Reproducibility and responsiveness of airway impedance measures derived from the forced oscillation technique across different operating lung volumes

Background: The forced oscillation technique (FOT) enables non-invasive measurement of respiratory system impedance. Limited data exists on how changes in operating lung volume (OLV) impact FOT-derived measures of airway resistance (Rrs) and reactance (Xrs). Objectives: This study examined the reproducibility and responsiveness of FOT-derived measures of Rrs and Xrs during simulated changes in OLV. Methods: Participants simulated breathing at six OLVs: total lung capacity (TLC), ~50% of inspiratory reserve volume (IRV 50 ), ~two-times tidal volume (VT 2 ), tidal volume (VT), ~50% of expiratory reserve volume (ERV 50 ), and residual volume (RV), on a commercially available FOT device. Each simulated OLV manuever was performed in triplicate and in random order. Total Rrs and Xrs were recorded at 5, 11, and 19 Hz. Results: Twelve healthy participants (2 female) completed the study (weight: 76.5 ± 13.6 kg, height: 178.6 ± 9.7 cm, body mass index: 23.9 ± 3.1 kg/m 2 ). Reproducibility of Rrs and Xrs at VT, VT 2 and IRV 50 was good to excellent (Range: ICC: 0.89 – 0.98, 95% confidence interval (CI): 0.70 – 0.98), while reproducibility at TLC, RV, and ERV 50 was poor to excellent (Range: ICC: 0.60 – 0.98, 95% CI: 0.36 – 0.97). Rrs and Xrs were not different between VT and VT 2 at any frequency ( P > .05). With lung hyperinflation from VT to TLC, Rrs and Xrs decreased at all three frequencies (e.g., At 5 Hz Rrs: mean difference (MD): (cid:0) 0.89, 95%CI: (cid:0) 0.03 to (cid:0) 1.75, P = .04; Xrs: MD: (cid:0) 0.56, 95%CI: (cid:0) 0.25 to (cid:0) 0.86, P < .01). With lung hypoinflated from VT to RV, Rrs increased, and Xrs decreased for all frequencies (e.g., MD at 5 Hz, Rrs: MD: 2.31, 95%CI: 0.94 – 3.67, P < .01; Xrs: MD: (cid:0) 2.53, 95% CI: (cid:0) 4.02 to (cid:0) 1.04, P < .01). Conclusion: FOT-derived measures of airway Rrs and Xrs are reproducible across a range of OLV ’ s, and are responsive to hyper-and hypo-inflation of the lung. To further understand the impact of lung hyper-and hypo-inflation on FOT-derived airway impedance additional study is required in individuals with pathological variations in operating lung volume.


Introduction
The forced oscillation technique (FOT) enables non-invasive measurement of respiratory system impedance via the application of an oscillating pressure signal at the mouth during tidal breathing (King et al., 2020).Mechanical impedance of the respiratory system measured via FOT comprises two elements: the resistance of the respiratory airways and tissues to the flow of gases (Rrs), and the reactance (elastance) of the respiratory system (Xrs) due to the stiffness of the lung and chest wall tissues.Physiologically, FOT-derived measures of Rrs and Xrs are impacted by the specific oscillatory frequency imposed over tidal breathing (King et al., 2020;Oostveen et al., 2003), and thus current standards recommend frequencies of 5-35 Hz (King et al., 2020).A broad FOT frequency range enables assessment of both proximal (higher frequencies) and peripheral (lower frequency) airways, with the difference in Rrs between 5 and 19 Hz considered a marker of peripheral airway resistence (Cho et al., 2020;Grimby et al., 1968).Beneficially, as FOT is effort-independent it is easier to perform than spirometry in children, the acutely sick, and in frail and elderly individuals (Kaminsky et al., 2022).
Despite the widespread applications of FOT, recently published technical standards have stipulated the need for additional research investigating the impact of varying the specific FOT measurement settings (such as oscillatory frequency and tidal breathing conditions) on Rrs and Xrs measures in health and disease (King et al., 2020).While FOT-derived measures of Rrs and Xrs can quantify airway responses to long-acting bronchodilators (Cottee et al., 2020), exercise-induced bronchodilation (Di Mango et al., 2006), and dyspnoea (Veneroni et al., 2022), and aid in the differential diagnosis of respiratory conditions such as COPD and asthma, there is limited data demonstrating how changes in operating lung volume (OLV) influences FOT.Given that changes in OLV, i.e., hyperinflation (Laveneziana et al., 2015;O'Donnell and Webb, 2008) or hypoinflation (Spottswood et al., 2004), are characteristic of certain cardiopulmonary diseases, ascertaining the direct impact of a shift in OLV on FOT would be beneficial for separating the influence of disease pathophysiology, per se, from secondary disease effects such as a shift in OLV.Accordingly, this study examined the reproducibility and responsiveness of FOT derived measures of Rrs and Xrs during simulated changes in OLV.It was hypothesised that FOT parameters would be reproducible across all OLV's, and that Rrs and Xrs would be altered by shifts in OLV.

