Your PAP machine gave you an AHI of 1.2. The app said it was a great night. You woke up exhausted anyway. If that pattern is familiar, you're not imagining it — and the explanation is almost certainly hiding in two things AHI was never designed to measure: flow limitation and RERAs.
This article explains what these events are, why they don't show up in your AHI, how they fragment sleep without you knowing, and how tools like AirwayLab can surface them from the data already on your SD card.
What AHI Counts (and What It Ignores)
The Apnea-Hypopnea Index counts two types of events per hour of recorded sleep:
- Apneas: complete cessation of airflow for at least 10 seconds
- Hypopneas: airflow reduction of 30% or more for at least 10 seconds, typically with a significant oxygen drop
Everything below those thresholds is invisible to AHI. Partial airway narrowing that restricts but does not block flow, brief arousals from sub-threshold breathing effort — none of it appears in the number your machine reports.
Why AHI was designed this way
AHI was developed to capture the most severe airway events associated with oxygen desaturation and cardiac risk. It does that job well. It was not designed to capture every source of sleep fragmentation, particularly sub-threshold events that affect sleep architecture without causing desaturation.
Flow Limitation: The Event AHI Was Not Built to See
During normal sleep, air moves through your upper airway in a smooth, rounded arc with each breath. When the airway partially narrows, that arc changes shape: the top flattens out into a plateau instead of a peak. Air still flows — the airway hasn't closed — but it's restricted.
That shape change is flow limitation. Your body responds to the extra breathing effort even when the airway restriction isn't severe enough to register as a hypopnea.
Inspiratory flow waveform shape
Normal
Smooth, rounded peak
Flow-limited
Flattened plateau
airwaylab.app — illustrative waveform shapes only, not clinical measurements
Your ResMed SD card records breath-by-breath flow waveform data in EDF files. Tools like AirwayLab can score each breath for the degree of flattening using the Glasgow Index (breath shape scoring) and the FL Score (percentage of breaths with significant flattening). These metrics make breath-shape patterns visible across entire sessions.
RERAs: When Flow Limitation Breaks Your Sleep
A Respiratory Effort-Related Arousal (RERA) is what happens at the end of a sequence of flow-limited breaths. After several seconds of restricted airflow, the increasing breathing effort crosses a threshold and triggers a brief arousal — a micro-waking that restores normal airflow and lets you settle back into sleep, often without conscious awareness.
The difference between flow limitation and a RERA:
Flow limitation
The continuous partial narrowing event. Multiple consecutive flow-limited breaths build respiratory effort.
RERA
The brief arousal that terminates a bout of flow limitation. It resets normal airflow but fragments sleep architecture in the process.
When RERAs occur frequently, the condition is sometimes called Upper Airway Resistance Syndrome (UARS). UARS is characterised by normal or near-normal AHI alongside significant sleep fragmentation from sub-threshold respiratory events. AHI alone cannot detect it.
Why Flow Limitation and RERAs Leave You Exhausted
The fatigue mechanism operates through three overlapping pathways:
Sleep architecture fragmentation
Each RERA pulls you briefly toward lighter sleep or wakefulness. Frequent RERAs prevent the deep, consolidated sleep stages where physical and cognitive restoration occurs.
Autonomic nervous system activation
Each arousal triggers a sympathetic stress response — a brief increase in heart rate and blood pressure. Repeated activation across a night accumulates physiological load even when you feel like you slept through it.
Cumulative respiratory effort
Hours of working against a partially narrowed airway is physically tiring, independent of sleep fragmentation. Your respiratory muscles have been working harder all night.
These are descriptions of physiological patterns, not diagnostic criteria. Whether these patterns are contributing to your symptoms is a question for your clinician.
How to Find Flow Limitation and RERAs in Your PAP Data
Your ResMed SD card contains EDF files with full breath-by-breath waveform data. This is the same data that research tools use to calculate flow limitation metrics. AirwayLab analyses these files in your browser and computes:
Glasgow Index
Scores each breath's shape on 9 components (0–1 each) for an overall score from 0 (normal) to 9 (severely flow-limited). Session averages show trends over time.
FL Score
The percentage of breaths in a session with clinically significant flow limitation. Lower values are typical of less flattened breath shapes; elevated values indicate more breath-shape flattening across the session.
NED + estimated RERA
Normalised Event Density and an estimated RERA count derived from flow waveform analysis, giving a picture of respiratory event patterns beyond AHI.
None of this requires uploading your data. AirwayLab processes everything locally in your browser using Web Workers. Your breathing data never leaves your device.
What You Can Do With This Information
Understanding your flow limitation and RERA patterns lets you do several useful things:
- Track trends over time. A Glasgow Index or FL Score that has been gradually increasing is a pattern worth tracking — your clinician can help interpret these findings in context.
- Investigate H2 positional effects. Some users find flow limitation scores differ substantially between the first and second halves of the night, which can reflect positional or REM-related factors.
- Correlate with symptoms. Nights with elevated flow limitation scores alongside poor subjective sleep quality give your clinician richer data to work with than AHI alone.
- Share with your clinician. AirwayLab's session reports include flow limitation metrics alongside AHI data, giving your sleep specialist a fuller picture to discuss.
These are informational uses of your data. They do not substitute for clinical evaluation or advice about your therapy.
When to Discuss With Your Sleep Physician
Patterns in your data worth bringing to a clinical appointment include:
- Persistently low AHI alongside unresolved fatigue, morning headaches, or unrefreshing sleep
- Elevated Glasgow Index or FL Score that has remained elevated or is trending upward over weeks
- High estimated RERA counts alongside subjective sleep fragmentation
- A large gap between H1 and H2 flow limitation scores that may reflect positional factors
Bring your AirwayLab session data to the appointment. Having objective metrics to point to gives your clinician more to work with than symptom description alone.
Further Reading
Understanding Flow Limitation in PAP Therapy — a deep dive into what the Glasgow Index and FL Score actually measure.
Beyond AHI: Why Your Sleep Apnea Score Might Be Misleading You — the research case for looking past the headline number.
Your AHI Is Normal But You're Still Exhausted — a practical guide to investigating persistent fatigue with PAP data.
Arousals vs Flow Limitation: What's Actually Waking You Up? — understanding the relationship between respiratory effort and sleep fragmentation.
Medical disclaimer:AirwayLab helps you understand your PAP data, but it is not a diagnostic tool and does not provide medical advice. The metrics described here are informational and intended to support conversations with your clinician — not to replace clinical evaluation. Always discuss your breathing data, symptoms, and therapy with a qualified sleep specialist.
See What AHI Is Hiding in Your Data
Your PAP machine already records flow waveform data. AirwayLab makes flow limitation and RERA patterns visible — for free, in your browser, with nothing uploaded.
AirwayLab is a free, open-source analysis tool. It is not a medical device and does not provide medical advice, diagnosis, or treatment recommendations. All analysis is informational — always discuss your breathing data and therapy with a qualified sleep specialist. Your data never leaves your browser.