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.

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

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

A fan-based device to generate pressure oscillations during spontaneous breathing has been developed and is presented in [2]. This decreases the disturbances on the pressure oscillation (pb).

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].

Currently hospital measurements are employed aimed at deriving estimates of the time varying behaviour of low frequent Zrs during the measurement.


[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.

[1]   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.

[2]   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

[3]   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, Januari 2017. Available online

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