Respiratory reactance in forced oscillation technique reflects disease stage and predicts lung physiology deterioration in idiopathic pulmonary fibrosis

BACKGROUND
Idiopathic pulmonary fibrosis (IPF) is a chronic progressive disease. Although pulmonary function test (PFT) is useful for evaluating the progression of IPF, obtaining adequate results in advanced cases can be challenging. Conversely, the forced oscillation technique (FOT) can be noninvasively performed, even in patients with severely deteriorated lung function. In this study, the usefulness of FOT for the evaluation of IPF disease status was investigated.


METHODS
We analyzed the PFT and FOT data of 97 patients with IPF.


RESULTS
The respiratory reactance (Xrs) components of FOT, especially in the inspiratory phase, correlated with the PFT values. Patients with advanced disease had significantly lower reactance at 5 Hz (X5), higher resonant frequency (Fres) and low-frequency reactance area (ALX). The longitudinal deterioration of Xrs was also observed. Moreover, X5 in the inspiratory phase predicted subsequent lung capacity deterioration.


CONCLUSION
The Xrs components of FOT, especially in the inspiratory phase, reflected restrictive ventilatory impairment and disease severity in patients with IPF.


Introduction
Interstitial lung disease (ILD) is a generic term for diseases that present with inflammation and fibrosis in the alveolar septum, causing restrictive ventilatory impairment (American Thoracic Society and European Respiratory Society, 2002). Idiopathic pulmonary fibrosis (IPF) is the most common phenotype of ILD and has a chronic progressive course with poor prognosis (Raghu et al., 2011). In patients with IPF, the pulmonary function test (PFT) variables, including vital capacity (VC), forced vital capacity (FVC), and diffusing capacity of the lung for carbon monoxide (DLCO), have been associated with prognosis (Martinez and Flaherty, 2006). The gender, age, and physiologic (GAP) disease staging model, which is scored by gender, age, FVC percent predicted (%FVC) and DLCO percent predicted (%DLCO), has been widely used for the prognostication of patients with IPF . Although PFT is important in IPF management, it requires breathing effort from patients and sometimes cannot be successfully performed, owing to severely deteriorated lung capacity or advanced age.
The forced oscillation technique (FOT) applies the pulse or artificial noise vibration that is electrically generated in the air to the intraoral direction of the subject and measures the returning airflow and intraoral pressure. This device enables quantitative evaluation of the mechanical factors, such as viscous resistance due to friction and elasticity or inertia of the airway and air, which prevent ventilation (Oostveen et al., 2003). Moreover, FOT allows noninvasive measurement of respiratory resistance and reactance during normal breathing, even in patients with severely impaired lung function.
Respiratory impedance (Zrs), which is measured during FOT, can be divided into respiratory resistance (Rrs) and reactance (Xrs), according to the following equation: obstructive pulmonary disease (COPD) and bronchial asthma, and many studies have confirmed its usefulness (Dellacà et al., 2004;Paredi et al., 2010;Shirai et al., 2013;Mikamo et al., 2013). On the other hand, only few reports have shown the usefulness of FOT in ILD (van Noord et al., 1989;Sugiyama A et al., 2013;Fujii M et al., 2015). Moreover, to the best of our knowledge, there have been no reports that focused on the use of FOT in IPF.
In this study, the usefulness of FOT was evaluated by investigating the relationship between PFT and FOT results, the differences in FOT values according to disease stage, and the longitudinal change in FOT values in patients with IPF.

Subjects
A total of 113 patients with IPF and who have undergone PFT and FOT at the Sapporo Medical University Hospital from March 2012 to March 2017 were retrospectively investigated. IPF was diagnosed by a committee that comprised three ILD specialists, based on the 2011 American Thoracic Society/European Respiratory Society/Japanese Respiratory Society/Latin American Thoracic Association statement (Raghu G et al., 2011). Patients who had lung cancer and/or those who underwent lung resection were excluded. Patients with combined pulmonary fibrosis and emphysema, which was diagnosed according to the criteria by Cottin et al. (2005), were also excluded from the study. A total of 97 patients with IPF were finally included in this study. This study was conducted in accordance with the Declaration of Helsinki and was approved by the institutional review board of the Sapporo Medical University Hospital (approval number 282-236; ref. April 13, 2017).

