Review
Impulse oscillometry in the evaluation of diseases of the airways in children

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Objective

To provide an overview of impulse oscillometry and its application to the evaluation of children with diseases of the airways.

Data Sources

Medline and PubMed search, limited to English language and human disease, with keywords forced oscillation, impulse oscillometry, and asthma.

Study Selections

The opinions of the authors were used to select studies for inclusion in this review.

Results

Impulse oscillometry is a noninvasive and rapid technique requiring only passive cooperation by the patient. Pressure oscillations are applied at the mouth to measure pulmonary resistance and reactance. It is employed by health care professionals to help diagnose pediatric pulmonary diseases such asthma and cystic fibrosis; assess therapeutic responses; and measure airway resistance during provocation testing.

Conclusions

Impulse oscillometry provides a rapid, noninvasive measure of airway impedance. It may be easily employed in the diagnosis and management of diseases of the airways in children.

Introduction

Asthma is reported to be the most common chronic respiratory disease in children, and its prevalence appears to be high worldwide.1 Accordingly, significant efforts have been made to develop noninvasive techniques that accurately measure lung function in children. Examples of such techniques include spirometry, plethysmography, interrupter technique (discussed later), tidal breathing measurements, multiple-breath inert gas washout technique, forced oscillation technique (FOT) and impulse oscillometry (IOS).2, 3, 4 The forced oscillation technique is the general name for airway mechanic measurements using the noninvasive superimposition of pressure fluctuations on the airway over the subject's normal, quiet, tidal breathing. More than 50 years ago, FOT was first determined by Dubois et al5 and has developed with regard to configuration, standardization, and application. Impulse oscillometry is one type of FOT. Other techniques of FOT use only one frequency or change the frequency “pseudo randomly.” Impulse oscillometry delivers a regular square wave of pressure 5 times per second, which has the advantage of generating a larger sample during measurements and emitting a continuous spectrum of frequencies that may provide more detailed characterization of respiratory function.6 Impulse oscillometry has been used in adults as well as in preschool children to identify lung dysfunction, such as in asthma.3, 4 It is noninvasive, easy to perform, and requires only passive patient cooperation.3 IOS systems are finding increased use in the clinical diagnostic testing of patients with airway hyperreactivity and airway obstruction. Impulse oscillometry also may be used during challenge and provocation testing. The IOS apparatus generates small pressure oscillations that are applied at the mouth and transmitted into the lungs, to help determine the impedance (Zrs) of the respiratory system.7 Pulmonary resistance and reactance are the key components of impedance8 and are measured and graphically displayed.

The purpose of this article is to provide a comprehensive overview of the application of IOS to the evaluation of respiratory disease in children. A complete search in Medline and PubMed was performed for articles on impulse oscillometry, in peer-reviewed journals, using the key words forced oscillation, impulse oscillometry, and asthma. The articles included in this study were based on the expert opinions of the authors.

Section snippets

Measurement Parameters

The IOS apparatus generates pressure oscillations at the mouth that propagate via movement of the air column in the conducting airways, which is followed by distension and recoil of the elastic components of lung tissues and creation of backpressure. The pressure oscillations are applied at a fixed (square wave) frequency of 5 Hz, from which all other frequencies of interest are derived. Low-frequency signals (5 Hz) penetrate out to the lung periphery, whereas high-frequency signals (20 Hz)

Comparison of Lung Patterns Using IOS and Spirometry in Pathologic Conditions

Figure 3 illustrates different patterns of abnormalities that may be identified using IOS in comparison with those patterns seen with spirometry. As mentioned, distal obstructive diseases, such as asthma and chronic bronchitis, result in a frequency-dependant increase in resistance (high R5–R20) because the pressure signal wave propagating out to the lung periphery (R5) encounters greater resistance than the more proximal higher-frequency (R20) impulse. Distal obstruction also results in a

Clinical Applications of IOS

Impulse oscillometry is used to diagnose, evaluate disease severity, and assess therapeutic responses20, 21, 22 in chronic lung diseases23 such as asthma3 and cystic fibrosis.24 Impulse oscillometry may help distinguish between asthma, chronic bronchitis, and emphysema based on differences in pulmonary resistance, frequency dependence of resistance, and pulmonary reactance.25 It also has been used to determine lung function in individuals with stable asthma and during provocation by

Comparison With Other Techniques

In addition to IOS, a number of techniques are routinely used to assess lung dysfunction. These include standard spirometry as mentioned,34 body plethysmography,35 and the interrupter technique,36 as well as tidal breathing measurements and multiple-breath inert gas washout technique. Standard spirometry continues to be the mainstay in the clinical assessment of lung function of school-aged children and adults. The technique is well known and easily performed, and standards of performance and

Clinical Scenarios

Impulse oscillometry may provide objective information that is beneficial to patient care in cases in which spirometry was either normal or could not be performed. Figure 4 outlines several instructive cases of children that were evaluated in the National Institutes of Health Pediatric Allergy Clinic. Panel A shows the impedance curves from a 3-year-old white boy with a history of food-induced systemic reactions and skin test positivity to wheat, egg, peanuts, and tree nuts; and who suffered

Conclusions

Impulse oscillometry is a noninvasive, rapid, safe and validated technique that measures respiratory impedance that is used as an indicator of lung function. It requires minimal cooperation from the subject and is therefore of great utility in preschool children, as well as in older children and adults. IOS may be used to diagnose, evaluate, and determine treatment response in those with asthma or other pulmonary disease states, with an accuracy and reproducibility comparable to spirometry and

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    Disclosures: Authors have nothing to disclose.

    Funding Source: This work was supported by the DIR Intramural Research Program of the NIAID.

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