Oscillometry is a non-invasive method for measuring the mechanical properties of the respiratory system. This test can enhance our understanding and management of lung diseases, such as asthma and COPD. Oscillometry offers special advantages in situations where spirometry and other lung function tests are not suitable for patients. This includes infants, people with neuromuscular diseases, those who have difficulty following instructions (such as children or those with neurological problems). It also applies to patients with sleep apnea, and those in need of critical care. In this article, we focus on the clinical applications of oscillometry for the diagnosis, treatment, and monitoring of asthma and COPD.
By América Torres
Physiology of Oscillometry
Oscillometry, or the Forced Oscillation Technique (FOT), measures the mechanical properties of the respiratory system. This includes the upper and intrathoracic airways, lung tissue, and chest wall. It does so during resting breathing by applying an oscillating pressure signal.
Oscillometry measures the mechanical impedance of the respiratory system (Zrs). This impedance represents the resistive and reactive forces that must be overcome to introduce an oscillating flow signal into the respiratory system. Those forces arise from three sources in the respiratory system:
- The resistance of the airways and tissues to flow (Rrs).
- The elastance (stiffness) of the lung parenchyma and chest wall in response to volume changes (included in the reactance, Xrs).
- The inertia of the accelerating gas in the airways (Irs).
The impedance of the respiratory system is generally reported at a single frequency. Or within the frequency range of 5 to 40 Hz on average, throughout the entire respiratory cycle. Reports also present it separately during the inspiratory and expiratory phases.
Spectrum of Respiratory Impedance
The impedance of the respiratory system (Zrs) evaluates the relationship between pressure and flow changes during oscillatory flow into and out of the lungs. Zrs has two basic components: resistance (Rrs) and reactance (Xrs). The following image and explanation illustrate these concepts in greater detail.
The plot shows respiratory system impedance (Zrs) against frequency. Zrs consists of a real component represented by resistance (Rrs) and an imaginary component expressed as reactance (Xrs). Rrs and Xrs Rrs values correspond to the frequencies at which they are measured (for example, Rrs5 = Rrs at 5 Hz, Rrs20 = Rrs at 20 Hz). The frequency at which Xrs crosses zero is the resonance frequency (fres). Below fres, Xrs is dominated by elastance, and above fres, Xrs is dominated by inertia.
The area under the Xrs and Zrs = 0 curve is an integrated measure of low-frequency Xrs. It starts from the lowest frequency up to fres, known as the area under the Xrs curve, AX. The lowest frequency of AX shows at 5 Hz, but it can be estimated starting from any frequency. Note that the Zrs spectrum shown in the image above is characteristic of a healthy adult. In healthy young children, Rrs values and frequency dependence would be relatively higher. Xrs would be more negative, and fres would notably shift to the right (resulting in an increase in AX).
Applications of Oscillometry in Managing Asthma
Oscillometry is a useful test for diagnosing pediatric and adult asthma patients, to detect the degree of obstruction affecting the airways, and to distinguish asthma from COPD. It can also help support an asthma diagnosis, predict future asthma control loss, or guide clinical treatment modifications. Furthermore, oscillometry provides insights into asthma pathophysiology through the effects of lung volume on oscillatory mechanics, as well as short-term and long-term variations in mechanics over time. These variations can serve as markers of instability, making them potentially valuable for detecting exacerbations or loss of control.
Intrabreath changes in oscillometry parameters can also provide additional information beyond conventional parameters. For example, in preschool-aged children, the use of this test improved the detection of patients with acute obstruction and recurrent wheezing compared to healthy controls. Meanwhile, in adults with severe asthma, it allowed differentiation between patients with poor disease control and those who were well controlled.
On the other hand, various studies have shown that a bronchodilator response (BDR) based on oscillometric parameters is better than one based on forced expiratory volume in 1 second (FEV1) for distinguishing asthmatic children from healthy children.
Benefits of Oscillometry for Evaluating Asthma Treatment Response
Some recent studies show that oscillometry and other parameters related to peripheral airway function can correlate with symptom improvement in patients with poorly controlled asthma receiving inhaled corticosteroids and long-acting β2 agonists (ICS/LABA). Oscillometric indices are also sensitive to improvements in asthma in response to mepolizumab therapy. Therefore, these findings suggest that oscillometry plays an important and complementary role to spirometry in identifying and monitoring treatment response in asthmatic patients.
