What Is PAP Flow Limitation?
Flow limitation occurs when your airway partially narrows during PAP therapy, restricting airflow even though it doesn't fully collapse. It's one of the most under-recognised issues in sleep therapy — your AHI can look perfect while significant flow limitation quietly degrades your sleep quality.
Why flow limitation matters
Most PAP machines report the Apnea-Hypopnea Index (AHI) as the primary measure of therapy effectiveness. AHI counts only complete airway closures (apneas) and significant reductions in airflow (hypopneas). But the airway can be substantially narrowed — reducing airflow and increasing respiratory effort — without triggering either threshold.
This is flow limitation: the airway is open enough that AHI stays low, but narrow enough that your body works harder to breathe. Over time, this increased effort fragments sleep, causes micro-arousals, and can leave you feeling unrefreshed despite “great” AHI numbers.
Flow limitation is closely related to Upper Airway Resistance Syndrome (UARS) and Respiratory Effort-Related Arousals (RERAs) — conditions that are increasingly recognised in sleep medicine but often missed by standard home sleep testing.
Signs your therapy may have flow limitation
Low AHI but poor sleep quality
Your machine reports AHI < 5 but you still wake unrefreshed, experience daytime fatigue, or have morning headaches.
Flattened flow waveform
In OSCAR or AirwayLab, inspiratory flow shows a flat plateau rather than a smooth rounded peak — the classic flow limitation signature.
Frequent micro-arousals
Brief awakenings (3–15 seconds) that fragment sleep architecture without producing full apnea or hypopnea events.
Cycling pressure patterns
AutoSet machines may show pressure "hunting" — repeatedly raising and lowering pressure — as the algorithm tries to respond to partial obstruction.
How AirwayLab detects flow limitation
AirwayLab reads the raw flow waveform data from your ResMed SD card and analyses every breath using three independent engines. Each method detects different manifestations of flow limitation, giving you a comprehensive picture that goes far beyond AHI.
Glasgow Index
Scores each breath on 9 shape descriptors (skew, flat top, spike, etc.) to produce a composite flow limitation score. Typical scores range from 0 to about 3. Best for overall therapy quality assessment.
Learn more →WAT (Wobble Analysis Tool)
Analyses ventilation patterns using three complementary metrics: FL Score (tidal volume variance), Regularity Score (Sample Entropy), and Periodicity Index (FFT spectral analysis for periodic breathing detection).
Learn more →NED Analysis
Measures the ratio of peak-to-mid inspiratory flow (Negative Effort Dependence) to quantify airway narrowing. Also detects M-shaped breaths and automated RERA event sequences.
Learn more →What you can do about it
Always consult your sleep physician
AirwayLab can identify flow limitation patterns, but treatment decisions should always involve your healthcare provider. Common approaches include:
- Pressure adjustment — Increasing minimum EPAP/IPAP or adjusting AutoSet min/max ranges to keep the airway more open.
- EPR / pressure relief settings — Sometimes reducing EPR (Expiratory Pressure Relief) can help maintain airway patency during the transition from expiration to inspiration.
- Bilevel therapy (VPAP/BiPAP) — For persistent flow limitation that CPAP alone cannot resolve, bilevel pressure support provides separate inspiratory and expiratory pressures.
- Positional therapy — Flow limitation is often worse in certain sleeping positions. AirwayLab's first-half vs second-half analysis can reveal positional patterns.