Impulse Oscillometry: Interpretation & Applications

[ Topics in Practice Management ]
Impulse Oscillometry
Interpretation and Practical Applications
Scott Bickel, MD; Jonathan Popler, MD, FCCP; Burton Lesnick, MD, FCCP; and Nemr Eid, MD, FCCP
Simple spirometry and body plethysmography have been routinely used in children aged
. 5 years. New techniques based on physiologic concepts that were first described almost
50 years ago are emerging in research and in clinical practice for measuring pulmonary function in children. These techniques have led to an increased understanding of the pediatric lung
and respiratory mechanics. Impulse oscillometry (IOS), a simple, noninvasive method using
the forced oscillation technique, requires minimal patient cooperation and is suitable for use
in both children and adults. This method can be used to assess obstruction in the large and
small peripheral airways and has been used to measure bronchodilator response and bronchoprovocation testing. New data suggest that IOS may be useful in predicting loss of asthma
control in the pediatric population. This article reviews the clinical applications of IOS, with an
emphasis on the pediatric setting, and discusses appropriate coding practices for the clinician.
CHEST 2014; 146(3):841-847
ABBREVIATIONS: ATS 5 American Thoracic Society; AX 5 area of reactance; BPD 5 bronchopulmo-
nary dysplasia; FEF25%-75% 5 forced expiratory flow, midexpiratory phase; Fres 5 resonant frequency;
IOS 5 impulse oscillometry; R5 5 resistance at 5 Hz; R10 5 resistance at 10 Hz; R20 5 resistance at
20 Hz; X5 5 reactance at 5 Hz
Pulmonary function testing is used to evaluate
respiratory mechanics and physiology in both
children and adults with suspected respiratory
diseases. Spirometry is perhaps the most commonly used pulmonary function test with the
advantage of being readily available in both
inpatient and outpatient settings, including
many primary care offices. Lung volume measurement by body plethysmography or gas
dilution requires more-expensive equipment
that often requires a dedicated pulmonary
function testing laboratory. Simple spirometry
and body plethysmography often can be performed successfully in children but may be
Manuscript received August 11, 2013; revision accepted January 13,
2014.
AFFILIATIONS: From the Department of Pediatrics (Drs Bickel and
Eid), University of Louisville, Louisville, KY, and Georgia Pediatric Pulmonology Associations, PC (Drs Popler and Lesnick), Atlanta, GA.
CORRESPONDENCE TO: Nemr Eid, MD, FCCP, Department of Pediatrics,
University of Louisville, 571 S Floyd St, Ste 414, Louisville, KY 40202;
e-mail: [email protected]
journal.publications.chestnet.org
limited by the child’s ability to follow directions and provide maximal, reproducible
efforts. Some of the challenges in performing
spirometry in younger children, especially
those aged 2 to 5 years, may have led to this
diagnostic modality being significantly underused.1,2 Current data suggest that only 21% of
primary care practitioners use spirometry in
the diagnosis of asthma in children.3
Impulse oscillometry (IOS) is an effortindependent modality based on the welldescribed forced oscillation technique4,5
and has emerged as a method to measure
© 2014 AMERICAN COLLEGE OF CHEST PHYSICIANS. Reproduction of
this article is prohibited without written permission from the American
College of Chest Physicians. See online for more details.
DOI: 10.1378/chest.13-1875
841
pulmonary function in both preschool- and school-aged
children in whom reliable spirometry is difficult to
obtain.6 Similarly, adults with both physical and cognitive limitations may benefit from this methodology.7
This article reviews the clinical use of IOS, potential
limitations, and appropriate coding practices for the
clinician.
IOS Fundamentals
IOS is a simple, noninvasive method requiring only passive patient cooperation that allows for the evaluation of
lung function through the measurement of both airway
resistance and airway reactance.1,8 Current IOS procedures are based on the physiologic concepts of the forced
oscillation technique originally described by Dubois et al4
in 1956. IOS uses sound waves to rapidly detect airway changes and requires only normal tidal breathing
from the patient. Pulmonary mechanics are determined
by superimposing small external pressure signals on the
spontaneous breaths of the patient. When analyzed,
these pressure signals separately quantify the degree of
obstruction in the central and peripheral airways. Forced
oscillation works by having a loudspeaker generate harmonic sound waves that flow through a conduit tube and
mouthpiece into the respiratory tract of the patient. In
forced oscillometry, these sound waves can be of single
or multiple frequencies, which usually range from
between 2 and 4 Hz to between 30 and 35 Hz.9,10 IOS is a
form of the forced oscillation technique in which the
pressure oscillations are applied at a fixed (square wave)
frequency of 5 Hz and from which all other frequencies
of interest are derived.8 Pressure and flow transducers
measure amplitude and phase differences to determine
the impedance of the respiratory system.
