If you have opened your CPAP data in OSCAR, you have probably seen an FL (flow limitation) channel that cycles between three values: 0, 0.5, and 1.0. These are ResMed's categorical assessment of inspiratory flow limitation, recorded by your machine during therapy.
What the Three Values Mean
ResMed AirSense 10 and AirCurve 10 devices record a flow limitation value approximately every two seconds throughout the night. The scale has three levels:
No flow limitation detected
The shape of your inspiratory flow waveform at that moment was normal -- a smooth, rounded curve without detectable flattening.
Moderate flow limitation
The device detected partial flattening of the inspiratory waveform. The airway is narrowing enough to restrict airflow, but not at maximum severity. This is the intermediate state.
Severe flow limitation
Significant flattening was detected. The top of the inspiratory curve is flat rather than rounded, indicating strong airway narrowing during inhalation.
These three values are assigned by ResMed's proprietary firmware and stored in the FLOW_LIMIT channel of your device's EDF data file. They are a device-level snapshot updated every two seconds, not a per-breath calculation.
What You See in OSCAR vs AirwayLab
In OSCAR, the FL channel is plotted as a step graph cycling between the three values. The proportion of time spent at 0.5 or 1.0 gives a rough picture of how often flow limitation was recorded during the night.
AirwayLab takes a different approach. Instead of using ResMed's categorical snapshot, AirwayLab's WAT engine calculates its own continuous FL Score directly from the raw inspiratory waveform, breath by breath. It is an uncalibrated relative index currently under review, not a percentage of the night, and it is not dependent on ResMed's firmware logic.
ResMed FL channel (OSCAR)
Three-level snapshot updated every ~2 seconds. Assigned by device firmware. Categorical: 0, 0.5, or 1.0.
AirwayLab FL Score
Continuous relative index. Calculated per breath from raw EDF waveform, open-source and auditable. Uncalibrated: scores cluster roughly 50-65 even for unobstructed breathing, so compare your own nights over time rather than the absolute number.
The two approaches are complementary. The ResMed FL channel is a fast device-level check built into the machine. AirwayLab's FL Score is an independent, open-source waveform analysis running in your browser that you can verify yourself -- cross-check it against NED and Glasgow, which measure flow limitation differently.
Neither replaces clinical evaluation -- they are two different ways of describing the same underlying flow signal.
What the FL Score Does NOT Tell You
A high proportion of 0.5 or 1.0 readings -- or a high AirwayLab FL Score -- does not automatically indicate a clinical problem or a need for therapy adjustment. Flow limitation is present to some degree in many PAP users and its significance depends on context only a clinician can assess.
Discuss your data with your clinician for clinical interpretation.
See AirwayLab's Continuous FL Score
Upload your ResMed SD card data to see AirwayLab's continuous FL Score alongside your Glasgow Index, NED, and WAT metrics -- all in your browser, nothing uploaded.
Related reading
What Is the WAT Score in CPAP Data? -- the FL Score is one of the three WAT metrics.
What Is the Glasgow Index in CPAP/BiPAP Data? -- nine-component breath shape analysis.
AirwayLab Glossary -- definitions of all metrics used in AirwayLab.
see all four metrics AirwayLab tracks -- how the FL Score fits into AirwayLab's broader measurement framework.
Understanding Flow Limitation in Your PAP Data -- what flow limitation is and how it differs from apneas and hypopneas.
AirwayLab is a free tool for analysing PAP flow data, built on an open-source analysis engine. Your data never leaves your browser. Nothing on this page constitutes medical advice -- always discuss your results with a qualified sleep specialist.