Chest. Sep;(3) doi: /chest Impulse oscillometry: interpretation and practical applications. Bickel S(1), Popler J(2), Lesnick. Impulse oscillometry (IOS), a simple, noninvasive method using the forced oscillation technique, requires minimal patient cooperation and is suitable for use in. This improvised technique of FOT that could use multiple sound frequencies at one time was called the impulse oscillometry system (IOS). The temporal.

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Impulse oscillometry IOS is a variant of forced oscillation technique, described by Dubois over 50 years ago, which permits passive measurement of lung mechanics. In this method, sound waves are superimposed on normal tidal breathing, and the disturbances in flow and pressure caused by the external waves are used to calculate parameters describing the resistance to airflow and reactive parameters that mostly relate to efficient storage and return of energy by the lung.

It requires minimal patient cooperation and can be done impulsee in subjects who are unable to perform spirometry. Importantly, IOS can differentiate small airway obstruction from large airway obstruction and is more sensitive than spirometry for peripheral airway disease. It has im;ulse used to study various respiratory disorders, especially asthma and is suitable for measuring bronchodilatory response as well as bronchoprovocation testing.

IOS parameters seem to be able to pick up early changes in lung functon such that they are superior to spirometry in predicting loss of control in asthmatic patients and possibly in identifying early airway disease in smokers.

Such comparisons, especially for chronic obstructive pulmonary disease, are made difficult by widespread use of spirometric parameters as the diagnostic gold standard.

Here, we discuss the principles and technique of IOS and review its application in obstructive airway diseases. Mechanical properties of the lung are important determinants as well as indicators of lung function and thus help in the diagnosis and monitoring of several lung disorders.

Spirometry, the most commonly performed lung function test in clinical practice, is considered to be the gold standard diagnostic test for OAD. However, the forceful expiratory and inspiratory maneuvers of spirometry require patient cooperation and physical capacity that is usually lacking in young children below 4 years age, elderly, and those with physical and cognitive limitations.

Other than technical difficulties, there are also some fundamental limitations. However, this too is shown to be inadequate. Importantly, the presence of small airway disease was firmly established using IOS in this group.

IOS is a noninvasive method, which uses sound waves to measure respiratory mechanics. It is based on the principle of forced oscillation technique FOTfirst described by Dubois et al.

This does not require any effort from the subject and hence is feasible to do in many situations like in children, very elderly people, in subjects who are on ventilators, who underwent surgery or when oscilloetry related bronchospasm is a concern.

The second advantage is that IOS can detect subtle changes in the small airway function even in the setting of normal spirometry, as illustrated above, thus providing valuable information for early diagnosis and monitoring of airway diseases.

In this review, we describe the principles of IOS and discuss the progress in adopting this relatively new methodology in OAD. In FOT, the sound waves, generated with the help of a loudspeaker are transmitted into the lungs of the subject.

These sound waves, which oscollometry essentially pressure waves, cause changes in the pressure and this change in pressure drives changes in impuls.

By measuring the magnitude of change in the pressure and flow, one can determine the mechanical properties of the lung. Waves of lower frequencies travel deep into lungs till alveoli and are reflected back while those of higher frequencies are reflected from the larger airways. Thus, the parameters calculated at different frequencies give measures of different regions in the lungs. The main difference is that in FOT, the sound waves of different frequencies were transmitted sequentially, whereas in IOS, an impulse, which can be mathematically decomposed into different impukse, is transmitted.

This helps in reducing the time of test and also provides a high signal to noise resolution. An impulse consisting of a mixture of sound waves of different frequencies is generated by the loud speaker at the mouth. As this wave passes into the lungs, it causes changes in pressure as well as in the flow of air.


The frequencies of the waves delivered in IOS ranges from 5 osillometry 30 Hz. A pressure transducer and a pnuemochromatograph are present at the mouthpiece, to measure the pressure and flow, respectively [ Figure 1 ]. During testing, the Subject should be in sitting position with the head in neutral or slightly extended position with a nose clip.

A technician or the subject should firmly support the cheeks of the subject during measurement. Impulse oscillometry system showing loud speaker Ascreen flap BY-adapter Cpnuemochomatograph Dmouth piece E and subject wearing nose clip and supporting cheeks with the hands F. For elucidation of the mathematical aspect of this technique, let us consider a simplistic simulated scenario in which, sound waves at two frequencies 5 Hz and 20 Hz were passed into the lung sequentially.

Flow recordings before the superposition of the waves show normal airflow during tidal breathing [ Figure 2c impluse. When the sound waves are overlapped on the tidal breathing, they result in a change in the flow and now flow recording shows a complex signal consisting of both respiratory and sound wave induced components, i. These are separated using baseline approximation technique. In short, straight line segment is inserted between the start and end points of the flow recordings due to each wave osckllometry consider this as the baseline.

The same procedures apply to the recordings of pressure, and now, we have the recordings of flow and pressure with respect to time. These recordings in time scale have to be converted into the frequency scale to further calculate the parameters of our interest.

Impulse oscillometry: interpretation and practical applications.

Fast Fourier transform, a mathematical technique is used to convert this time scale to frequency scale. Respiratory input impedance Zrs is calculated as the ratio of the resulting pressure and flow changes due to the external pressure waves.

