Flatness Index
The ratio of mean inspiratory flow to peak inspiratory flow per breath. Values of 0.85 or above indicate significant flow limitation with a flat-topped breath pattern.
What Is Flatness Index?
The Flatness Index measures how rectangular (flat-topped) each inspiratory breath is by computing the ratio of mean flow to peak flow. A perfectly rectangular breath, representing complete flow limitation where the airway acts as a fixed-resistance bottleneck, would have a Flatness Index of 1.0. A normal rounded breath typically has a Flatness Index around 0.65 to 0.75.
When the Flatness Index reaches 0.85 or above, it indicates significant flow limitation. The inspiratory flow plateau is essentially flat, meaning airflow cannot increase further despite continued respiratory effort. The airway has become a flow-limiting orifice.
The Flatness Index complements NED by detecting a different aspect of flow limitation. NED specifically measures the peak-to-mid flow drop (progressive narrowing during inspiration), while Flatness Index measures overall breath rectangularity. A breath can be very flat (high FI) without much NED if the flow plateau is consistent throughout inspiration. AirwayLab uses both metrics together with the Glasgow Index and FL Score to provide the most complete assessment of airway restriction.
Normal Ranges
How AirwayLab Measures This
The NED engine computes Flatness Index for every breath in the flow waveform as mean inspiratory flow divided by peak inspiratory flow. It is reported as part of the NED Analysis tab, and breaths with FI above 0.85 are counted towards the Combined FL Percentage.
Try it with your dataFrequently Asked Questions
What does a high Flatness Index mean?
A Flatness Index above 0.85 means the inspiratory flow waveform is nearly rectangular. During inspiration, airflow quickly rises to a plateau and stays flat, indicating the airway is acting as a fixed-resistance bottleneck. This is a clear sign of flow limitation.
How is the Flatness Index different from NED?
Flatness Index measures overall breath rectangularity (mean flow divided by peak flow). NED measures the specific drop from peak to mid-inspiratory flow. A breath can be flat (high FI) but have consistent flow throughout (low NED), or vice versa. Both are markers of flow limitation but capture different patterns.
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Medical Disclaimer
AirwayLab is not a medical device and is not FDA-cleared or CE-marked. It is provided for educational and informational purposes only. The analysis results should not be used as a substitute for professional medical advice, diagnosis, or treatment. Always consult qualified healthcare providers regarding your sleep therapy and any changes to PAP settings.