Most of us started our CPAP journey staring at one number: AHI. If it's below five, therapy is working. That's the conventional wisdom, and it's a useful starting point. But AHI measures one thing — apneas and hypopneas — and your airway does a lot more than just collapse and recover.
There's a whole layer of breathing data sitting in your CPAP's recordings that AHI doesn't capture. Flow limitation, effort patterns, sleep continuity, arousal signals — these are the patterns that often explain why therapy feels “off” even when AHI looks fine.
AirwayLab surfaces four of these metrics. Here's what each one describes.
Flow Limitation Score
The Flow Limitation Score describes the shape of your breath, not just whether a breathing event occurred. A full, round breath gets a score of 0. A flattened breath — one where the peak airflow is blunted, indicating partial upper airway resistance — scores 0.5. A severely flattened breath scores 1.0.
Why does shape matter? Because flow limitation can produce respiratory effort and micro-arousals without ever triggering a formal apnea or hypopnea. A night with an AHI of 2 can still involve significant flow limitation — and the fatigue and fragmented sleep that goes with it.
AirwayLab calculates this score breath-by-breath across your session and shows you how it distributes over the night.
Glasgow Index
The Glasgow Index measures the cumulative burden of flow limitation across a session, expressed as a single number. Rather than describing individual breaths, it shows the overall picture: how much of your night involved partial airway obstruction?
A lower Glasgow Index indicates fewer flow-limited breaths over the course of the session. A higher number indicates more. The metric is used in sleep research as a way to quantify upper airway resistance burden in a form that's comparable across nights.
In AirwayLab, the Glasgow Index sits alongside your AHI and RDI so you can see whether flow limitation is a consistent pattern or an occasional feature of your nights.
WAT Score
The WAT (Wobble Analysis Tool) bundles three independent metrics that describe breathing stability during PAP therapy. FL Score measures inspiratory flatness — how flow-limited each breath is. Regularity uses sample entropy to quantify how variable your minute ventilation is over time. Periodicity Index uses spectral analysis to detect cyclical breathing patterns in the 30–100 second range.
Together, these three metrics describe breathing “wobble” — the instability that sits below the threshold of formal apneas and hypopneas. A stable night shows low FL scores, low entropy, and no periodic pattern. An unstable night shows elevated values across one or more of these dimensions.
WAT is particularly useful as a cross-session trend: if one or more of these stability metrics is consistently elevated, that's worth discussing with your clinician.
NED — Negative Effort Dependence
NED (Negative Effort Dependence) describes a specific breathing pattern where increased inspiratory effort produces decreasedairflow. It's a hallmark of upper airway collapsibility: the harder you try to breathe in, the more the airway narrows in response.
NED analysis shows the relationship between your estimated respiratory effort and your airflow. Where that relationship is inverse — more effort, less flow — NED is present. It's one of the indicators researchers use to characterise upper airway anatomy and behaviour during sleep.
AirwayLab's NED analysis surfaces this pattern from your flow waveform data and shows it as a trend across sessions.
How These Four Metrics Work Together
AHI tells you how many times your breathing fully stopped or significantly reduced. The Flow Limitation Score and Glasgow Index describe what's happening to your airway between those events. The WAT score shows how stable your breathing is across the session — flatness, variability, and cyclical patterns. NED shows whether your airway anatomy is contributing to resistance under increased respiratory effort.
None of these metrics is a diagnosis. Together, they give you a richer picture of what your therapy data contains — and a more informed set of questions to bring to your clinician.
All analysis in AirwayLab runs in your browser. Your data never leaves your device.
Discuss your data with your clinician for clinical interpretation.
See Your Data
Upload a CPAP recording to see your Flow Limitation Score, Glasgow Index, WAT score, and NED analysis. Free and always will be. Discuss your data with your clinician for clinical interpretation.
Related reading
CPAP Flow Limitation Score: What 0, 0.5, and 1.0 Mean — the three-point scale your ResMed device uses and how AirwayLab extends it.
What Is the Glasgow Index in CPAP Data? — a nine-component breath shape score that captures cumulative flow limitation.
What Is the WAT Score in CPAP Data? — FL Score, regularity, and periodic breathing in one bundle.
What Is NED (Negative Effort Dependence)? — a breath-by-breath measure of airway resistance during PAP therapy.
The metrics shown by AirwayLab are informational and describe patterns in your therapy data. They are not clinical assessments, diagnoses, or treatment recommendations. Always discuss your therapy data with a qualified clinician.