Oscillometry, also known as impulse oscillometry (IOS) or the forced oscillation technique (FOT), is a non-invasive method used to assess lung function by measuring the mechanical properties of the respiratory system. It analyzes how air moves through the airways, providing insights into their condition during natural, quiet breathing. This technique offers a comprehensive evaluation of airway resistance, reactance, and compliance, key measures of lung mechanics. Oscillometry provides a more nuanced assessment of lung function than traditional methods.
Understanding Oscillometry
Oscillometry superimposes gentle sound waves or pressure oscillations onto a patient’s normal breathing pattern. These small, rapid pressure changes, typically within a frequency range of 4 to 50 Hz, are introduced at the mouth. As the patient breathes quietly, transducers measure the resulting pressure and flow signals within the respiratory system. The device then separates these oscillatory signals from spontaneous breathing, precisely measuring how the lungs respond to the applied waves.
The “speed” of these oscillations is measured in Hertz (Hz), indicating pressure/flow cycles per second. Lower frequencies, such as 5 Hz, penetrate deeper into the peripheral airways, while higher frequencies, like 20 Hz, primarily assess the central airways. By analyzing the relationship between applied pressure and resulting airflow, oscillometry calculates respiratory impedance.
This impedance has two main components: resistance (Rrs) and reactance (Xrs). Resistance reflects the opposition to airflow, indicating how hard air moves through the lungs, and can be increased by factors like bronchoconstriction or mucus. Reactance represents the elastic and inertial properties of the lung tissue and chest wall, providing information on lung stiffness and how well air reaches peripheral areas.
Applications in Respiratory Health
Oscillometry plays an important role in diagnosing and monitoring various respiratory conditions by providing specific insights into airway mechanics. It is particularly useful in managing obstructive lung diseases such as asthma and chronic obstructive pulmonary disease (COPD). Oscillometry can help differentiate asthma from COPD and categorize the severity of COPD, as patients with this condition often show higher resistance and more negative reactance values. Measurements like the difference between resistance at 5 Hz and 20 Hz (R5-R20) indicate small airway dysfunction, common in both asthma and COPD.
Oscillometry also helps detect subtle changes in lung function that might not be apparent with other tests. It can identify airway narrowing or stiffness, before symptoms fully manifest. The technique assesses the effectiveness of treatments, such as bronchodilators, by showing changes in airway resistance and reactance after medication. This provides clinicians with objective data to guide treatment modifications and improve patient outcomes. It also assesses lung function impairments caused by occupational hazards and smoking, and detects central airway obstructions.
Advantages and the Patient Experience
Oscillometry offers several advantages, making it suitable for a wide range of patients, including those who struggle with traditional lung function tests. Its non-invasive nature means it only requires the patient to breathe normally into a mouthpiece, eliminating forced breathing maneuvers. This makes it a comfortable and effortless test, particularly beneficial for young children, elderly individuals, or those with physical or cognitive limitations who may find forced maneuvers challenging.
During the test, the patient typically sits upright with a nose clip applied and their mouth sealed around the mouthpiece. They then breathe quietly and steadily, usually for 20 to 30 seconds per measurement. Multiple measurements, typically four to six, are taken to ensure reproducible results. The simplicity and minimal patient cooperation allow for accurate and reliable measurements in populations where other tests might be difficult to perform, such as infants or those with severe respiratory conditions.
Oscillometry Versus Other Lung Tests
Oscillometry provides distinct information compared to other common lung function tests, such as spirometry, and often serves as a complementary tool. While spirometry relies on forced expiratory maneuvers and primarily assesses large airway function, oscillometry measures lung mechanics during quiet, tidal breathing. This difference allows oscillometry to provide separate measurements of airway resistance and reactance, offering a more detailed analysis of the entire respiratory system, including both central and peripheral airways.
Oscillometry is particularly sensitive in detecting small airway dysfunction, which spirometry may miss, especially in individuals with normal spirometry results but persistent symptoms. It can detect airway abnormalities in symptomatic individuals even when spirometry results are normal. Although both tests are used for diagnosis, they are not interchangeable; oscillometry can provide additional information about the airways and lung parenchyma that complements spirometry findings, aiding in early detection and comprehensive understanding of lung health.