Applications of Oscillometry for Treating COPD
Oscillometry is also a valuable test for early detection of smoking-related adverse effects before COPD diagnosis. Several studies found that many smokers with normal spirometry show abnormalities in Zrs. In fact, up to 60% of those patients exhibited some anomaly in oscillometry results.
Oscillometry also helps clinicians better categorize the severity of COPD. Patients with this disease have significantly higher values of Rrs and more negative values of Xrs than healthy individuals. These changes are proportional to the degree of airway obstruction. In fact, intrabreath oscillometry examination helps demonstrate evidence of tidal expiratory flow limitation (EFLT) in COPD. In those patients, reactance and resistance are higher during expiration compared to inspiration, reflecting dynamic airway compression and expiratory flow limitation. The underlying mechanisms of airway collapse during tidal breathing remain uncertain. However, researchers have empirically defined thresholds based on the difference between inspiratory and expiratory Xrs. These thresholds provide a sensitive and specific method for detecting EFLT.
The inspiratory-expiratory difference in Xrs and its variability over time also associate with increased dyspnea. Additionally, this index links to accelerated impairment of exercise capacity and a higher likelihood of exacerbations, independently of the degree of impairment shown by spirometry. In COPD patients, oscillometry also reveals greater variations in lung function over time, as well as greater bronchodilator responses than expected by spirometry alone. This reinforces that the disease pathogenesis extends beyond fixed airway obstruction and reversibility in large airways.
Oscillometry + Spirometry
Oscillometry proves useful as a standalone pulmonary function test. Therefore, combining it with spirometry maximizes the benefits of each of each test and provides much more information. Oscillometry and spirometry help diagnose and monitor many lung diseases, such as asthma and COPD, two of the most common diseases affecting millions of people. According to the GINA 2024 report, asthma affects approximately 300 million people worldwide and causes 1000 deaths daily.
Regarding COPD, while obtaining definitive figures is challenging, the GOLD 2024 guideline estimates a global prevalence of COPD of 10.3% (95% confidence interval (CI): 8.2%, 12.8%).
SCHILLER-GANSHORN supports efforts to deliver accurate and early diagnoses. We design and provide advanced pulmonary function testing equipment to help clinicians achieve this goal. tremoflo® is an oscillometer that measures resistance and reactance in the small airways. It complements spirometry to help detect and manage asthma and COPD. SpiroScout is a high-precision ultrasonic spirometer. It is a reliable diagnostic tool and a key component in distinguishing between asthma and COPD early and effectively. We invite you to discover their advantages and why they are complementary tests.
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SOURCES
[1] David A. Kaminsky et al. Clinical significance and applications of oscillometry. European Respiratory Review 2022 31: 210208; DOI: 10.1183/16000617.0208-2021. https://err.ersjournals.com/content/31/163/210208#sec-5
[2] Gregory G. King et al. Technical standards for respiratory oscillometry. European Respiratory Journal 2020 55: 1900753; DOI: 10.1183/13993003.00753-2019. https://erj.ersjournals.com/content/55/2/1900753?ijkey=edbce9b7913b4126828986b14ff8f0165c3d4910&keytype2=tf_ipsecsha
Frequently Asked Questions About Oscillometry in Asthma and COPD Management
What is oscillometry and how does it work?
Oscillometry is a non-invasive pulmonary function test that measures the mechanical properties of the respiratory system during normal breathing. It applies small oscillating pressure signals to assess airway resistance and reactance without requiring forced maneuvers.
How does oscillometry differ from spirometry?
Oscillometry evaluates airway mechanics during resting breathing, while spirometry requires forced inhalation and exhalation. Because of this, oscillometry is especially useful for patients who cannot perform spirometry reliably, such as young children, elderly patients, or those with neuromuscular or neurological conditions.
How does oscillometry help in managing asthma?
In asthma, oscillometry helps detect airway obstruction, assess peripheral airway involvement, and monitor treatment response. Oscillometric parameters can correlate with symptom control and may identify loss of asthma control or exacerbation risk earlier than spirometry alone.
What role does oscillometry play in COPD assessment?
Oscillometry supports early detection of COPD-related airway abnormalities, even when spirometry results are normal. It also helps categorize COPD severity, detect expiratory flow limitation, and assess dynamic airway collapse, providing insights beyond fixed airway obstruction.
Why is combining oscillometry with spirometry important?
Combining oscillometry with spirometry provides a more comprehensive evaluation of lung function. While spirometry measures airflow limitation, oscillometry complements it by adding information on airway mechanics and small airway involvement. Together, they improve diagnosis, monitoring, and clinical decision-making in asthma and COPD.