The impulses generated by the loudspeaker travel superimposed on normal tidal breathing through the large
and small airways, with higher frequencies reflecting
back from the large airways to the mouth and lower
frequencies traveling deeper into the lung before returning. A pressure and flow transducer measures inspiratory
and expiratory flow and pressure. The resultant signals of
pressure and flow are separated from the breathing pattern by signal filtering. Respiratory impedance is the sum
of all the forces (resistance and reactance) opposing the
pressure impulses (oscillations) and is calculated from the
ratio of pressure and flow at each frequency.9,10
Interpreting IOS results requires one to be familiar with
the specific attributes of the test. Resistance is the
in-phase component of lung impedance and reflects information about the forward pressure of the conducting
842 Topics in Practice Management
airways, whereas the reactance is the out-of-phase component of lung impedance and reflects the capacitive
and inertive properties of the airways. Capacitance can
be thought of as reflecting the elasticity of the airway,
whereas inertance reflects the mass-inertive forces of the
moving air column.10 Reactance can be viewed as the
rebound resistance, or an echo, giving information
about the distensible airways.4,10 At low frequencies,
capacitive pressure loss is large compared with inertive
pressure loss, whereas at higher frequencies, the inertive
properties dominate. Therefore, the reactance at 5 Hz (X5)
reflects the combined effect of tissue elastance and inertance, although at this lower frequency, the effect of
tissue elastance would dominate. Because the ability of
the lungs to store capacitive energy is primarily manifest
in the small airways, reactance at low frequencies can
provide important information about the distal airways.
Although the mathematical model behind resistance
and reactance has been described in detail elsewhere9,11,12
and is beyond the scope of this article, it is helpful to
understand that inertance represents a positive value
and capacitance a negative value. States that reduce the
elasticity of the lung, such as fibrosis and hyperinflation,
make capacitance increasingly negative.10
The intermediate frequency at which the total reactance
is null is known as the resonant frequency (Fres) and is
illustrated by a sample IOS result in Figure 1. The Fres
occurs when the magnitudes of the capacitive and
inertive pressure loss are the same. This value can be of
use in discriminating between low-frequency and highfrequency reactance values: Below the Fres, the elastic
properties of the lung (represented by capacitance)
dominates, whereas above the Fres, inertance dominates.10 The Fres tends to be higher in children, to
decrease with age, and to be elevated in both restrictive
and obstructive states.
The area of reactance (AX) is another common parameter used in interpreting IOS. AX represents the total
reactance (area under the curve) at all frequencies
between 5 Hz and Fres (Fig 1).10,13 This single value,
therefore, comprises all the frequencies measured by
IOS where the elastic properties of the lung (again, represented by capacitance) dominates over inertance. As
with X5, this value also provides information regarding
peripheral airway obstruction.
The resistance at 5 Hz (R5) represents the total airway
resistance, and the resistance at 20 Hz (R20) represents
the resistance of the large airways. One can infer the
resistance of the small airways by subtracting R20 from
[ 146#3 CHEST SEPTEMBER 2014 ]
neous intrathoracic pressure.8 The procedure takes
approximately 20 to 90 s to complete.
Clinical Applications of IOS
Figure 1 – Sample impulse oscillometry result illustrating key impulse
oscillometry attributes. F 5 frequency; R5 5 resistance at 5 Hz;
R20 5 resistance at 20 Hz; X5 5 reactance at 5 Hz.
R5, which can be used with X5, Fres, and AX to reflect
changes in the degree of obstruction in the peripheral
airways. Airway resistance decreases with age. In
patients with small airways disease, changes in resistance at low frequencies (R5) become apparent.3,13
Coherence is an important IOS parameter for the practitioner to recognize and review in interpreting the
validity of IOS results. Coherence, a value between
0 and 1, reflects the reproducibility of impedance measurements and is based on a comparison between the
airflow entering the lungs and the pressure wave
reflected back from the respiratory system.8 To ensure
accurate testing, coherence at 5 Hz should ideally be
. 0.8 cm H2O, with coherence at 20 Hz between 0.9
and 1.6,14 Coherence is decreased by improper technique,
swallowing, glottis closure, obstruction of airflow by the
tongue, or irregular breathing.8 It is important to note
that coherence cutoff values have not been validated in
younger children.8,15
IOS Methodology
IOS can be performed in an inpatient or outpatient
setting. The device should be calibrated daily as
directed by the manufacturer.15 IOS is typically performed with the patient sitting and breathing at tidal
volume, the head held in neutral position, a nose clip
in place, and the cheeks firmly supported by either
the patient or another individual such as the examiner or caregiver.9 This positioning is important to
limit the influence of the compliance of the cheeks
and prevent shunting of the applied impulses through
the upper airway.9,16 The maneuver should also be
performed with the legs uncrossed to reduce extra-
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Because IOS does not require the forced expiratory
maneuvers needed to generate spirometric data, it can
be easily used in the pediatric population17,18 and in
adults who may be too weak or otherwise unable to
perform spirometry. IOS may also be useful where
spirometry is contraindicated, such as in patients who
recently underwent surgery or who have had recurrent
pneumothoraces or in cases where spirometry-related
bronchospasm is a concern.17 IOS has been used in both
adult and pediatric patients for the diagnosis of airway
hyperreactivity and airway obstruction14,19 and may be
used during bronchoprovocation challenges.20 As with
spirometry, IOS values are correlated with clinical
symptoms and asthma control,18 although an advantage
of IOS may be the detection of subtle changes in a
patient’s airway function earlier than with conventional
spirometry.17,21 Some data suggest that IOS can be used
to assess abnormal distal airway function, even in the
setting of normal spirometry.21,22 Indeed, Shi et al13
found that neither forced expiratory flow, midexpiratory phase (FEF25%-75%) nor FEV1 was as effective as small
airway IOS indexes in detecting poorly controlled
asthma in children.
