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 (IRV₅₀), ∼two-times tidal volume (VT₂), tidal volume (VT), ∼50% of expiratory reserve volume (ERV₅₀), 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/m²). Reproducibility of Rrs and Xrs at VT, VT₂ and IRV₅₀ was good to excellent (Range: ICC: 0.89–0.98, 95% confidence interval (CI): 0.70–0.98), while reproducibility at TLC, RV, and ERV₅₀ 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 VT₂ 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): − 0.89, 95%CI: − 0.03 to − 1.75, P = .04; Xrs: MD: − 0.56, 95%CI: − 0.25 to − 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: −2.53, 95%CI: −4.02 to −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.