Advanced Oscillometry Insights: Managing Asthma and COPD

01.07.24 10:42 PM Comment(s) By America

By América Torres

Oscillometry is a non-invasive method for measuring the mechanical properties of the respiratory system, which can enhance our understanding and management of lung diseases such as asthma and COPD.

This technique offers special advantages in situations where spirometry and other lung function tests are not suitable for patients, such as infants, people with neuromuscular diseases, those who have difficulty following instructions (such as children or those with neurological problems), those 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.

Physiology of Oscillometry

Oscillometry, or the Forced Oscillation Technique (FOT), measures the mechanical properties of the respiratory system (i.e., the upper and intrathoracic airways, lung tissue, and chest wall) during resting breathing by applying an oscillating pressure signal.

Oscillometry measures the mechanical impedance of the respiratory system (Zrs), which represents the resistive and reactive forces that must be overcome to introduce an oscillating flow signal into the respiratory system. These forces arise from three sources in the respiratory system:

1. The resistance of the airways and tissues to flow (Rrs).

2. The elastance (stiffness) of the lung parenchyma and chest wall in response to volume changes (included in the reactance, Xrs).

3. 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 (i.e., both inspiration and expiration). It has also been reported 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). To provide a clearer understanding of respiratory impedance, we offer the following image and a brief explanation:

respiratory impedance in asthma

The respiratory system impedance (Zrs) is plotted against frequency. Zrs consists of a real component represented by resistance (Rrs) and an imaginary component expressed as reactance (Xrs). Rrs and Xrs at specific frequencies are denoted by the frequency 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 is shown at 5 Hz, but it can be estimated starting from any frequency. It should be noted 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 have shown 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 can also be a valuable test for early detection of smoking-related adverse effects before COPD diagnosis. Several studies have found that many smokers with normal spirometry show abnormalities in Zrs; in fact, up to 60% of them exhibited some anomaly in oscillometry results.

Oscillometry can also aid in categorizing the severity of COPD. Patients with this disease have significantly higher values of Rrs and more negative values of Xrs than healthy individuals, with these changes proportional to the degree of airway obstruction. In fact, intrabreath oscillometry examination has been used to demonstrate evidence of tidal expiratory flow limitation (EFLT) in COPD.

In these 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 are uncertain, although empirically defined thresholds based on the difference between inspiratory and expiratory Xrs have been shown to be a sensitive and specific method for detecting EFLT.

The inspiratory-expiratory difference in Xrs and its variability over time are also associated with increased dyspnea. Additionally, this index is linked 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 has also revealed greater variations in lung function over time, as well as greater bronchodilator responses than expected by spirometry alone. This reinforces that the disease's pathogenesis extends beyond fixed airway obstruction and reversibility in large airways.

Oscillometry + spirometry

Oscillometry is a pulmonary function test that has proven to be useful on its own. Therefore, by combining it with spirometry, the benefits of each of these tests are maximized to provide 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 2024 GINA Report, asthma affects approximately 300 million people worldwide and causes 1000 deaths daily.

Regarding COPD, while obtaining definitive figures is challenging, the 2024 GOLD Report estimates a global prevalence of COPD of 10.3% (95% confidence interval (CI): 8.2%, 12.8%).

SCHILLER-GANSHORN contribute to efforts to offer accurate and early diagnoses, providing equipment with advanced technology for pulmonary function testing. tremoflo®is an oscillometer that measures resistance and reactance of small airways. SpiroScoutis an ultrasonic spirometer. We invite you to discover their advantages and why they are complementary tests.


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