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

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  • ERS TASK FORCE

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

    E. Oostveen*, D. MacLeod#, H. Lorino}, R. Farrez, Z. Hantos, K. Desager, F. Marchal**,on behalf of the ERS Task Force on Respiratory Impedance Measurements

    The forced oscillation technique in clinical practice: methodology, recommendationsand 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 ImpedanceMeasurements. #ERS Journals Ltd 2003.ABSTRACT: The forced oscillation technique (FOT) is a noninvasive method with whichto measure respiratory mechanics. FOT employs small-amplitude pressure oscillationssuperimposed on the normal breathing and therefore has the advantage over conventionallung 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 aroutine 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 airwayobstruction, but do not discriminate between obstructive and restrictive lung disorders.There is no consensus regarding the sensitivity of FOT for bronchodilation testing inadults. Values of respiratory resistance have proved sensitive to bronchodilation inchildren, although the reported cutoff levels remain to be confirmed 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 adeep inspiration is needed, forced oscillation technique does not modify the airway smoothmuscle tone. Forced oscillation technique has been shown to be as sensitive as spirometryin detecting impairments of lung function due to smoking or exposure to occupationalhazards. Together with the minimal requirement for the subject9s cooperation, this makesforced oscillation technique an ideal lung function test for epidemiological and fieldstudies. Novel applications of forced oscillation technique in the clinical setting includethe monitoring of respiratory mechanics during mechanical ventilation and sleep.Eur Respir J 2003; 22: 10261041.

    *Dept of Pulmonary Medicine, UniversityHospital Antwerp, Belgium. #Scottish Inter-collegiate Research Training Network, NHSEducation for Scotland, UK. }Service dePhysiologie, Hopital Henri Mondor, Creteil-Paris, France.

    zUnitat de Biof sica i Bioengi-

    nyeria, Facultat de Medicina, Barcelona,Spain.

    Dept of Medical Informatics, Univer-

    sity of Szeged, Hungary. Dept of paediatrics,University Hospital Antwerp, Belgium.**Laboratoire d9Explorations FonctionnellesPediatriques, Hopital d9Enfants, CHU deNancy, Vandoeuvre, France.

    Correspondence: E. Oostveen, Dept of Pulmo-nary Medicine, University Hospital Antwerp,Wilrijkstraat 10, B-2650 Edegem-Antwerp,Belgium.Fax: 32 38214447E-mail: Ellie.Oostveen@uza.be

    Keywords: Forced oscillations, guidelines,respiratory impedance, respiratory mechanics,respiratory resistance, standardisation

    Received: August 4 2003Accepted: August 6 2003

    CONTENTS

    Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . 1027

    Potential and limitations . . . . . . . . . . . . . . . . . 1027

    Respiratory impedance . . . . . . . . . . . . . . . . . . 1027Measurement arrangements . . . . . . . . . . . . . . . 1027

    Oscillation frequencies . . . . . . . . . . . . . . . . . . . 1027

    Recommendations for measurements . . . . . . . . . . . 1028

    Set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1028

    Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . 1028

    Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . 1028

    Input signals. . . . . . . . . . . . . . . . . . . . . . . . . . 1028

    Signal processing. . . . . . . . . . . . . . . . . . . . . . . 1029Report of results . . . . . . . . . . . . . . . . . . . . . . . 1029

    Measurement conditions . . . . . . . . . . . . . . . . . 1029

    The upper airway artefact . . . . . . . . . . . . . . . . . . 1029

    Clinical applications . . . . . . . . . . . . . . . . . . . . . . 1030

    Reference values . . . . . . . . . . . . . . . . . . . . . . . 1030

    Reproducibility . . . . . . . . . . . . . . . . . . . . . . . . 1030

    Diagnostic capacity . . . . . . . . . . . . . . . . . . . . . 1031Follow-up and field studies . . . . . . . . . . . . . . . 1032

    Identification of airway reactivity . . . . . . . . . . . 1033

    Reversibility . . . . . . . . . . . . . . . . . . . . . . . . . . 1033

    Bronchial hyperresponsiveness . . . . . . . . . . . . . 1034

    Forced oscillation technique in

    infancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1035

    New developments . . . . . . . . . . . . . . . . . . . . . . . . 1035

    Applications of the forced oscillation technique inmonitoring respiratory mechanics . . . . . . . . . . 1035

