49,50]. However, baseline FEV₁% pred. is proving to be a good predictor of the future risk of exacerbations [30]. The FEV₁/FVC ratio (Tiffeneau index) is more sensitive when defining the severity of bronchial obstruction [25]. In addition, FEV₁/FVC, along with FEF_25-75, are the most commonly affected indicators of childhood asthma characterized by stored FEV₁, regardless of the severity of the disease [25,51].

We agree with the published strong evidence that underestimating the severity of asthma leads to suboptimal treatment and impaired quality of life [52,53]. For the purpose of the study, we defined the severity of asthma according to three criteria: according to the stage of GINA control treatment, according to the baseline spirometry, and according to the degree of asthma symptoms outside the attack (daytime, nighttime, use of rescue medication, restriction in physical activity taken from the 7-point ACQ scale). We found a significant discrepancy in the classification of children by severity using the three methods. Asthma severity, defined by the stage of control treatment and based on symptoms (ACQ and GINA score), is significantly lower than that based on spirometry results. Our results are consistent with the literature data on the lack of association between symptom severity, medication intensity, and FEV₁% of predicted values [25]. Nair et al. also demonstrate that the use of spirometry identifies a large percentage of children with abnormal pulmonary function who have been evaluated as having mild asthma according to history and physical examination [54]. Schifano et al. investigate the concordance between spirometry and asthma symptoms when assessing asthma severity and initiating control treatment in children. The results of their study show that in 36% of the children tested, the severity of asthma determined by the basis of symptoms, was lower than that based on the results of spirometry [26]. Ming-Sheng Lee et al. also found a weak correlation between lung function and the level of symptoms assessed by ACT/C-ACT (asthma control test, childhood asthma control test) [55]. According to the authors, pulmonary function and symptoms at the age of 5-11 years have different implications for asthma control, with Lung function tests (LFT) reflecting the condition of the respiratory tract on the day of the study, whereas asthma control questionnaires (ACQ/АCT) provide information about the patient’s symptoms for the previous month/week. Our results also confirm that the combination of LFT and ACT/ACQ allows the detection of more patients with inadequate asthma control, therefore we strongly advocate this method to be included in everyday practice of pediatric pulmonologists.

It was noted that more than half of studied children (65%, n=124) showed a normal baseline FEV₁≥80% during exacerbation or poor asthma control. Our results confirmed the results of a number of studies in children, in which baseline FEV₁ and spirometry in general show low sensitivity in detection of bronchial obstruction, and 80% of asthmatic attacks occur in children with normal FEV₁ [56]. Restriction on use of FEV₁ alone in pediatric asthma is demonstrated and by Bacharier et al., with a lack of association between symptom severity, drug intensity, and FEV₁% of predicted value [25]. Consistent with literature, our results suggest that making a clinical diagnosis based on a single measured value of baseline FEV₁ may underestimate the diagnosis, severity, and choice of control treatment [56,57].

When dividing baseline spirometry indices from those reflecting caliber/function mainly of large airways (FEV₁ and PEFR - Peak expiratory flow rate) and those reflecting caliber/function mainly of small airways (MMEF_25/75 and MMEF₇₅), we found a stronger correlation of the Tiffeneau index (FEV₁/FVC) with indicators reflecting small airways. Francisco et al. demonstrate that indicators reflecting the caliber of small airways (FEF_25-75, FEF₅₀, FEF₇₅) in childhood are more sensitive in detecting bronchial obstruction than those reflecting large airways (FEV₁ and PEFR), and FEV₁/FVC correlates better with small lung volumes [58]. Similar to our observations, Vilozni et al. found low sensitivity of the most commonly used parameters (FEV₁, FEV_0.5, FVC and PEFR) in the detection of bronchial obstruction compared to mean FEF_25/75% and FEF₅₀% flow rates [59].

