The forced oscillation technique in clinical practice: methodology, recommendations and future developments. E. Oostveen, D. MacLeod, H. Lorino, R. Farre´ , Z. Hantos, K. Desager, F. Marchal, on behalf of the ERS Task Force on Respiratory Impedance Measurements. #ERS Journals Ltd 2003. ABSTRACT: The forced oscillation technique (FOT) is a noninvasivemethod with which to measure respiratory mechanics. FOT employs small-amplitude pressure oscillations superimposed on the normal breathing and therefore has the advantage over conventional lung function techniques that it does not require the performance of respiratory manoeuvres. The present European Respiratory Society Task Force Report describes the basic prin- ciple of the technique and gives guidelines for the application and interpretation of FOT as a routine lung function test in the clinical setting, for both adult and paediatric populations. FOT data, especially those measured at the lower frequencies, are sensitive to airway obstruction, but do not discriminate between obstructive and restrictive lung disorders. There is no consensus regarding the sensitivity of FOT for bronchodilation testing in adults. Values of respiratory resistance have proved sensitive to bronchodilation in children, although the reported cutoff levels remain to be cone rmed in future studies. Forced oscillation technique is a reliable method in the assessment of bronchial hyper- responsiveness in adults and children. Moreover, in contrast with spirometry where a deep inspiration is needed,forced oscillation techniquedoes not modifytheairway smooth muscle tone. Forced oscillation technique has been shown to be as sensitive as spirometry in detecting impairments of lung function due to smoking or exposure to occupational hazards. Together with the minimalrequirement for the subject 9s cooperation, this makes forced oscillation technique an ideal lung function test for epidemiological and e eld studies. Novel applications of forced oscillation technique in the clinical setting include the monitoring of respiratory mechanics during mechanical ventilation and sleep.

https://erj.ersjournals.com/content/erj/22/6/1026.full.pdf

The forced oscillation technique in clinical practice: methodology, recommendations and future developments

Oscillometry (also known as the forced oscillation technique) measures the mechanical properties of the respiratory system (upper and intrathoracic airways, lung tissue and chest wall) during quiet tidal breathing, by the application of an oscillating pressure signal (input or forcing signal), most commonly at the mouth. With increased clinical and research use, it is critical that all technical details of the hardware design, signal processing and analyses, and testing protocols are transparent and clearly reported to allow standardisation, comparison and replication of clinical and research studies. Because of this need, an update of the 2003 European Respiratory Society (ERS) technical standards document was produced by an ERS task force of experts who are active in clinical oscillometry research.The aim of the task force was to provide technical recommendations regarding oscillometry measurement including hardware, software, testing protocols and quality control.The main changes in this update, compared with the 2003 ERS task force document are 1) new quality control procedures which reflect use of "within-breath" analysis, and methods of handling artefacts; 2) recommendation to disclose signal processing, quality control, artefact handling and breathing protocols (e.g. number and duration of acquisitions) in reports and publications to allow comparability and replication between devices and laboratories; 3) a summary review of new data to support threshold values for bronchodilator and bronchial challenge tests; and 4) updated list of predicted impedance values in adults and children.