    Low-frequency oscillations. . . . . . . . . . . . . . . . 1035

    High-frequency oscillations . . . . . . . . . . . . . . . 1035

    Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1036

    As a tool for the investigation of respiratory mechanicsin clinical practice, the forced oscillation technique (FOT) iswell supported theoretically and has the advantage of beinga noninvasive, versatile method and demanding minimal

    cooperation of the patient. The most attractive feature ofFOT is that the forced oscillations are superimposed on thenormal breathing, avoiding the need for any special breathingmanoeuvre or any noticeable interference with respiration.

    Eur Respir J 2003; 22: 10261041DOI: 10.1183/09031936.03.00089403Printed in UK all rights reserved

    Copyright #ERS Journals Ltd 2003European Respiratory Journal

    ISSN 0903-1936

  • During the past decade, advances in basic research and FOTapplications, as well as new developments in technology, haveevoked new interest from both the clinical and industrialfields. To address demands for further standardisation ofFOT, a European Respiratory Society Task Force wasestablished to update the standardisation work carried outduring the Commission of the European CommunitiesBiomedical Engineering Advisory Committee (COMAC-BME) programme of respiratory impedance (Zrs) measure-ment development [1], and to develop clinical guidelines forZrs measurement. The present report summarises the mostimportant underlying concepts of the FOT, offers guidelinesfor its implementation and use in the clinical environment,and gives a brief overview on the latest developments ofpotential clinical impact.

    Methodology

    Since the first FOT measurements by DUBOIS et al. [2],numerous variants of the FOT have been developed in termsof measurement configuration, oscillation frequencies andevaluation principles. This short review is focussed on theroutine clinical applications, addressing the most basicconcepts only, and the reader is referred for more detailedinformation to monograph articles [37].

    Potential and limitations

    The essence of the FOT can be elucidated by contrasting itsprinciple with that of the respiratory mechanical measure-ments that depend on spontaneous breathing activity orrespiratory manoeuvres. Uniquely, for the FOT, externaldriving signals (i.e. forced oscillations) are used to determinethe mechanical response of the respiratory system and theinvestigator uses specifically developed forcing waveforms toexplore the respiratory mechanical properties, relying on thewell-developed arsenal of linear system analysis. FOT thuspossesses solid theoretical foundations and a high degree ofversatility, which are far beyond the capability of conven-tional respiratory mechanical tests. However, the requirementof linearity necessitates the use of small-amplitude oscilla-tions, which may leave undisclosed some energetically andfunctionally important nonlinear properties that manifestduring tidal breathing, and assumes methodological rigor inboth data collection and analysis.

    Respiratory impedance

    The key concept of the forced oscillatory respiratorymechanics is the "impedance" (Z), the spectral (frequencydomain) relationship between pressure (P) and airflow (V9)(see Appendix). In simple terms, Z can be conceived as ageneralisation of resistance, since it embodies both the in-phase and out-of-phase relationships between P and V9. Thein-phase component is called the real part of Z (or resistance(R)), whereas the out-of-phase relationship is expressed bythe imaginary part (or reactance (X)), and both appear asfunctions of the frequency of oscillation (f). In other words, Rdescribes the dissipative mechanical properties of the respira-tory system, whereas X is related to the energy storagecapacity and thus determined jointly by the elastic properties(the relationship between P and volume) dominant at lowoscillation frequencies and the inertive properties (therelationship between P and volume acceleration), whichbecome progressively more important with increasing f.

    Measurement arrangements

    Depending on the sites of the P and V9 measurements andof the application of the forced oscillations, different kinds ofimpedance of the respiratory system can be defined. Mostcommonly, the forced oscillations are applied at the airwayopening, and the central airflow (V9ao) is measured with apneumotachograph attached to the mouthpiece, face mask orendotracheal tube (ETT). Pressure is also sensed at the airwayopening (Pao) with reference to body surface (in this case,atmospheric) pressure (Pbs). The input impeda