Oscillometry, also known as the Forced Oscillation Technique (FOT), is a non-invasive method used in pulmonary function testing. It evaluates the mechanical properties of the respiratory system by measuring the response of the lungs and airways to an external signal. Unlike traditional tests like spirometry, oscillometry avoids forced maneuvers, requiring the patient only to breathe normally. This provides clinicians with unique insights into airway function.
The Underlying Principle of Operation
The core of oscillometry involves superimposing small, rapidly oscillating pressure waves onto the patient’s normal tidal breathing. These pressure waves are generated by a loudspeaker and delivered via a mouthpiece. Frequencies typically range from 5 to 35 Hertz, which is significantly faster than the normal breathing rate. Highly sensitive transducers measure the pressure and resulting airflow simultaneously at the mouth.
The system calculates respiratory impedance, the complex ratio of the pressure signal to the flow signal across the respiratory system. Impedance quantifies the opposition the lungs present to the movement of air at different frequencies. Since the pressure oscillations are very small, they do not disrupt the patient’s regular breathing pattern.
The use of multiple frequencies is foundational because waves penetrate the airways to varying depths. Lower-frequency oscillations travel deeper into the peripheral airspaces, while higher-frequency oscillations are reflected by the central airways. This frequency-dependent behavior allows the test to compartmentalize the mechanical properties of the respiratory system.
Clinical Utility for Difficult-to-Test Populations
The minimal requirement for patient cooperation makes oscillometry suitable for populations unable to perform forced respiratory maneuvers. Standard spirometry demands a maximal, sustained exhalation, which is often challenging or impossible for many patients. Oscillometry requires only that the patient breathe normally into the device, greatly expanding the application of lung function testing.
Young children, especially those of preschool age, often struggle to execute the complex instructions for spirometry. Oscillometry removes this barrier, enabling earlier diagnosis and monitoring of conditions like asthma and cystic fibrosis in pediatrics. The elderly or severely frail patients who lack the muscular strength or cognitive capacity for forceful maneuvers can also easily complete the test. This includes individuals with significant neuromuscular compromise or those experiencing an acute exacerbation of a respiratory disease.
Oscillometry is also useful in detecting small airway obstruction, which conventional spirometry may miss. The small airways, defined as those less than two millimeters in diameter, are often the first site of pathology in conditions such as Chronic Obstructive Pulmonary Disease (COPD) and asthma. By providing a sensitive measure of mechanical changes in these peripheral zones, oscillometry offers a complementary diagnostic tool.
Understanding Resistance and Reactance Measurements
Oscillometry provides two main physiological measurements derived from respiratory impedance: Resistance (R) and Reactance (X). Resistance represents the opposition to airflow resulting from friction within the airways and lung tissue. An increase in resistance suggests narrowing or obstruction of the airways. This measurement is reported at different frequencies, providing insight into the anatomical location of the obstruction.
Resistance at 5 Hertz (R5) reflects the total resistance of the entire respiratory system. Resistance at 20 Hertz (R20) primarily measures the resistance of the central, larger airways, as higher-frequency waves do not penetrate deeply. Clinicians calculate the difference (R5 minus R20) to determine the resistance specific to the peripheral or small airways. A high R5-R20 difference suggests obstruction located deep within the lungs, often an early sign of disease.
Reactance (X) reflects the elastic and inertial properties of the respiratory system. The elastic component relates to the stiffness, or compliance, of the lung and chest wall. Reactance at 5 Hertz (X5) is particularly informative as it is sensitive to the elastic recoil of the peripheral airspaces and the presence of air trapping. Air trapping causes the lungs to become hyperinflated and stiffer, which is reflected as a more negative X5 value.