Oscillometry:
How it works and what it can measure
During an oscillometry measurement, the respiratory system is stimulated with external pressure that oscillates faster than the normal breathing rate. This allows the flow generated by the stimulus to be measured and separated from the flow generated by the respiratory muscles.
The “speed” of an oscillation is measured in Hz that is the number of times pressure/flow oscillates in a second.
Slower oscillations (i.e. 8 Hz in preschoolers and 5 Hz in the remaining age range) are the most informative since they travel longer into the airways and can reach small airways.
Rrs – Respiratory Resistance
For each respiratory segment, resistance is a measurement of its degree of obstruction. The tracheobronchial tree is a complex branching structure of respiratory segments and respiratory resistance measured by oscillometry is the result of the combination of the resistances of all the stimulated segments. This complex structure makes Resistance more sensitive to central airways pathology but less sensitive to peripheral changes and influenced by heterogeneous distribution of resistances and reactances across the airway tree1.
Rrs is increased in clinical situations such as:
- during bronchoconstriction
- in presence of excessive mucus production or mucous plugging
- airway inflammation
- other causes of airway narrowing or obstruction2
Xrs – Respiratory Reactance
At low stimulating frequencies (5 Hz – 8 Hz), Xrs serves as a measure of the elasticity of the air and the structures in the airways and alveoli being stimulated. A stiffer lung or a smaller volume of air accessible by the stimulating pressure both contribute to a more negative reactance.
Low-frequency Xrs provides valuable information for clinicians to assess how effectively the lung is being ventilated in its distal areas or how well air reaches the peripheral areas.
Xrs falls below predicted values in the following clinical situations:
- peripheral obstruction3
- presence of tidal expiratory flow limitation4
- alveolar gas trapping and/or closing of alveolar units5,6
Impedance spectra, or “oscillogram”: Rrs and Xrs values measured at different oscillometry speed or “frequencies”
Frequently used parameters:
Rrs at 5 Hz (or 8 Hz, in preschoolers)
Xrs at 5 Hz (or 8 Hz, in preschoolers)
Fres: resonant frequency, frequency where Xrs=0
Ax: area of Xrs between 5 Hz and Fres
R5-19: frequency dependence of Rrs, difference between Rrs at 5 Hz and at 19 Hz
Rrs and Xrs are usually expressed as cmH2O/(L/s) or hPa/(L/s)
Within-breath oscillometry
In within-breath oscillometry Rrs and Xrs are analyzed in ther temporal changes during the breathing cycle
✓ Rinsp: average Rrs during inspiration
✓ Xinsp: average Xrs during inspiration
✓ Rexp: average Rrs during expiration
✓ Xexp: average Xrs during expiration
✓ ∆Xrs: Xinsp-Xexp for the detection of tidal Expiratory Flow Limitatation (EFLt)
