You check your PAP machine every morning. AHI: 1.2. The app says "Great night!" with a smiley face. But you still wake up exhausted, foggy, and reaching for your third coffee by 10 AM. Sound familiar? The culprit might be something your machine tracks but never tells you about: flow limitation.
What Is Flow Limitation, Exactly?
During normal breathing, air flows freely through your upper airway. The flow signal looks like a smooth, rounded wave — air in, air out, nice and symmetrical. But when your airway starts to narrow (without fully collapsing), the flow pattern changes. Instead of a smooth curve, you get a flattened top — as if someone put a lid on your breath.
This partial narrowing is flow limitation. Your airway hasn't collapsed enough to count as an apnea (complete blockage) or even a hypopnea (significant reduction with oxygen drop). But it's working harder than it should, and your body notices.
Think of it like a garden hose
An apnea is when someone steps on the hose completely — no water flows. A hypopnea is a partial step — water trickles. Flow limitation? That's a kink in the hose. Water still flows, but the pressure pattern changes. Your sprinkler (or in this case, your body) knows something is off.
Why Your AHI Says You're Fine
The Apnea-Hypopnea Index only counts events that meet specific criteria: a breathing pause of at least 10 seconds (apnea) or a 30%+ airflow reduction accompanied by an oxygen desaturation of 3-4% (hypopnea). Flow limitation fits neatly into neither category.
Research published in the European Respiratory Journal has shown that patients can have significant flow limitation throughout the night with a perfectly normal AHI. These patients often report symptoms identical to those with untreated sleep apnea: daytime sleepiness, unrefreshing sleep, morning headaches, and cognitive fog.
The clinical term for these events is Respiratory Effort-Related Arousals (RERAs). They're breathing disturbances that cause brief awakenings — enough to fragment your sleep architecture — but don't show up in the AHI. Some sleep physicians use the Respiratory Disturbance Index (RDI), which includes RERAs, but your PAP machine doesn't report this.
Detecting Flow Limitation in Your Data
The good news: your ResMed machine already records the data needed to detect flow limitation. It's stored on your SD card in detailed flow waveforms. The machine just doesn't analyze it for you in a meaningful way.
Researchers have developed several approaches to quantify flow limitation from these waveforms:
Glasgow Index
Developed at the University of Glasgow, this index scores breath shapes on a 0-1 scale. A score above 0.3 suggests significant flow limitation. It detects flattened tops, skewed peaks, and other distortion patterns.
NED (Negative Effort Dependence)
Measures whether increasing breathing effort actually decreases airflow — a hallmark of a narrowed airway. Higher NED values indicate more severe limitation.
Flatness Index
Compares the peak flow to the mean flow of each breath. A perfectly round breath has a specific ratio; flattened breaths deviate from this, signaling obstruction.
RERA Detection
By combining flow shape analysis with patterns of breathing effort and arousal signatures, it's possible to estimate RERA events from PAP flow data alone.
What Causes Flow Limitation on PAP?
If flow limitation is happening despite PAP therapy, it usually points to one of a few issues:
- Pressure too low: The most common cause. Your prescribed pressure may not be enough to fully splint the airway open, especially during REM sleep when muscles relax further.
- EPR too high: Expiratory Pressure Relief (EPR) reduces pressure during exhalation for comfort. But if set too high (e.g., EPR 3), the lower expiratory pressure may allow airway narrowing.
- Positional factors: Sleeping supine (on your back) increases gravitational collapse of the airway. Flow limitation often worsens in certain positions.
- Mask leak: Significant mask leak reduces effective pressure delivery, which can allow the airway to narrow even when the set pressure would otherwise be adequate.
What You Can Do About It
Understanding your flow limitation profile is the first step. Here's a practical approach:
Analyze your actual data
Tools like AirwayLab can read your SD card and calculate Glasgow Index, NED, and estimated RERA counts — metrics your machine's app won't show you.
Look at trends, not single nights
Everyone has occasional flow-limited breaths. What matters is the pattern across weeks. Consistently high Glasgow scores (>0.3) warrant attention.
Discuss with your sleep physician
Bring your flow limitation data to your next appointment. Many physicians are receptive to adjusting pressure, EPR, or mode settings when presented with objective evidence.
Further Reading
Palombini et al. (2013). "Upper airway resistance syndrome: still not recognized and not treated." Sleep Science, 6(1), 19-26.
Farré et al. (2004). "Noninvasive monitoring of respiratory mechanics during sleep." European Respiratory Journal, 24(6), 1052-1060.
Clark et al. (2017). "Automated detection of inspiratory flow limitation from CPAP devices." Journal of Clinical Sleep Medicine, 13(2).
Ready to See Your Flow Limitation Data?
AirwayLab analyzes your ResMed SD card data for flow limitation, RERAs, and breathing patterns — all in your browser, with nothing uploaded to any server.