Measurement of respiratory impedance and PFT
Respiratory impedance was measured with a broadband FOT using MostGraph-01 (Chest M.I. Co., Ltd, Japan) and met the standard recommendations (Oostveen et al., 2003). Impulse oscillatory signals that were generated by a loud speaker were applied to the respiratory system through the mouthpiece during tidal breathing for approximately 30 s. During the measurements, the subjects supported their cheeks to reduce upper airway shunting and were asked to wear a nose clip to avoid air leaks while sitting with their neck in a comfortable neutral posture. In this study, we measured and analyzed Rrs at 5 Hz (R5) and 20 Hz (R20), the difference between R5 and R20 (R5-R20), Xrs at 5 Hz (X5), resonant frequency (Fres), and low-frequency reactance area (ALX). Oscillatory indices were expressed as the mean value during a respiratory cycle (whole breath), expiratory phase (Ex), inspiratory phase (In), and difference between expiratory and inspiratory phases (Δ).
VC, FVC, forced expiratory volume in one second (FEV1), and DLCO were measured using CHESTAC-8900 (Chest M.I. Co., Ltd, Japan), according to recommendations (Miller et al., 2005). FOT and PFT were performed on the same day, and FOT measurements were performed before PFT.

Relationship between the measured values of PFT and FOT
Correlation between the PFT and FOT values was investigated using the Spearman's rank correlation coefficient. For patients who underwent several PFT and FOT evaluations, the initial measurement was used.

PFT and FOT values according to IPF disease severity
Based on the GAP disease stage , patients with IPF were classified in two groups: GAP stage I (n = 47) and GAP stage II/III (n = 50). The PFT and FOT results were compared between the two groups using the Mann-Whitney U test.

Longitudinal variations of the PFT and FOT values
Next, we assessed the longitudinal variations in PFT and FOT in patients who underwent the evaluations more than twice. The test values at the initial and second measurements were evaluated. Because the interval from the initial to the second measurement differed between cases, patients with a measurement interval of 12 ± 3 months were included in the longitudinal analysis (n = 41). For each patient, the initial PFT and FOT values were compared with the second values using the Wilcoxon signed-rank test. Data are presented as median (IQR). p values were calculated using the Chi square test or Mann-Whitney U test. ALX, low-frequency reactance area; Δ, difference between expiratory and inspiratory phases; DLCO, diffusing capacity of the lung for carbon monoxide; Ex, expiratory phase; FEV1, forced expiratory volume in one second; FOT, forced oscillation technique; Fres, resonant frequency; FVC, forced vital capacity; In, inspiratory phase; IPF, idiopathic pulmonary fibrosis; Rrs, respiratory system resistance; R5, Rrs at 5 Hz; R20, Rrs at 20 Hz; R5-R20, difference between R5 and R20; VC, vital capacity; Xrs, respiratory system reactance; X5, reactance at 5 Hz. a DLCO was measured in 84 cases.

Predictive factors of ≥10 % FVC decline
Additionally, the predictors of FVC decline after 12 months from when the initial measurements were investigated. Cases in which the interval between the initial and second measurements of PFT and FOT was 12 ± 3 months were included in this analysis (n = 41). Patients were divided into two groups according to the rate of FVC decline (≥10 % or < 10 %) over 12 ± 3 months. FVC decline rate was calculated by the following formula: FVC decline rate (%) = (Second value − Initial value)/ Initial value × 100 Logistic regression analyses were performed to identify the predictive factors of ≥10 % FVC decline. The variables used in the univariate analysis included age, gender, smoking status, DLCO, %DLCO, and FOT values. The values that had p values of < 0.20 in the univariate analysis were included in the multivariate analysis.

Statistical analysis
Statistical analyses were performed using SPSS Statistics software (SPSS Statistics Version 22; IBM, Chicago, IL) and GraphPad Prism v7 software (GraphPad, Inc., San Diego, CA, USA). A p-value of < 0.05 was considered statistically significant.

