Table 1.

Impulse Oscillometry Terminology

Impedance (Xrs)	A calculation of the total force needed to propagate a pressure wave through the pulmonary system, comprising resistance and reactance
Resistance (R)	Energy required to propagate a pressure wave through the airways; to pass through the bronchi and bronchioles, and to distend the lung parenchyma. Resistance is determined when a pressure wave is unopposed by airway recoil and is in phase with airflow.
Reactance (X)	Energy generated by the recoil of the lungs after distention by a pressure wave out of phase with airflow
Area of reactance (AX or XA)	Area under the curve between the reactance values for 5Hz and the resonance frequency
Coefficient of variability (CV)	Statistical determinant of the trial-to-trial variability serving as an index of reproducibility
Coherence	An estimate of the quality of impedance measurements. Provides an index of discrepancy between input and measured signals
Compliance	An indicator of the ability of the lung tissue to distend in response to the pressure wave
Frequency-independent change	When resistance values do not vary at different frequencies. If overall resistance is increased, this may be indicative of proximal obstruction.
Frequency-dependent change	When resistance varies with frequency more than age-dependant normal values. This may be indicative of distal obstruction.
Resonance frequency	The frequency at which the lung tissue moves from passive distention to active stretch in response to the force of the pressure wave signal; graphically when reactance is zero.