We found that children with peripheral obstruction (SAO, FEV₁>80%, and MMEF_25/75<65%, regardless of baseline FEV₁/FVC) had a 2.388-fold greater risk of developing any of the risk domain elements (OR 95% CI 1.077-5.294). The same result was obtained with the logistic regression method (HR 2.27 95% CI 1.120-4.603). Children in this group are 5.9 times (OR 95% CI 2.487-13.998) more likely to be positive BDR for MMEF_25/75 and target 6.171 times (OR 95% CI 2.523-15.096) for ΔFEV₁ above 12% compared to children with normal baseline FEV₁≥80% without peripheral obstruction (MMEF_25-75≥65%). We have also shown a tendency of poor asthma control in children with peripheral obstruction.

Several studies have linked small airways function to asthma symptoms [60]. Recently, Schiphof-Godart et al. selected patients with SAO based on FEF₅₀ from spirometry and R5-R20 from IOS and found that patients with SAO have more frequent symptoms induced by physical exertion, allergen contact and climate change [61]. Sirux V et al. found that reduced baseline FEF_25-75 levels increased the risk of long-term asthma persistence and more severe BHR (bronchial hyperreactivity), regardless of FEV₁ levels, i.e., regardless of the effect of large airways [62]. According to Kanchongkittiphon et al. results, 79% of children with persistent asthma have normal FEV₁≥80% and 63% normal FEV₁/FVC≥80%, and a low FEF_25-75 provides additional information on asthma control regardless of FEV₁ and FEV₁/FVC [63]. Using logistic regression, Gibb et al. found that FEF_25-75<60% was associated with a 2.50-fold higher likelihood of hospitalization in the previous year (OR 2.50, CI 1.17-5.35) than FEF_25-75≥60%, again regardless of baseline FEV₁ [64].

Tosca et al. demonstrate a correlation between FEF_25-75, sIgE for house dust, and FeNO in children with AP and/or asthma and suggest a likely direct link between the markers of allergic inflammation and SAO [65]. The inversely proportional relationship between FEF_25-75 (≤65%) and FeNO in children with asthma, as well as the significance of FEF_25-75 as an indirect marker of inflammation in airways has been confirmed by other authors [66-68]. Similar to the results of Tosca et al., we found a significantly higher percentage of children sensitized to microarrays in house dust (D. farinae and D. pteronyssimus) in the peripheral obstruction (SAO) group compared to normal function children, P=0.037.

The MFVL (Maximal flow volume loop) evaluation of spirometry interpretation process also includes visual inspection of the shape of the curve. An objective indicator providing information on the form of an MPC is the FEF_25-75/FVC ratio (a surrogate marker for the ratio of airway size to lung size) [58,59,69,70]. Vilosny et al. examined the form of MFVL (FEF_25-75/FVC) and found that the contour of the curve in children differs significantly from that in adults and may appear “normal” in children with mild obstruction [59]. Our results showed a statistically significant difference between the MMEF_25-75/FVC index of healthy children and children with asthma (P<0.0001). As expected in patients with asthma, the curve is markedly obstructive, regardless of the baseline FEV₁ value. A significant difference in MFVL shape (MMEF_25-75/FVC) was also found in the asthma group in response to administration of a bronchodilator (P<0.0001), which, however, maintained its markedly obstructive nature (MMEF_25-75/FVC below 0.75 IQR [0.57-0.91]).

A significant difference was observed in mean values of baseline spirometry, BDR (ΔFEV₁ and ΔMMEF_25-75) and MMEF_25-75/FVC in the group of peripheral obstruction (SAO) and normal function children. Children with SAO had significantly lower values of all studied indices and higher BDR (P<0.0001).

