Detection of Small Airway Dysfunction in Asthmatic Patients by Spirometry and Impulse Oscillometry System

The small airways are defined as the airways with an internal diameter less or equal than 2 mm in diameter with significant differences from large airways: they have no cartilaginous supports, mucous glands, and they are lined by surfactant, which reduces surface tension and helps prevent them from closing on expiration at low lung volumes [1]. The small airways are the major site of airflow obstruction both in asthma and chronic obstructive pulmonary disease, where they are early involved in the course of these diseases [2]. In asthmatic patients, chronic inflammatory infiltrate consisting of eosinophils, T-lymphocytes, neutrophils, and macrophages may frequently occur in the small airways [3‒5]. Additionally, longitudinal data between airway inflammation types and SAD could already be established even in patients with stable FEV1 [6]. This persistent inflammation leads to small airways disfunction (SAD), that contributes to a more severe bronchial hyper-responsiveness, worse disease control, and a higher number of exacerbations [7].

Spirometry is the most used lung function test to diagnose and stratify severity of lung disease, and forced expiratory volume in 1 s (FEV₁) is used to assess the severity of airway obstruction. However, FEV₁ mainly reflects large airway obstructions, and a significant amount of SAD must accumulate before FEV₁ becomes abnormal. Among spirometry parameters, forced expiratory flow between 25 and 75% (FEF_25-75) of the forced vital capacity (FVC) is one of the most used measures to assess SAD [8‒10]. Recently, in a large cohort of subjects, Xiao et al. [11] used three indicators of spirometry to assess SAD, namely FEF_25-75, forced expiratory flow at 50% (FEF₅₀), and at 75% (FEF₇₅) of FVC; SAD was diagnosed when at least two of these three indicators were less than 65% of predicted values.

Spirometry provides additional parameters of SAD. Since the last part of forced exhalation better reflects smaller airway patency, its measurement may be more sensitive to intercept the decrease in terminal expiratory flows [12]. It is of note that the ratio of forced expiratory volume in 3 and in 6 s (FEV₃/FEV₆) less than lower limit of normal (LLN) was effective to detect a distinct population of current or ex-smokers with evidence of SAD at the quantitative CT scanning [13]. Accordingly, FEV₃/FEV₆ may be considered a reliable tool to assess SAD.

In clinical practice, besides spirometry, the Impulse Oscillometry System (IOS) is used to assess lung function. IOS measures the lungs’ mechanical properties during quiet tidal breathing by the application of minimal pressures at the mouth [14]. The fall in resistance from 5 Hz to 20 Hz (R5–R20) is considered an IOS measure of the small airway resistances [14, 15]. Interestingly, in asthmatic patients with normal FEV₁ values, R5–R20 detected SAD, which was related to poor disease control and a history of asthma exacerbations [16]. Ventilation heterogeneity is also a sign of small airway dysfunction that can be assessed using, e.g., washout tests [17] or CT scans [18].

In spite of the importance to detect SAD, the measurement of small airway function is challenging because of their relative inaccessibility and currently makes use of different techniques, by lacking a gold standard. In addition, spirometry and IOS parameters to assess SAD are based on different maneuvers, i.e., forced maximal expiration versus tidal breathing, and it is not yet known whether they provide complementary or supplementary information on SAD. The aim of the present study is, therefore, to ascertain in a cohort of asthmatic patients, whether spirometry and IOS indicators agree to detect SAD, acting as supplementary tools. Moreover, we assessed the relationship between spirometry and IOS indicators of SAD and clinical characteristics of the patients.

This was an observational cross-sectional single-center study, conducted in a 1-day visit between January 2018 and December 2021 at the Asthma Outpatient Clinic of the Respiratory and Lung Function Unit of University Hospital of Parma. We prospectively recruited patients with age ≥18 years, affected by asthma diagnosed according to the GINA criteria [19]. The exclusion criteria were patients unable to perform pulmonary functional tests, either patients with severe comorbidities or with other coexisting lung diseases.

