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 19Hz.

RESULTS: Twelve healthy participants (2 female) completed the study (weight: 76.5±13.6kg, height: 178.6±9.7cm, body mass index: 23.9±3.1kg/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 to 0.98), while reproducibility at TLC, RV, and ERV50 was poor to excellent (Range: ICC: 0.60-0.98, 95% CI: 0.36 to 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 frequenices (e.g., At 5Hz 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 5Hz, Rrs: MD: 2.31, 95%CI: 0.94 to 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.