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 impedance of therespiratory system (Zrs,in) is then the spectral (frequencydomain) relationship between transrespiratory pressure(Prs=Pao-Patm) and V9ao: Zrs,in(f)=Prs(f)/V9ao(f). When Zrsis partitioned into pulmonary (ZL) and chest wall impedance(Zw) on the basis of the measurement of intraoesophagealpressure (Pes), ZL and Zw are obtained from ZL=(Pao-Pes)/V9ao and Zw=(Pes-Pbs)/V9ao, respectively. A special version ofthe input FOT is the head generator technique, where Pao isapplied around the head, in order to minimise upper airwaywall shunting [8]. An alternative instrument that can be usedto estimate Zrs,in and that does not require the recording offlow (V9), is a wave tube connecting the source of forcedoscillations (usually a loudspeaker) and the subject; Zrs,in ismeasured as the load impedance on the wave tube, on thebasis of the geometric and physical properties of the tube andthe inside air, and the pressure recorded at the inlet and outletof the tube [9]. Transfer impedance is obtained when theoscillations are imposed and P and V9 are measured atdifferent sites of the respiratory system; accordingly, variousmeasurements of transfer impedance can be instrumented.However, if the impedance of the total respiratory system(Zrs,tr) is considered, either the oscillatory excitation at theairway opening is combined with the plethysmographicmeasurement of output, "body surface" flow, or the oscilla-tions are imposed in a "head-out" plethysmograph on thebody surface, with the measurement of V9ao. As Zrs,in andthese two variants of Zrs,tr are affected differently by theparallel elements of the respiratory system, such as alveolargas compressibility and upper airway wall movements, theycan be selected or combined to obtain more reliable estimatesof the airway and tissue impedance. The present review isrestricted to the most easily implementable FOT, namely Zrs,in.

    Oscillation frequencies

    For routine clinical applications of FOT it is usual to applya medium frequency range, i.e. the imposed oscillations startfrom 24 Hz, roughly 1 decade above the spontaneous breath-ing rate, and extend up to a few times 10 Hz. In this frequencyrange, the healthy respiratory system exhibits a largely frequency-independent respiratory resistance (Rrs) whose major compo-nent is airway resistance (Raw) (fig. 1). Respiratory reactance(Xrs) undergoes the transition from negative values (when theelastic reactance dominates) to positive values increasing withf (the dominance of inertial reactance). At the characteristicresonant frequency (fres), where Xrs crosses zero, the elasticand inertial forces are equal in magnitude and opposite. Thelow-frequency oscillations include the frequencies of sponta-neous breathing and, accordingly, can be applied duringapnoeic conditions only, whereas the high-frequency rangecontains oscillations up to several 100 Hz. Use of low-frequency and high-frequency forced oscillations reveals dif-ferent mechanical properties of the respiratory system, andthese techniques are promising as lung function test methods;for this reason they are considered in the "New developments"section. The present section of this report focuses on the mostcommonly used medium-frequency range FOT.

    1027CLINICAL APPLICATION OF FOT

  • Both single-frequency and composite signals have beenused in clinical practice. When the FOT is applied to explorethe patterns or mechanisms of frequency dependence of Zrs inhealth and disease, the simultaneous application of severalfrequency components, i.e. the use of composite signals, suchas pseudorandom noise or recurrent impulses, is preferred.The single-frequency FOT may be used in the tracking ofrelatively rapid changes in Zrs, e.g. those occurring within therespiratory cycle, or as an accessory device for monitoringairway patency, and it may also be useful in the evaluation ofchanges in the bronchomotor tone.

    Recommendations for measurements

    Set-up

    The subject is connected via a mouthpiece to the set-up thatmost commonly utilises a loudspeaker to deliver the forcedoscillatory signal (fig. 2). The P and V9 signals are measurednext to the mouthpiece. To enable spontaneous breathing ofthe subject, a shunt pathway open to the atmosphere isnecessary; this is usually a wide-bore side tube (with a highimpedance to present a small leak for the high oscillatoryfrequencies and a low resistance against spontaneous breath-ing) placed in parallel to the loudspeaker. A mechanicalresistor may also be used for this purpose. A bias flow to flush

    the dead space is optional and can preferably be introducedbetween the loudspeaker and the pneumotachograph. When abacterial filter is placed between the set-up and the patient forhygienic purposes, the measured Zrs should be corrected forthe impedance of this filter. The dead space of the filter shouldbe minimal to avoid shunting effects at high Zrs.

    Apparatus

    The FOT system should impose a load against spontaneousbreathing ofv0.1 kPa?s?L-1 below 5 Hz. When using compositesignals, the loudspeaker should be able to develop a peak-to-peak pressure variation of 0.2 kPa at the airway opening. Thelargest P developed in the system should not exceed 0.5 kPa.

    The differential pressure transducer used for flow measure-ment together with its connections to the pneumotachographshould be symmetrical and of low compliance, providing...

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