Age, sex, smoking habit, BMI (kg/m²), current therapy, allergy, duration of diagnosis, and number of asthma exacerbations in the past year were recorded. The disease control was assessed by using the Italian version of the Asthma Control Test (ACT) [20]. Patients subjectively evaluated the degree of impairment caused by their disease during the preceding 4 weeks by responding to five questions using a five-point scale. The sum of the scores of the five questions gave the total ACT score (range 0–25). A cut-off score of 19 or less identifies patients with poorly controlled asthma. All patients performed pulmonary function tests, including spirometry and IOS measurements. The study protocol was approved by the Ethics Committee for the Province of Parma, Italy, and was conducted in accordance with Good Clinical Practices and the Declaration of Helsinki.

Spirometry was performed by using a flow-sensing spirometer connected to a computer for data analysis (V_max 22 and 6,200; SensorMedics, Yorba Linda, CA, USA) according to the international guidelines [21]. We selected spirometry maneuvers with largest sum of FVC + FEV₁ to determine other indices [21]. FEV₁, FVC, FEF_25-75, FEF₅₀ and FEF₇₅ were recorded and expressed as absolute values (liters) and as a percentage of predicted value (% pred.) [22]. The presence of at least two of three forced expiratory flows, i.e., FEF_25-75, FEF₅₀, and FEF_75, less than 65% of predicted values (FEF+) was considered as index of SAD [11].

The FEV₁/FVC and FEV₃/FEV₆ values were recorded and expressed as a ratio. The lower limit of normal for FEV₃/FEV₆ values was calculated according to the equations of Hansen et al. [23]. A FEV₃/FEV₆ value less than the lower limit of normal (FEV₃/FEV₆+) was considered index of SAD [13].

IOS was performed by using the Jaeger MasterScreen-IOS (CareFusion Technologies; San Diego, CA, USA), following standard recommendations [14]. All subjects underwent IOS before spirometry because forced expiration maneuver might change per se airway tone. Briefly, patients were asked to wear a nose clip and were seated during tidal breathing with their necks slightly extended and their lips sealed tightly around the mouthpiece while firmly supporting their cheeks with their hands. At least three trials, each lasting 30 s, were performed, and mean values were taken for each value. According to reference values, a conservative upper limit of normal for R5–R20 was chosen at 0.070 kPa s·L⁻¹ to define the presence of SAD [9, 24, 25] (R5-R20+). The 5 Hz reactance (X5, kPa·s·L⁻¹) and reactance area (AX, kPa·L⁻¹) were also measured because these are considered a hallmark of peripheral airway dysfunction [9, 14].

A Shapiro-Wilk test was used to assess the distribution of the variables. Data were reported as mean ± standard deviation (SD) for the variables with normal distribution and as median [25th–75th percentile] for those with a nonnormal distribution.

Unpaired t test, Mann-Whitney test, and Pearson’s χ² test were used for comparisons when appropriate. Relationships between variables were assessed by Pearson correlation coefficient (r) or by Spearman’s rank correlation coefficient (ρ), depending on distribution. Effect size was interpreted according to Cohen’s conventions: the effect size was low if the value of r was around 0.1, medium if r was around 0.3 and large if r was more than 0.5 [26]. Linear regression analysis was carried out for values reporting significant correlation.

To test the interrater agreement, Cohen’s Kappa (κ) was calculated [27]. κ <0.00 indicates “poor,” 0.00≤ κ ≤0.20 “slight,” 0.21≤ κ ≤0.40 “fair,” 0.41≤ κ ≤0.60 “moderate,” 0.61≤ κ ≤0.80 “substantial,” and 0.81≤ κ ≤1.00 “almost perfect” agreement. A p value <0.05 was considered significant.

We enrolled 301 asthmatic patients (179 females) with age range 18–85 years. In all patients, severity of airway obstruction according to FEV₁ and FEV₁/FVC ranged from 40 to 140% of predicted and from 44 to 91%, respectively. The anthropometric, clinical, and functional characteristics of patients are listed in Table 1. Spirometric and IOS parameters of SAD were strongly related, and relationships are reported in Table 2.

One hundred and eighty-eight (62%) out of 301 patients had at least two of three forced expiratory flows, i.e., FEF_25-75, FEF₅₀, and FEF_75, less than 65% of predicted values (FEF+). In addition, 121 (40%) and 123 (41%) out of 301 patients had a FEV₃/FEV₆ value less than the lower limit of normal (FEV₃/FEV₆+) and a R5-R20 value greater than the upper limit of normal (R5-R20+), respectively.