Elucidation of Impulse oscillometry methodology. Sine waves at 5 Hz a and at 20 Hz b.

Flow recording of normal tidal breathing c. Flow recording when tidal breathing is superimposed by 5 Hz waves d and 20 Hz waves e. In IOS, rather than sending the pressure waves of different frequencies sequentially, an impulse that mathematically consists of all frequencies from 5 Hz to 30 Hz is sent into the lungs. The disadvantage is that the impulse used in the IOS can be a little forceful to the subject when compared to the gentler plain sinusoidal waves of FOT and may even change the lung mechanics slightly.

In addition, if one desires to track within-breath changes of Zrs, the discontinuous nature of IOS can reduce temporal resolution. However, the advantages overcome these limitations for most pulmonologists. First, with IOS we can calculate the impedance at every frequency from 5 to 30, whereas, in FOT, we can calculate only at the frequencies of sine waves we use. Second, this results in improved signal to noise ratio and makes it a better tool for detecting regional abnormalities that have small effects on lung mechanics.

Last but not least, this also decreases the duration of the test. Together, this leads to increased efficiency for diagnostic applications in a PFT laboratory.

Respiratory impedance is the sum of all forces which oppose the generated impulse. Impedance measured at any frequency is the ratio of the difference in pressure and changes in the flow at that frequency. Depending on the region where the pressure is measured, the impedance varies.

For example, pressure difference at the mouth and in the alveoli gives impedance of the airways and the difference at the mouth, and pleural pressures give a total impedance of the lung. In IOS, the pressure measured at the mouth is compared to atmospheric pressure, which is the pressure outside the chest wall. This defined as respiratory system Zrs and includes the in-phase real component which is the resistive component Rrs and an out-of-phase imaginary component which is a reactive component Xrs.

Simply put, Rrs can be viewed as the energy dissipation whereas Xrs as energy storage. Since IOS measures input impedance, abnormalities of chest wall and skeletal muscles will also be reflected in the measurement. The resistance derived from impedance includes the resistance due to central airways, peripheral airways, lung tissue, and chest wall, although the latter two are usually negligible.


However, in children the contribution of small airways is higher than in adults. Resistance is independent of the frequency in healthy subjects.

In central airway obstruction, the resistance at all frequency increases, while, in small airway obstruction, the resistance at lower frequencies increases but is unchanged at higher frequencies that do not reach the small airways [ Figure 3a ].

Impulse oscillometry: interpretation and practical applications.

This frequency dependency of resistance might be normal in children, but indicates small airway obstruction in adults. Measurements of resistance a. Bold, long-dash and short-dash lines represent measurements in normal, central airway obstruction and peripheral airway obstruction respectively. Reactance includes two components, the inertia of the air column to move inertance and the capacitance of the lung.

Capacitance can be interpreted as a property which reflects elasticity of the lung. The capacitance component of the reactance is defined to be negative in sign and inertance is defined as positive. Unlike resistance, reactance is frequency dependent. Since, the oscillometrh properties of lungs majorly reside at the periphery, at low frequencies, the capacitance component dominate, and total lung reactance is negative, whereas, at higher frequencies, the inertia of the air column in larger airways dominates making the total reactance positive [ Figure 3b ].

Importantly and perhaps counterintuitively, here elastance or capacitance refers to energy return properties of the lung, similar to electric circuits, not stiffness during inflation — the more intuitive definition for clinicians. Therefore, in either fibrosis or emphysema or small airway disease, the reactance at lower frequencies would change in the same direction, i. Osciplometry, the direction of change of reactance does not differentiate between obstructive or restrictive diseases.

Resonant frequency fres is defined as the frequency at which the inertial properties of airway and the capacitance of lung periphery are equal [ Figure 3b ], i. We cannot attribute fres to a specific mechanical property of lungs, but it can be used to separate low frequencies where capacitance component dominates impuulse high frequencies where the inertial component takes over.

The normal value of fres in adults is Hz. In children, it is higher and increases with decreasing age. In lung diseases, both obstructive and restrictive, fres is increased above normal.

This is because of reactance becoming more negative at low frequencies in each of these diseases, as discussed above. Area of reactance AX includes the area under the reactance curve from lowest frequency to the fres.

It is the area between X-axis below zero and reactance curve in the graph [ Figure 3b ]. It includes the total area dominated by the capacitance and reflects the elastic properties of the lung.

As seen with reactance and fres, this also increases in any disease of lung periphery. AX is a single measurement that summarizes the above parameters and is also shown to be correlated with resistance at lower frequencies.

Coherence is another important parameter and is used to determine the validity and quality impupse the test results. It reflects the reproducibility the impedance measurements. However, it is important to note that these values are for adults, and there are no standard values reported in children. Coherence can be decreased because of improper technique, irregular breathing, glottis closure, and swallowing.

Normal values for adult and pediatric population are essential for easy interpretation of the test.

However, studies that aim to determine predictive equations for IOS parameters were few worldwide, and there were no studies done on the Indian population. There is an urgent need for conducting such studies in many areas around the globe, including India. Among those studies, many were done in children. All those studies found standing height as the single most important parameter which helps in predicting resistance. Age is also shown to have strong correlation with resistance and reactance values.

The resistance and Frequency of resonance decrease with increasing height and age, whereas, reactance increases.