Baseline characteristics
The baseline characteristics of the study population are shown in Table 1

Comparison between the PFT and FOT values according to GAP disease stage
On PFT, the VC, %VC, FVC, %FVC, FEV1, DLCO, and %DLCO were significantly lower in the GAP stage II/III than in the GAP stage I group. No significant Rrs difference was found between both groups. Conversely, X5 (whole breath, In) was significantly lower, whereas Fres (whole breath, Ex, In) and ALX (whole breath, In) were significantly higher in the GAP stage II/III than in the GAP stage I group (Table 3).  Values are presented as correlation coefficients. *p value < 0.01, # p value < 0.05. ALX, low-frequency reactance area; Δ, difference between expiratory and inspiratory phases; DLCO, diffusing capacity of the lung for carbon monoxide; Ex, expiratory phase; FEV1, forced expiratory volume in one second; FOT, forced oscillation technique; Fres, resonant frequency; FVC, forced vital capacity; In, inspiratory phase; IPF, idiopathic pulmonary fibrosis; PFT, pulmonary function test; Rrs, respiratory system resistance; R5, Rrs at 5 Hz; R20, Rrs at 20 Hz; R5-R20, difference between R5 and R20; VC, vital capacity; Xrs, respiratory system reactance; X5, reactance at 5 Hz. a DLCO was measured in 84 cases.

Comparison between the initial and second values of PFT and FOT
Among the patients included in the longitudinal analysis, the median duration of the initial and second measurements was 12 months (range, 11-15 months; IQR, 11-13 months). In the longitudinal analysis, VC, %VC, FVC, %FVC, FEV1, DLCO, and %DLCO significantly decreased, whereas FEV1/FVC significantly increased (Table 4). Although no significant change was observed in the Rrs during the clinical course, X5 (whole breath, In) significantly decreased, whereas Fres (whole breath, Ex, In) and ALX (whole breath, Ex, In) significantly increased. Fig. 1. Correlation between FVC/%FVC and Xrs in the inspiratory phase. X5 (In) has significant positive correlations with FVC and %FVC. Fres (In) and ALX (In) have significant negative correlations with FVC and %FVC. ALX (In), low-frequency reactance area in the inspiratory phase; Fres (In), resonant frequency in the inspiratory phase; FVC, forced vital capacity; %FVC, forced vital capacity % predicted; Xrs, respiratory system reactance; X5 (In), respiratory system reactance at 5 Hz in the inspiratory phase.

Predictive factors of FVC decline over 12 months
On univariate analysis, the Xrs values in the inspiratory phase were significantly associated with ≥10 % FVC decline over 12 ± 3 months (p < 0.05) ( Table 5). Because almost all Xrs indices had p values of < 0.20 and were found to be strongly correlated (r = 0.8-0.9), X5 (In) was included in the multivariate analysis as the representative index of Xrs. The multivariate analysis revealed that low X5 (In) was significantly associated with ≥10 % FVC decline over 12 ± 3 months [odds ratio (OR) 0.137, 95 % CI 0.021-0.875, p = 0.036) ( Table 6).