When the agreement between FEF+, FEV₃/FEV₆+, and R5-R20+ was assessed, we found that κ values were 0.20, 0.49, and 0.07 between FEF+ and R5-R20+, FEF+ and FEV₃/FEV₆+, and FEV₃/FEV₆+ and R5-R20+, respectively (Fig. 1). Accordingly, the relative strength of agreement associated with κ statistics was slight between FEF+, FEV₃/FEV₆+, and R5-R20+ and moderate between FEF+ and FEV₃/FEV₆+.

Fifty-four (18%) out of 301 patients had poorly controlled asthma (ACT score ≤19). When we compared the percentage of R5-R20+ with poorly controlled asthma to that of R5-R20+ with well controlled asthma, we found a significant difference (28% vs. 12%; χ² = 12.01, p < 0.001) (Fig. 2). No difference was found when we compared the percentages of FEV+ and FEV₃/FEV₆+ with poorly controlled asthma to those of FEV+ and FEV₃/FEV₆+ with well-controlled asthma. In Table 3, correlations between ACT score and spirometric and IOS values are listed.

In the present study, we enrolled 301 adult asthmatic patients, whose anthropometric, clinical, and functional characteristics were recorded. The degree of airway obstruction ranged from normal to moderately severe, and SAD was diagnosed in 62% of patients through FEF_25-75, FEF₅₀, and FEF₇₅ measures; in 40% with FEV₃/FEV₆ values; and in 41% through IOS parameters, such as R5-R20. As expected, we found a correlation between spirometry and IOS parameters in assessing SAD. Surprisingly, when agreement among variables was analyzed, we observed only a slight concordance between spirometry and IOS indicators of SAD, probably due to different execution techniques. Finally, in line with previous results [28, 29], our findings showed that IOS values were significantly related to asthma control, unlike spirometric measures.

Diagnosis of SAD is not based on a single and standardized method and cannot be restricted to a single technique or parameter [30]. Multiple tests are available for assessment of SAD, including functional tests such as spirometry, impulse oscillometry, single and multiple breath washout; washout testing do not include only nitrogen washout, as this is associated with some methodological problems (e.g., NO back diffusion and measurement times) [31]. Regarding imaging techniques useful in assessing SAD, both HRCT and quantitative computed tomography (QTC) are important for differentiation of, e.g., functional small airway disease and emphysema [32]. Finally, xenon magnetic resonance imaging (MRI) has been demonstrated to be useful for understanding the underlying pathophysiology [33]. Moreover, invasive procedures like surgical biopsy and transbronchial biopsy can be performed when non-invasive procedures are not satisfactory in achieving the diagnosis [34].

Spirometry is the commonly used test to assess SAD, even if it is not completely reliable. The FEF_25-75 is the referring measure of small airways, as it examines the middle portion of expiratory flow of non-cartilaginous airways, that usually collapse during forced expiration. FEF_25-75 values are suggestive of SAD when lower than 60% or LLN [8, 35]. However, FEF_25-75 is dependent on FVC and is a poorly reproducible measure, especially when FVC is abnormal [34]. Due to the unreliability of a single parameter, Xiao et al. [11] proposed a diagnosis of SAD based on three indicators of small airways, FEF_25-75, FEF 50%, and FEF 75%; when two of three parameters are lower than 65% of predicted, diagnosis of SAD can be established. FEV₃/FEV₆ seems to be a reliable measure to evaluate SAD in adults, superior to the FEV₁/FEV₆, FEV₃/FVC, and FEV₁/FVC [36]. FEV₃/FEV₆ is considered impaired when its values are lower than LLN. Anyway, consensus regarding appropriate spirometry parameters to diagnose SAD has not been achieved [35]. IOS, applying oscillating pressure variation to the respiratory system, is used to assess lung function and SAD; in particular, the fall in resistance from 5 Hz to 20 Hz (R5–R20) in kPa·s·L⁻¹ is considered a measure of the small airway resistances [14, 15].

