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.