IOS has been studied in a number of disease states,
including asthma, COPD,23 cystic fibrosis,24-26 bronchopulmonary dysplasia (BPD),27 OSA,28,29 central airway
obstruction,30 adult interstitial lung disease,31 and occupational and environmental irritant exposure.21,32 IOS
has been found to be useful in measuring response to
bronchodilators, such as salbutamol and ipratropium, in
patients with asthma and COPD.33-35
The American Thoracic Society (ATS) recently released
a statement summarizing optimal pulmonary function
tests in children age , 6 years.36 Regarding cystic
fibrosis and IOS, the ATS noted limited data, with
recent studies not demonstrating a change in airway
resistance or reactance in young children.25 Studies thus
far have been unable to establish an association between
IOS parameters and ongoing infections or increased
coughing.24-26
The forced oscillation technique has been used for several decades in patients with BPD.27 The ATS36 noted
one study indicating that children with a history of BPD
have elevated resistance, decreased reactance, and
higher Fres.37 Udomittipong et al38 demonstrated in a
multivariate analysis that a significant relationship exists
843
between length of oxygen requirement and a patient’s
IOS parameters.
Although studied in other disease states, IOS has perhaps been best characterized to date for its use in
asthma. Studies have demonstrated that patients with
asthma have an elevated R5,39-41 an elevated AX,39,41 an
elevated Fres,39,41 and a more negative X539-41 compared
with control subjects. Additionally, improvement in airway resistance after bronchodilator administration appears
to correlate well with a bronchodilator response by standard spirometry.40 When using IOS to assess a patient
with asthma, one should look at multiple parameters. A
change in R5 by 30% to 35% has been found to reflect a
positive response to bronchodilators.14,42 Komarow et al17
reported that resistance at 10 Hz (R10) and AX had the
best profiles based on the receiver operating characteristic for detecting a significant bronchodilator response.
A 28.6% change in R10 and a 229.1% change in AX
were reported as the optimal cutoff points.
IOS has also been used in bronchoprovocation testing.
A typical response will show an increase in R5, R20, and
Fres with a decrease in X5.43 Bailly et al43 reported that
X5 was the only parameter sensitive enough to detect
bronchial hyperreactivity and that a 20% decline in
FEV1 correlated with a 50% decrease in X5. Other
studies have reported similar findings,44,45 whereas
Vink et al46 also demonstrated a correlation between
decreased FEV1 and increased R5 and R10. When used
with IOS, lower doses of bronchoprovocative agents are
required to induce measurable and significant bronchoconstriction.47,48 Indeed, Schulze et al49 showed significant increases in resistance well before a response was
seen in FEV1 at lower doses of methacholine, suggesting
that oscillation techniques are more sensitive than
spirometry.
One area of difficulty in the management of pediatric
asthma is the prediction of loss of disease control.50
Some pediatric patients may have difficulty verbalizing
respiratory symptoms, whereas others may have difficulty perceiving a change in their respiratory status.51
Objective parameters to predict loss of asthma control,
including traditional spirometry and exhaled nitric
oxide levels, do not appear to accurately reflect a decline
in asthma control.52-54 In a recent study, Shi et al50 demonstrated that children with controlled asthma who
have increased peripheral airway IOS indexes are at risk
for losing asthma control, which suggests that monitoring small airway function by IOS can be useful in
identifying patients who are at risk for losing asthma
844 Topics in Practice Management
control and in assisting with clinical decisions and
treatment plans. Regarding treatment strategies,
Rabinovitch et al55 recently published data correlating
elevated AX with positive response to combined longacting b-agonist therapy vs increasing the dosage of
inhaled corticosteroids, although the exact reasoning
behind the association remains unclear and further
research is needed. These reports add to existing data
emphasizing the importance of the small airways in
asthma control.20,22
Clinical applications of IOS may best be illustrated by
reviewing case examples based on actual patients evaluated in a pediatric pulmonary clinic. The first case is that
of a 6-year-old girl who presented with a chronic cough
of several years. She had normal spirometry (FEV1,
114%; FEF25%-75%, 79%) without significant change to
bronchodilators. IOS in this patient showed normal
R5 and R20 but elevated AX (26.97 cm H2O/L) and
depressed X5 (23.7597 cm H2O/L/s, 130% of normal).