Methods
Experimental procedures were approved by the relevant Institutional Human Research Ethics Committees in accordance with the Declaration of Helsinki, and all participants provided written informed consent.Twelve healthy individuals (age: 27.7 ± 5.5 years old) who had no known respiratory, cardiovascular, or neuromuscular limitation and were not taking any prescribed medication participated in the study.
Participants were initially coached to perform tidal breathing at six different OLV's while breathing through a spirometer (Breeze, Medgraphics, USA) with visual feedback.The six different OLVs are represented in Fig. 1 and include 1) total lung capacity (TLC), 2) ~50% of inspiratory reserve volume (IRV 50 ), 3) ~two-times tidal volume (VT 2 ), 4) tidal volume (VT), 5) ~50% of expiratory reserve volume (ERV 50 ), 6) residual volume (RV).Participants breathed on a commercially available FOT device (Resmon Pro 2.6.1,RESTECH, MGC Diagnostics) with data collected for > 10 breaths and in triplicate for each OLV.The order of tests was randomised, and participants practised breathing at each OLV with visual feedback before performing them on the FOT device.Participants rested for one minute between repeat tests and five minutes between each OLV.Total Rrs and Xrs across the respiratory cycle (i.e., during inspiration and expiration) were recorded at 5, 11, and 19 Hz as recommended by the European Respiratory Society (King et al., 2020), and the difference in Rrs between 5 and 19 Hz (Rsr 5-19 Hz) was computed as a marker of peripheral airway resistance.
Reproducibility of Rrs and Xrs at each OLV were evaluated via intraclass correlation coefficients (ICC) (two-way random, absolute agreement, single measures), coefficient of variation (CoV), bias and 95% limits of agreement (LoA).A repeated measures ANOVA was performed to examine differences in Rrs and Xrs at each OLV with Tukey post-hoc adjustment to account for multiple pairwise comparisons.Statistical testing was performed with SPSS version 28 (SPSS, Chicago, Illinois) and significance was accepted at P < .05.All data are presented as mean ± standard deviation.

