2.2. Measurement of Resistance and Reactance in Central and Small Airways

Lung function analysis was performed using TremoFlo^® c-100 (THORASYS Thoracic Medical Systems Inc., Montreal, QC, Canada). Hereby, airwave oscillometry (AO) with oscillation frequencies between 5 and 37 Hz was used. Since this frequency is higher than the tidal breathing frequency, a high-frequency wave was superimposed onto the breathing waveform, allowing the device to calculate resistance (R) and reactance (X) [29].

To allow the oscillatory waves to pass directly through the trachea, the test person needed to be in a seated, upright position, with the head slightly extended. They also need a nose clip to seal the nasal passage, with their hands supporting their cheeks on both sides and their lips enclosing the mouthpiece. Three consecutive 20 s. measurements during tidal breathing were performed. The average was used for analysis. The value obtained prior to the intervention, referred to as the baseline value, was used as a reference. For quality control reasons, the coefficient of variation (CV) ought to be lower than 15% [29].

Resistance is defined as the energy needed to move the pressure wave through the airways [30]. It is determined at 5 and 19 Hz, representing the total (R5) and central (R19) airway obstruction. R5–R19 is the calculated difference of the total airway resistance (R5) and central airway resistance (R19), representing the peripheral airway obstruction. Reactance is a complex term representing the energy determined by flow airflow dynamics in the airways, influenced by the elasticity of tissue and the interstitial forces [30]. AX is defined as the reactance area. The increased values are correlated with distal obstruction [30]. The tidal volume (VT) represents the ventilated air volume during physiological in- and expiration [31].