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Forced oscillation technique
How oscillometry measures the airway.
The physics behind the test, the parameters it reports, and how to read them — separated from the product pages so the method can be explained properly.
What makes oscillometry different
Spirometry asks the patient to force air out. Oscillometry asks nothing of them at all.
- The patient breathes normally. Gentle pressure oscillations are superimposed on tidal breathing, and the system measures how the respiratory system responds. There is no manoeuvre to coach and no effort to sustain.
- It measures impedance, not volume. Spirometry reports how much air moved and how fast. Oscillometry reports the opposition the airway presents — resistance and reactance — across a range of frequencies.
- Low frequencies reach further. A 5 Hz wave travels the whole bronchial tree; a 20 Hz wave damps out before the periphery. The difference between them is what exposes small-airway disease.
- It works where spirometry cannot. Children from three years, frail and elderly patients, the acutely breathless, and anyone who cannot coordinate a forced expiration.
- It complements rather than replaces. FEV₁ remains the parameter most clinicians reason with. Oscillometry adds a measurement axis beside it, from the same session.
- Standards apply. Parameter definitions and acceptability criteria follow the ERS technical standards for respiratory oscillometry (King GG, et al. Eur Respir J 2020;55:1900753).
Reading a report
What each parameter measures, and which way it moves when the airway is obstructed.
| Parameter | What it measures | In obstruction |
|---|---|---|
| R5 | Total respiratory resistance at 5 Hz. Low-frequency waves reach the whole bronchial tree, so R5 reflects central and peripheral airways together. | Rises |
| R20 | Resistance at 20 Hz. High-frequency waves damp out before the periphery, so R20 reflects the large, central airways alone. | Rises only with central obstruction |
| R5−R20 | The frequency dependence of resistance — the part of the total the large airways do not explain. The most-used oscillometry marker of small-airway dysfunction. | Widens |
| X5 | Reactance at 5 Hz: the elastic and inertive behaviour of the respiratory system. Becomes more negative as peripheral airways stiffen or close. | Falls (more negative) |
| AX | Reactance area — the integral of the reactance curve between 5 Hz and the resonant frequency. A single number for the total low-frequency reactance burden. | Increases |
| Fres | Resonant frequency: where elastic and inertive forces cancel and reactance crosses zero. | Shifts higher |
| Coherence | A signal-quality index rather than a clinical result. Low coherence flags artefact — a mouth leak, swallowing, glottic closure or vocalisation — so a run can be repeated instead of reported. | Quality check |
Reference values are applied by age, height and sex, including Indian reference equations.
Measurement background
alveoflow Forced Oscillometry System
alveoflow's quiet-breathing assessment is a game-changer for our pediatric patients. No more struggling with forced maneuvers or inconsistent data due to patient effort
Director of Pulmonology, St. Jude Medical
The precision of the resistance and reactance metrics has uncovered airway dysfunction in our research subjects that traditional spirometry completely missed
Senior Research Fellow, Global Respiratory Institute
Portable, precise, and patient-friendly. It’s the first oscillometry platform that truly scales with clinical demand. The reporting is detailed and actionable
Chief of Staff, Metropolitan Pulmonary Lab
US FDA 510(k) Pending
Clinical Utility & Small-Airway Sensitivity The Forced Oscillation Technique (FOT) provides objective, effort-independent respiratory impedance measurements, enabling high-sensitivity detection of small-airway dysfunction and frequency-dependent resistance across diverse clinical and research cohorts
Reactance (Xrs): Represents the reactive storage component dictated by pulmonary elastance and inertance. Low-frequency reactance (X5) serves as a sensitive marker for reduced lung compliance and peripheral airway closure
Effort-Independent Analysis: Delivers precise clinical lung profiles during quiet breathing, eliminating maneuver-related variability and deep-breath bias across fragile, pediatric, and adult populations
Advanced High-Fidelity Respiratory Impedance
Frequency-dependent resistance mapping for early-stage small-airway involvement detection
High-fidelity clinical metrics synchronized via secure, medical-grade cloud architecture
Effort-independent diagnostic profiles optimized for longitudinal chronic care monitoring
The alveoflow platform transitions medical-grade Forced Oscillation Technique (FOT) from centralized laboratories to the point-of-care. By utilizing effort-independent assessment of total respiratory impedance (Zrs), clinicians can obtain precise measurements of resistance (Rrs) and reactance (Xrs) during tidal breathing, isolating central and peripheral airway mechanics with unprecedented fidelity
Clinical Synthesis & Diagnostic Implementation
Comprehensive Impedance Mapping: Simultaneous characterization of Rrs and Xrs across the 5–40 Hz spectrum
Small-Airway Precision: Enhanced sensitivity for distal airway dysfunction compared to traditional peak flow metrics
Effort-Independent Reliability: Optimal for clinical research and patients with limited maneuver compliance
Evaluates peripheral airway resistance and identifies frequency-dependent resistance changes. This sensitive biomarker monitors clinical bronchodilator response patterns earlier than conventional spirometry, detecting airway instability during quiet breathing
Quantifies respiratory impedance (Zrs) to evaluate flow limitations and small-airway involvement. This effort-independent assessment allows for precise staging of chronic obstructive conditions while mitigating the variability inherent in forced expiratory maneuvers
Facilitates sub-clinical detection of respiratory dysfunction in pediatric and geriatric cohorts where forced maneuvers are often suboptimal. Reliable tidal breathing analysis ensures high-fidelity diagnostics for researchers monitoring small-airway mechanics
The integration of Forced Oscillation Technique (FOT) into pulmonary practice provides a granular assessment of respiratory impedance (Zrs) by decoupling frictional resistance (Rrs) from reactive energy storage (Xrs). Unlike conventional spirometry, FOT enables a precise, effort-independent assessment, specifically identifying small-airway involvement and frequency-dependent resistance patterns through tidal breathing analysis
Clinical Utility of Oscillometric Mechanics
The Forced Oscillation Technique (FOT) provides a comprehensive evaluation of respiratory impedance (Zrs) during tidal breathing, isolating frequency-dependent resistance and reactance. This multi-frequency approach provides a granular assessment of lung function, detecting early small-airway involvement with a precision that exceeds standard pulmonary volumetry
True Tidal Breathing Breath naturally without effort-dependent maneuvers, making it easy for patients of all ages
Dual-Band Multi-Frequency: Waveform excitation from 5–40 Hz (up to 60 Hz optional)
Intra-Breath Resolution: Deep analysis of airway dynamics within every single breath
AI Interpretation Engine: Automated insights for clear clinical and research diagnostics
Next-Gen Forced Oscillation Precision FOT technology (5–40 Hz) enabling earlier detection of small-airway dysfunction
Intra-Breath Resolution High-fidelity intra-breath resolution for real-time visualization of resistance and reactance
AI Interpretation Engine Sophisticated automated analysis provides clear diagnostic insights from complex respiratory datasets
Fast Testing Workflow Streamlined workflow designed for high-patient throughput, completing tests in under 90 seconds
Class IIa Medical Diagnostic Device
Advanced Forced Oscillation Technique (FOT)
Six parameters, and what each one tells you
Select a parameter to locate it on the spectrum
What comes back after a test
Pre- and post-bronchodilator values side by side, with the change expressed against the reference set in use
R5 — total respiratory resistance
Resistance measured at 5 Hz. The low-frequency signal reaches the whole airway tree, so R5 reflects total resistance — central and peripheral together. Raised R5 indicates obstruction somewhere in the system; on its own it does not say where
See it measured on your own patients.
A 30-minute session with a clinical specialist, on your workflow.