She also demonstrated a positive response to bronchodilators, with a 244.5% change in AX and a 239%
change in X5 (Fig 2). The patient was, therefore, started
on inhaled corticosteroids to control her symptoms, and
she demonstrated good clinical improvement.
Another patient seen in the same clinic was a 9-year-old
boy with a history of corrected double aortic arch,
residual tracheomalacia, and suspected persistent asthma.
He had persistently abnormal spirometry (baseline
FEV1, approximately 60%, FEF25%-75%, approximately
40%), with flow-volume loops consistent with intrathoracic obstruction. These persistent abnormalities made
objective assessment of his underlying, coexistent
asthma difficult. Thus, the patient underwent IOS,
which showed normal values (R5, 113%; R20, 78%),
except for a mildly decreased X5 (23.25 cm H2O/L/s,
137% of normal), without significant response to bronchodilators in any parameter (Fig 3). As a result, the
patient was able to be weaned successfully from highdose combined inhaled corticosteroids and long-acting
b-agonists.
Limitations of IOS
Although IOS has many useful clinical applications, the
procedure has some limitations. First, the procedure is
effort-independent compared with spirometry, but
patients still need to be cooperative to generate valid
results. Second, spirometry is currently more widely
adopted and better studied15,35,36; therefore, interpretation of its results often are more straightforward to the
practitioner. Research remains ongoing about the
[ 146#3 CHEST SEPTEMBER 2014 ]
Figure 2 – Impulse oscillometry results in a 6-y-old girl with chronic cough and normal spirometry. Chg 5 change; CO 5 coherence;
pred 5 predicted; R 5 resistance; X 5 reactance. See Figure 1 legend for expansion of other abbreviations.
precise meaning, interpretation, and clinical application
of IOS parameters, especially in less common disease
states such as childhood interstitial lung disease or in
settings such as the ICU where the forced oscillation
technique may have applications for sedated patients15
or those receiving ventilation.56 Furthermore, the ATS
noted that reference values for IOS are primarily available
for non-Hispanic white children,36 although data from
other populations are becoming available.7,57-60 As discussed, IOS is well suited for conditions involving airway
obstruction, but it may not provide definitive informa-
tion on restrictive states,9 although more research into
this area is needed.
Coding and Billing Considerations
For US practitioners, IOS can be billed using Current
Procedural Terminology code 94728. At present, the
Centers for Medicare & Medicaid Services assigns 0.26
physician work relative value units for interpretation of
IOS. If the physician is only doing the interpretation and
does not own the equipment, a -26 modifier is used. When
performed in a nonfacility, the global reimbursement
Figure 3 – Impulse oscillometry results in a 9-y-old boy with asthma and persistent abnormal spirometry who had vascular ring repair during infancy.
See Figure 1 and 2 legends for expansion of abbreviations.
journal.publications.chestnet.org
845
is 1.35 relative value units, including both the technical component and the physician interpretation. In
general, a -25 modifier should be appended to the
evaluation and management code when performed on
the same day of service. It should also be noted that
most payers consider IOS to be for patients who are
unable to perform spirometry, although there are clinical
situations as discussed herein where the two tests may
be complementary. For this reason, IOS is not generally
billable on the same day as spirometry or other pulmonary function tests.
Conclusions
IOS is a useful tool in the diagnosis and evaluation of
pediatric and adult patients with asthma and obstructive
lung diseases. IOS can be performed easily in children
aged , 5 years, making it useful in children unable to
perform traditional spirometry. IOS may be more sensitive than spirometry at identifying pathology in the
peripheral airways and may have better predictive value
than spirometry in identifying patients with potential
loss of asthma control. IOS allows bronchoprovocation
testing at lower doses of methacholine. Research is
ongoing regarding the utility of IOS in other pulmonary
conditions.
Acknowledgments
Financial/nonfinancial disclosures: The authors have reported to
CHEST that no potential conflicts of interest exist with any companies/
organizations whose products or services may be discussed in this
article.
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