Reading the detail
Flow limitation
Inspiratory flow limitation
Flow limitation sits below the threshold of everything your AHI counts. The airway narrows, the shape of your inspiratory flow flattens, but airflow never drops far enough or long enough to be scored. You can therefore have an excellent AHI and still sleep badly, and flow limitation is the usual explanation.
The numbers that matter
- Scored in your AHI
- no
- Reported as
- a flow limitation index, median and 95th percentile
- Recognised by
- flattening of the inspiratory flow curve
- Clinical relevance
- linked to arousals and unrefreshing sleep
What the machine sees
An unobstructed breath in has a rounded flow curve. As the airway narrows, that curve flattens into a plateau: you pull harder and the flow stops increasing. The machine measures how flat the curve is and reports it as an index rather than as a count of events.
Because it is a shape measurement rather than an event count, it is a continuous signal. That makes the 95th percentile and the trend more informative than any single figure.
Why it explains tiredness with a good AHI
Sustained flow limitation makes you work harder to breathe. That effort can fragment sleep through brief arousals that never last long enough to become a scored event, and the result is a night that looks clean on paper and feels broken.
This is the gap between the number your machine advertises and how you actually feel. If your AHI is under 1 and you are still exhausted, flow limitation is the first place to look, along with your leak and your total sleep time.
What it means for your pressure
On an APAP, flow limitation is one of the signals the algorithm responds to: sustained flattening typically makes the machine raise pressure before any event occurs. Seeing flow limitation and pressure move together in your data is the algorithm working as designed.
On fixed CPAP nothing responds to it, so persistent flow limitation is worth showing to your prescriber. It is one of the more common reasons a switch from fixed pressure to an auto-adjusting mode gets considered.
Common questions
Is flow limitation included in my AHI?
+
No. It is deliberately below the scoring threshold for apneas and hypopneas, which is exactly why a low AHI can coexist with poor sleep. It is reported as its own index, separately.
How do I reduce flow limitation?
+
It usually responds to pressure, to sleeping position, and to nasal congestion. All three are worth discussing with your prescriber rather than adjusting alone, since raising pressure has its own trade-offs on leak and comfort.
Related terms
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