Discussion
This study aimed to evaluate the reproducibility and responsiveness of FOT-derived measures of Rrs and Xrs during simulated changes in OLV.The main findings are that FOT-derived measures of Rrs and Xrs are highly reproducible and responsive to changes in OLV, with the greatest magnitude of changes in Rrs and Xrs observed between VT and the extreme ends of OLV (i.e., at TLC and RV).
To our knowledge, this is the first study to show that total Rrs and Xrs are reproducible across a range of OLVs, and this suggests that FOT is a suitable measure of airway resistance and reactance under conditions of hyper-or hypo-inflation of the lungs.However, the increased  (Watts et al., 2016).Regardless of CoV, the high reproducibility of FOT at each OLV is supported by the ICCs, 95% CI, and limits of agreement.Interestingly, as OLV decreased from TLC to RV, there were progressive increases in Rrs at 5, 11 and 19 Hz.Physiologically, at greater OLVs the bronchi are hyperinflated due to the negative pressure generated by the expanded thoracic cavity.Hence, a lower resistance in the central airways, with limited impact of the peripheral airways, is likely driving the overall reduction in Rrs at higher OLV's (Milne et al., 2018;Milne et al., 2019).Conversely, as OLV decreases from VT towards RV we observed an increase in Rrs, alongside a rise in Rrs 5-19 Hz, which likely reflects a substantial contribution from the peripheral airways at lower OLV's (Nagels et al., 1980;Oostveen et al., 1989).
An increase in positive thoracic pressure applied by the weight of the rib cage and accessory respiratory muscles during expiration increases the resistance of central airways (Milne et al., 2018;Milne et al., 2019).In contrast to Rrs, Xrs progressively decreased during both hyper-and hypo-inflation (i.e., from VT to TLC and from VT to RV, respectively).At IRV 50 and TLC, hyperinflation of the lungs results in a reduced elastic recoil of central and peripheral airways, and increases the stiffness of the chest wall, resulting in a decreased Xrs (Petro et al., 1981).At the opposite extreme -ERV 50 and RV -the increased thoracic pressure and contraction of respiratory muscles necessary to mimic hypo-inflation caused pressure on the lung airways and a reduction in the elastic expansion of the alveoli (Petro et al., 1981).These physiological data and resproducbility findings suggest that FOT is responsive to changes in OLV and is an important method for investigating respiratory impedance in health and disease.

Limitations
Although currently available FOT devices use similar principles, considerable differences in impedance measurements are noted in the literature.Therefore, comparisons across a range of commercial devices should be examined to determine the applicability of these findings across different vendors.While the breathing manuvers were well tolerated by our participants (only 2-3 trials across the entire study needed to be repeated due to subject limitations, such as coughing, or device error), the study included atypical patterns of breathing that may have impacted the findings (e.g., breathing near TLC and RV is uncomfortable).Furthermore, our cohort included predominantly healthy individuals and only 2 females which precluded any sex-specific or health status comparisons.A similar study should be replicated in individuals with natural differences in OLV to better understand pathology, e.g., across severities of COPD and with varying levels of hyperinflation.

Conclusions
In conclusion, FOT-derived measures of airway Rrs and Xrs are reproducible across a range of OLV's, and are responsive to hyper-and hypo-inflation of the lung.To understand the impact of lung hyper-and hypo-inflation on parameters of FOT in clinical patients, and determine the clinical applications of FOT, further study in individuals with pathological variations in OLV are warranted.

Fig. 1 .
Fig. 1.A) Schematic of the breathing manoeuvres performed by participants at different operating lung volumes.Breathing manoeuvres were performed at: TLC, total lung capacity; IRV50, ~50% of inspiratory reserve volume; VT2, two-times tidal volume; VT, tidal volume; ERV50, ~50% of expiratory reserve volume; RV, residual volume.B) Resistance and reactance parameters across the different operating lung volumes at 5, 11, and 19 Hz (X axis values have been offset for visual clarity).Resistance decreased as the lung hyperinflated from VT towards TLC, and increased as the lung hypoinflated, from VT to RV. Reactance decreased from VT towards TLC as the lungs hyperinflated, and as the lungs hypoinflated from VT to RV.

Table 1
Reproducibility of FOT parameters at different operating lung volumes.Mean data are represented as average ± standard deviation.measurement variation at TLC and RV, and across the Xrs data, suggests some caution is needed when interpreting FOT-derived Rrs and Xrs at the extreme ends of hyper-and hypo-inflation.Our variability agrees with earlier work highlighting a wider CoV for Xrs than Rrs in healthy and diseased adults