Impulse oscillometry (IOS), also known asrespiratory oscillometry,forced oscillatory technique (FOT), or justoscillometry, is anon-invasivelung function test that measures themechanical properties of therespiratory system, particularly theupper andintrathoracic airways,lung tissue andchest wall, usually during the patient'stidal breathing (the way someone breathes when they arerelaxed).^[1][2][3]

Impulse oscillometry measures themechanical impedance of the respiratory system (Z_rs), which encompasses theresistance of the respiratory system to flow (R_rs), the reactance orstiffness of thelung parenchyma in response to changes in volume (X_rs) and theinertance of accelerating gas in the airways (I_rs).^[2][3]

The following relations hold between these parameters:${\displaystyle Z_{rs}=R_{rs}+i{X}_{rs}}$, where${\displaystyle i}$ is theimaginary unit (${\displaystyle \sqrt{-1}}$), and${\displaystyle X_{rs}=\omega I_{rs}-\frac{E_{rs}}{\omega }}$, where${\displaystyle E_{rs}}$ is the airwayelastance and${\displaystyle \omega }$ is theangular velocity such that${\displaystyle \omega =2\pi f}$, where${\displaystyle f}$ is thefrequency of the stimulus oscillation.^[2]

Z_rs is measured by comparing the magnitudes of mechanical stimuli, specifically oscillations ofpressure, i.e. pressurewaves, transmitted into the respiratory system with the magnitudes of the stimuli's effects on tidal airflow; this is done by superimposing these oscillations over spontaneous tidal breathing.^[1][2][3]

The stimulus is an oscillation of pressure of a particular frequency that is transmitted to the lungs of the patient.^[1] This is usually done bymouth, though the direct stimulation of the chest wall is also possible.^[2] These pressure waves cause changes in the airflow during tidal breathing; the magnitudes of the pressure waves and the changes in airflow they cause are then used to determine the airways' mechanical impedance.^[1]

Frequencies ranging from 4-50Hz are commonly generated by aloudspeaker, while frequencies between 0.5 and 4 Hz may alternatively also be generated by apiston orpneumatic proportional solenoid valves.^[2]

Different frequencies measure the mechanical properties of different parts of the respiratory system; the resistance at 5 Hz (R₅) represents total airway resistance, while the resistance at 20 Hz (R₂₀) represents the resistance of the central airways. The reactance at 5 Hz (X₅) reflects the elasticity of the peripheral airways.^[4]

• Pulmonary function testing
• Respiratory therapy
• Respiratory physiology

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