Estimation of respiratory impedance at low frequencies using the forced oscillation technique

The forced oscillation technique (FOT) is a non-invasive measurement technique to characterize the impedance of a respiratory system (Zrs). Zrs is defined as a frequency dependent ratio of pressure at the airway opening (pao) to air flow (qe) induced by Zrs.

It is emphasized in [1] that measurement and analysis of Zrs at lower physiological frequency ranges (< 10 Hz) is most sensitive to normal physical processes and pathological structural alterations. This work aims at measuring Zrs on spontaneously breathing patients at lower frequency ranges using FOT.

A pressure generating device with a low output impedance ZD is developed [2] and is used for measurements on patients with COPD, asthma and without respiratory complaints.

Measurement of Zrs

Figure : Measurement of Zrs. pe: wanted pressure excitation, ZD: output impedance of device, pao: pressure at airway opening, qao: flow at airway opening, consisting of the addition of response qe of respiratory system Zrs with breathing flow qb.


The main difficulty in the measurement of Zrs is the presence of the breathing flow qb which overlaps with qe in the low frequencies (see figure).

Elimination of the breathing disturbance from the estimation of Zrs has been done by means of nonlinear breathing models in [3] and by regularization techniques in [4].  Additionally, a measurement technique using adaptive excitation signals has been deployed and has proven to be effective during clinical trials at UZ Brussel. The technique has been patented under the name “Improved methods for determining respiratory properties” and the results are available in the PhD Thesis of Hannes Maes (October 2017) that is available on the ELEC website.

Improvement of the SNR has achieved by means of a collaboration with DEMCON Macawi respiratory systems. They developed a high dynamic turbine blower with high pressure efficiency, used in portable respiratory systems.

The measurement of Zrs is particularly valuable for the following important applications:

A) The diagnosis of lung diseases can be facilitated by lung impedance measurement. A promising way to get this diagnostic information is via the low frequency FOT, although further modelling of the respiratory system is required to establish FOT as a diagnostic tool.

B) A ventilation device is an essential tool in an Intensive Care Unit (ICU) to allow for appropriate oxygenation of the patient. This device requires the tuning of patient-specific respiratory control parameters. In this project, we will aim at replacing the current visual tuning by a tuning based on the FOT respiratory measurements, using pressure and flow measurements and data-driven modeling tools.


  1. D. W. Kaczka and R. L. Dellaca, “Oscillation mechanics of the respiratory system: applications to lung disease,” Crit Rev Biomed Eng., vol. 39, pp. 337–359, 2011.
  2. H. Maes, G. Vandersteen, M. Muehlebach, and C. Ionescu, “A fan-based, low-frequent, forced oscillation technique apparatus,” IEEE Transactions on Instrumentation and Measurement, vol. 63, no. 3, pp. 603–611, March 2014.
  3. H.Maes, G. Vandersteen and C. Ionescu, "Estimation of respiratory impedance at low frequencies during spontaneous breathing using the forced oscillation technique," Engineering in Medicine and Biology Society (EMBC), 2014 36th Annual International Conference of the IEEE , vol., no., pp.3410-3413, 26-30 Aug. 2014
  4. H.Maes, M. Zivanovic, J.Schoukens and G.Vandersteen, “Estimating Respiratory Impedance at Breathing Frequencies Using Regularized Least Squares on Forced Oscillation Technique Measurements” IEEE Transactions on Instrumentation and Measurement, Vol. 66, No. 3, March 2017, pp. 479-491
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