There is a great interest in BDR in children with normal baseline spirometry. A cut-off between 60% and 70% for FEF_25-75 is best in predicting positive BDR (ΔFEV₁≥12%) with normal baseline FEV₁≥80% [71]. According to Simon et al. FEF_25-75<68% showed 95% sensitivity and 63% specificity for predicting a 20% increase in FEV₁ after Albuterol inhalation [31]. In our sensitivity and specificity analysis of the main indicators of baseline spirometry for prediction of positive BDR by classical criteria (ΔFEV₁% init. ≥12%) we obtained similar results. The highest diagnostic power (AUC 0.843 CI 0.781-0.845) demonstrated MMEF_25/75 with the best cut-off combining maximum sensitivity and specificity (77.8%, or 78.8%) below 58.1%, which is close to the lower limit of reference value for this indicator reported by literature data (60%, 65%) [32,72]. MMEF_25/75 threshold <65% in the study population showed a high sensitivity of 82.7% but a lower specificity of 54.5%.

Assessment of small airways condition using spirometry has its advantages and disadvantages over the gold standard IOS and requires careful attention in the interpretation process. Use of FEF_25-75% in adults is not recommended because of high indicator variability in healthy subjects, whereas in childhood it is more widely used. FEF_25-75% is an indicator of high physiological sensitivity for predicting bronchial reversibility [32]. In childhood, FEF_25-75% provides additional information on clinical status and inflammation of PD, correlates well with BD in patients with normal baseline FEV₁ and is associated with morbidity and severity of pediatric asthma [71].

In addition to its high variability, another drawback of the FEF_25-75 indicator is the lack of consensus on its normal value [73]. Ciprandi et al. propose a threshold for “normal” FEF_25-75, showing that 45% of children with mild asthma have a value for FEF_25-75 below 65% of what is predicted. Simon et al. in the CAREN (Childhood Asthma Research and Education Network), using the ROC method, find that a threshold of 68% for FEF_25-75 may predict positive BDR (20% change in FEV₁) in patients with mild asthma and normal FEV₁ [32]. Mandadzhieva et al. found that healthy children exposed to secondhand smoke had lower values for FEF₅₀ and FEF₇₅, suggesting that initial chronic inflammation with this localization would lead to peripheral obstruction [74].

According to a recent large multinational and multicenter study by Quanjer et al., values of FEF_25-75% and FEF₇₅% below the LNN (<-1.645 z-score, according to the GLI 2012 reference equation) are only found in 2.75% and 1.29% respectively, with FEV₁, FVC and FEV₁/FVC within the reference range (above LLN). According to the same study, FEF_25-75% miss bronchial obstruction in 2.9% of cases and FEF₇₅% in 12.3% of cases. The authors conclude that indicators reflecting maximum flow rates in the middle part of the expiratory flow-volume loop do not provide additional information to those of FEV₁, FVC, and FEV₁/FVC when making clinical decisions [75].

In our real-life study, the analyzed values of spirometry indices are calculated as percentages of the predicted Zapletal reference equation, which is traditionally used in the Bulgarian population of children. The use of the GLI 2012 reference equation and the z-score method in the interpretation of spirometry has not yet been widely adopted and accepted in the daily practice of pediatric pulmonologists in Bulgaria. When working with older reference equations, and especially when using a fixed cut-off for LNN, the FEF_25-75% indicator provides valuable clinical information in children with asthma and normal baseline FEV₁.

Conclusions

Based on literature references and the results we have obtained, we can conclude that there is a small but essential group of asthmatic children with a normal baseline FEV₁ and an abnormal MMEF_25-75. Children in this group are at an increased risk of adverse outcome (exacerbations, hospitalizations, progressive reduction of pulmonary function, persistent BDR) and may need to undergo a higher step of control treatment after careful assessment of adherence to the therapeutic plan and evaluation of inhaler technique.

In children with asthma and normal baseline FEV₁ and Tiffeneau index (<LLN), MMEF_25-75 may be considered a marker that predicts BDR positivity (delta FEV₁% init.), asthma control severity, and risk of exacerbations, physical activity attacks, both in scientific research and in clinical practice. However, average debits and their formal inclusion in official guidelines remain limited due to their high variability. Baseline spirometry and Asthma Control Assessment Questionnaire (ACQ) correlate poorly, but administered in combination may better identify children at risk for loss of control, exacerbation, and progressive pulmonary function impairment.

Disclosure of conflict of interest

None.