Discussion
IPF is a chronic progressive disease of unknown etiology and has a poor prognosis (Natsuizaka et al., 2014). The PFT variables VC, FVC, and DLCO have been used to evaluate disease status (Travis et al., 2013) and were reported to predict the prognosis of patients with IPF (Martinez and Flaherty, 2006). However, some patients with advanced disease have difficultly performing PFT, which requires effort and a certain amount of VC to measure DLCO. Therefore, appropriate results cannot be obtained occasionally. On the other hand, FOT can be noninvasively performed during normal breathing, even in advanced cases.
In this study, the PFT and Xrs on FOT values were strongly correlated, particularly when Xrs was measured in the inspiratory phase. Moreover, the Xrs values predicted ≥10 % FVC decline over 12 months after performing FOT. Due to the short observation period of the current study, it was not possible to assess whether Xrs itself predicted the   Data are presented as median (IQR). p values were calculated using the Wilcoxon signed-rank test. ALX, low-frequency reactance area; Δ, difference between expiratory and inspiratory phases; DLCO, diffusing capacity of the lung for carbon monoxide; Ex, expiratory phase; FEV1, forced expiratory volume in one second; FOT, forced oscillation technique; Fres, resonant frequency; FVC, forced vital capacity; GAP, gender, age, and physiologic variables; In, inspiratory phase; IPF, idiopathic pulmonary fibrosis; PFT, pulmonary function test; Rrs, respiratory system resistance; R5, Rrs at 5 Hz; R20, Rrs at 20 Hz; R5-R20, difference between R5 and R20; VC, vital capacity; Xrs, respiratory system reactance; X5, reactance at 5 Hz. a DLCO was measured in 36 cases. Y. Mori, et al. Respiratory Physiology & Neurobiology 275 (2020) 103386 prognosis of patients with IPF. Nevertheless, ≥10 % FVC decline has been reported to predict mortality in patients with IPF (Du Bois et al., 2011;Richeldi et al., 2012) and has been used as the primary endpoint in a pirfenidone clinical trial (King et al., 2014). Therefore, Xrs values, especially in the inspiratory phase, may be used as a PFT substitute to predict disease progression and the prognosis of patients with IPF. The usefulness of FOT to evaluate the disease status in patients with IPF was also examined in the present study. Sugiyama et al. (2013) evaluated the differences in FOT values between healthy volunteers and patients with ILD, bronchial asthma, and COPD; they reported that patients with ILD showed lower X5 and higher Fres and ALX, compared with those in the healthy controls. In this study, comparison between GAP stage I and stage II/III revealed that X5 decreased and Fres and ALX increased as the disease stage progressed. Moreover, these values deteriorated in the longitudinal analysis. For the Xrs, three indices are mainly used: X5, which is Xrs at 5 Hz; Fres, which is the resonant frequency at point 0; and ALX, which is the low-frequency area (integral from X5 to Fres). These indices were reported to reflect lung parenchymal and airway abnormalities. × 5 is considered the reciprocal of lung compliance, and its value becomes more negative when the lung tissue has reduced compliance (Sugiyama et al., 2013). In addition, Fres reflects progression of lung fibrosis and increase in lung elastic recoil in ILD (Shirai and Kurosawa, 2016). According to these reports, X5 decrease, Fres increase, and ALX increase may be considered to indicate the progression of lung fibrosis in IPF.
In this study, patients with IPF were classified in two groups according to the GAP model. Ley et al. (2012) proposed the GAP model, which is scored by gender, age, and lung physiology (%FVC and % DLCO), to discriminate prognosis of patients with IPF. In this model, patients are classified into three stages of disease severity, and treatment is proposed according to the disease stage. In GAP stage I, patients may not require immediate enlisting for lung transplantation because of the low risk for one-year mortality. However, physicians should consider enlisting patients in GAP stage II/III for lung transplantation. Therefore, in this study, the differences in FOT results according to disease severity were assessed after classifying patients with IPF into GAP stage I or stage II/III.
The usefulness of FOT has been comprehensively investigated in obstructive airway diseases, such as bronchial asthma or COPD. The Rrs in FOT, especially in the expiratory phase, was reported to reflect the disease status of obstructive airway diseases (Ohishi et al., 2011). Conversely, Xrs was believed to be more important for the assessment of ILD pathophysiology, as the current study demonstrated. Fujii et al. (2015) found that Fres in the inspiratory phase correlated with the FVC, FEV1, DLCO, and fibrosis score in ILD. Sugiyama et al. (2013) reported that the presence of ILD was associated with ΔX5 and that ΔX5 was negatively correlated with VC and DLCO. Although several reports have focused on ILD, patients with ILD are considered to have small airway disease in varying degrees, depending on the ILD disease type (Fulmer and Roberts, 1980). Therefore, this study included only patients who were strictly diagnosed as IPF.
This study had some limitations. First, the reference values for MostGraph have not been established; therefore, the parameters of MostGraph cannot be evaluated using percent predicted values. Second, the number of patients who underwent multiple measurements was relatively small. Finally, there are FOT equipments other than MostGraph used in this study [e.g., Master Screen IOS (Eric Jaeger, Germany)], and slight differences in measured values depending on devices are reported. It is unclear whether results of this study could be also applied to other devices.

Conclusions
The FOT Xrs values, especially in the inspiratory phase, were useful in evaluating disease progression in IPF. Even in patients with advanced disease and who have difficulty performing PFT, FOT may be performed to noninvasively evaluate disease status and predict lung capacity decline. Further prospective studies are required to validate the use of Xrs for predicting the prognosis of patients with IPF.

Table 6
Multivariate logistic regression analysis of the factors that predicted 10 % or more decline in FVC within 12 months. *p value < 0.05. CI, confidence interval; Δ, difference between expiratory and inspiratory phases; FVC, forced vital capacity; In, inspiratory phase; R5, respiratory system resistance at 5 Hz; R20, respiratory system resistance at 20 Hz; X5, respiratory system reactance at 5 Hz.