Our results, showing a correlation between spirometric values (FEF_25-75, FEF₇₅, FEF₅₀, FEV₃/FEV₆) and IOS parameters (R5-R20) in detecting SAD, are in line with literature. Chiu et al. [37] demonstrated that IOS can be a helpful tool, along with spirometry, in evaluating SAD. Cottini et al. [38] found a strong correlation between R5-R20 and FEF_25-75, only in GINA step 5 asthmatic patients, while in GINA steps 1–4, an inverse but non-significant correlation was detected.

The original finding of our results was that, when agreements among parameters were evaluated, we observed a moderate agreement between spirometric measures (FEF+ and FEV₃/FEV₆+) but only a slight agreement between spirometric and IOS measures (FEF+ and R5-R20+, FEV₃/FEV₆+ and R5-R20+). To our knowledge, this is the first study that evaluated the agreements between these two different techniques in asthmatic patients. A recent study, by Lu et al. [39], found similar results about slight agreement between spirometry and IOS in assessing SAD but the study involved a large cohort of COPD patients.

The weak concordance between the two tests could be due to different maneuvers required for each test; spirometry demands a forced expiratory effort and implies the elastic recoil of the lungs, while IOS requires a tidal and more physiologic breathing without any effort or involvement of pulmonary elastic recoil. For this reason, both spirometry and IOS are useful and complementary evaluations in diagnosing SAD. However, the differences in forced versus tidal breathing maneuvers do not completely explain the weak concordance, which may be attributable to the low sensitivity of spirometry for diagnosing SAD.

Regarding asthma control determined with ACT, in our study a significant relationship was found between IOS values but not spirometry values and asthma control scores, with R5-R20+ in poor controlled asthma group, suggesting a greater sensitivity of IOS than spirometry in detecting the disease control. Takeda et al. [40] observed similar results, showing that IOS correlated better with clinical symptoms than spirometry in asthmatic patients. Furthermore, in patients with moderate to severe persistent asthma, Jabbal et al. [41] found that IOS measurements of R5-R20, AX, and resonant frequency, but none of the spirometry measurements, were significantly different in terms of worse disease control. Our findings are also in line with the results observed by Cottini et al. [42], who pointed out that the combination of FEF_25–75 and R5–R20 was a valid predictor of poor asthma control. By contrast, Manoharan et al. [43] defined IOS and spirometry as equivalent measurements in evaluating asthma control; however, they recorded oral steroid and short-acting beta-agonist use as surrogates for long-term asthma control.

The main limitation of our study is that some important techniques, useful in assessing SAD, were not evaluated: single and multiple breath washout tests were not performed because they are complex and demanding techniques. Likewise, HRCT/QCT was not performed and invasive procedures were not included in the study protocol because they are poorly used in clinical practice. Moreover, we performed a single-center study, and asthmatic patients were not stratified into GINA classes. On the other hand, we selected a wide cohort of patients ranging from mild to moderately severe disease.

SAD is a relevant comorbidity in asthmatic patients, and its detection could be helpful in tailoring a therapeutic choice. Due to their different maneuvers which differently affect lung mechanics, spirometry and IOS may be considered complementary techniques in evaluating SAD. However, we demonstrated that IOS is a more reliable and sensitive measure than spirometry in assessment of clinical control in mild and moderately severe asthmatic patients.

The study protocol was approved by the Ethics Committee for the Province of Parma, Italy (approval number 51364, dated December 15, 2021) and was conducted in accordance with Good Clinical Practices and the Declaration of Helsinki. Written informed consent was obtained from participants to participate in the study.

The authors declare that they have no conflicts of interest for this work.

This study was not funded.

Roberta Pisi, Marina Aiello, Annalisa Frizzelli, Davide Feci, Ilaria Aredano, Gaia Manari, Luigino Calzetta, Giovanna Pelà, and Alfredo Chetta made a significant contribution to the work reported, whether that was in the conception, study design, execution, acquisition of data, analysis, and interpretation, or in all these areas; took part in drafting, revising, or critically reviewing the article; gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work.

The data that support the findings of this study are not publicly available due to ethical and legal reasons, as they contain information that could compromise the privacy of research participants, but are available from the corresponding author upon reasonable request.