https://erj.ersjournals.com/content/erj/55/2/1900753.full.pdf

Technical Standards for Respiratory Oscillometry

Conventional methods of lung function testing provide measurements obtained during specific respiratory actions of the subject In contrast, the forced oscillation technique (FOT) determines breathing mechanics by superimposing small external pressure signals on the spontaneous breathing of the subject FOT is indicated as a diagnostic method
to obtain reliable differentiated tidal breathing analysis Because FOT is performed without closure of a valve connected to the mouthpiece, and without maximal or forced respiratory manoeuvres, it is unlikely that FOT itself will alter airways smooth muscle tone
FOT utilises the external applied pressure signals and their resultant flows to determine lung mechanical parameters These pressure–flow relationships are largely distinct from the natural pattern of individual respiratory flows, so that measured FOT results are, for the most part, independent of the underlying respiratory pattern Therefore, oscillometry minimises demands on the patient and requires only passive cooperation of the subject: maintenance of an airtight seal of the lips around a mouthpiece and breathing normally
through the measuring system with a nose-clip occluding the nares Potential
applications of oscillometry include paediatric, adult and geriatric populations, comprising diagnostic clinical testing, monitoring of therapeutic regimens, and epidemiological evaluations, independent of severity of lung disease Oscillometry is also applicable to veterinary medicine
The last two main sections Relevance of oscillometry in clinical practice and
Oscillometry in the clinical pulmonary laboratory emphasise clinical aspects of
application and interpretation of FOT rather than methodological details and
technological solutions, which are discussed in the two sections that follow immediately below Clinical application of FOT does not require mastery of the mathematical infrastructure of the technical methodology, and readers interested in the clinical use of
FOT may find it more useful to begin with these clinical sections and refer subsequently to methodological and technological details

Forced oscillation technique and impulse oscillometry

The mechanical impedance of the respiratory system defines the pressure profile required to drive a unit of oscillatory flow into the lungs. Impedance is a function of oscillation frequency, and is measured using the forced oscillation technique. Digital signal processing methods, most notably the Fourier transform, are used to calculate impedance from measured oscillatory pressures and flows. Impedance is a complex function of frequency, having both real and imaginary parts that vary with frequency in ways that can be used empirically to distinguish normal lung function from a variety of different pathologies. The most useful diagnostic information is gained when anatomically based mathematical models are fit to measurements of impedance. The simplest such model consists of a single flow-resistive conduit connecting to a single elastic compartment. Models of greater complexity may have two or more compartments, and provide more accurate fits to impedance measurements over a variety of different frequency ranges. The model that currently enjoys the widest application in studies of animal models of lung disease consists of a single airway serving an alveolar compartment comprising tissue with a constant-phase impedance. This model has been shown to fit very accurately to a wide range of impedance data, yet contains only four free parameters, and as such is highly parsimonious. The measurement of impedance in human patients is also now rapidly gaining acceptance, and promises to provide a more comprehensible assessment of lung function than parameters derived from conventional spirometry.

Oscillation mechanics of the respiratory system.

The understanding of the mechanical properties of the mammalian respiratory system and how they change under the influence of drugs and in disease are frequently pursued in small animals, since they can be easily obtained in large numbers as pure-bred strains. However, conventional experimental set-ups for studying small animals are generally limited in their ability to measure gas flow into the lungs. Here, the authors present a computer-controlled research ventilator for small animals which can provide conventional mechanical ventilation as well as arbitrary flow perturbations with a bandwidth from 0-55 Hz. Respiratory impedance is estimated from the displacement of the piston and the pressure it generates, thereby obviating the need for a direct flow measurement. The performance of the device was tested on mechanical loads whose impedances were calculated theoretically. The measured and predicted loads agreed within less than 5% up to 30 Hz. Furthermore, the measured impedance of two mechanical loads in series precisely matched the sum of their individual impedances. >

A computer-controlled research ventilator for small animals: design and evaluation

Respiratory flow is commonly obtained by measuring the pressure difference across a pneumotachograph. When respiratory input impedance is studied, that pressure difference may be very small with respect to the absolute pressure swings inside the pneumotachograph. Then the in-phase rejection of the differential pressure transducer is expected to markedly influence the accuracy of the data. The problem was investigated by computer simulation and by measurements on a mechanical analog of the respiratory system made of a resistance, an inertance, and a compliance arranged in series. Both studies demonstrated that comparatively small differences in the volumes of the chambers or in the lengths or diameters of the connecting tubes led to artifactual frequency dependence of resistance and serious misestimation of compliance and inertance. Errors were larger when the resistance of the pneumotachograph was smaller and the impedance of the subject larger. In practice, with usual pneumotachographs accurate impedance...

In-phase rejection requirements for measuring respiratory input impedance.

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In-phase rejection requirements for measuring respiratory input impedance.