Understanding Flow Limitation on Your CPAP

Your AHI Is Fine. So Why Are You Still Tired?

You check your CPAP data every morning. Your AHI is 1.2. Your leak rate is low. Your usage is over seven hours. By every standard metric, your therapy is working. And yet you wake up feeling like you barely slept.

If this sounds familiar, flow limitation may be the missing piece. It is one of the most clinically meaningful signals your CPAP machine records, and most users never see it because myAir doesn't show it.

What Is Flow Limitation?

Your airway exists on a spectrum. At one end, it is fully open: air flows freely with each breath. At the other end, it collapses completely: that is an apnea. Flow limitation lives in the middle of that spectrum. Your airway narrows enough to restrict airflow, but not enough to fully close.

Think of it like a garden hose. Pinch the hose halfway and water still flows, but the stream weakens and sputters. Your airway does the same thing. Air still gets through, but your body has to work harder to breathe. That extra effort fragments your sleep, even though no "event" gets scored.

This matters because the standard AHI metric only counts events where airflow drops by at least 30% (hypopnea) or stops entirely (apnea). Partial narrowing that reduces airflow by 10% or 20% doesn't register as an event at all. You could have hundreds of these partial restrictions in a single night and your AHI would still read zero.

How Your CPAP Machine Measures It

ResMed AirSense 10 and 11 machines continuously monitor the shape of your breathing waveform. A normal breath produces a smooth, rounded flow curve. When the airway narrows, the top of that curve flattens, creating what clinicians call "inspiratory flow limitation."

Your machine quantifies this flattening as a number between 0 and 1:

Value	Meaning	What It Looks Like
0.0 - 0.2	Normal	Round, unrestricted breathing waveform
0.2 - 0.5	Moderate	Noticeable flattening, airway is partially narrowed
0.5 - 1.0	Elevated	Significant flattening, airway is substantially restricted

This data is recorded every two seconds to your SD card. myAir does not display it. OSCAR shows it as a raw signal. CPAP Clarity shows it as a color-coded chart with green, amber, and red zones that match these thresholds, plus plain-English insights that explain what your numbers mean.

Reading Your Flow Limitation Chart

When you import your SD card data into CPAP Clarity, the flow limitation chart appears in the Detailed Charts section of your dashboard. Here's how to read it.

The Y-axis runs from 0 to 1. Lower is better.

The color zones correspond to the thresholds above:

• Green zone (0 - 0.2): Your airway is open. Normal breathing with no significant restriction.
• Amber zone (0.2 - 0.5): Moderate narrowing. Some restriction is present but not alarming on its own. Brief dips into this zone are common during position changes or REM sleep.
• Red zone (0.5 - 1.0): Significant narrowing. Your airway is substantially restricted. Sustained time in this zone can fragment sleep and reduce therapy effectiveness.

What a good night looks like: The line stays mostly in the green zone with occasional brief rises into amber. The average is below 0.2.

What a concerning night looks like: The line spends extended periods in amber or red, especially during the middle of the night when sleep is deepest. The average is above 0.3, or the P95 (95th percentile) is above 0.5.

What CPAP Clarity Tells You

CPAP Clarity evaluates your flow limitation data automatically and surfaces insights when something needs attention. There are three patterns the tool looks for:

Elevated average flow limitation. If your average flow limitation for the night is above 0.2, you will see a warning explaining that your airway was partially narrowing throughout the night. If it is above 0.5, the insight escalates to an alert. Even when your AHI looks acceptable, sustained flow limitation can fragment your sleep.

High peak flow limitation. If your 95th percentile flow limitation is above 0.5, it means you had sustained periods of significant airway narrowing. If it exceeds 0.7, that is a more serious pattern that may indicate upper airway resistance your standard AHI scoring misses.

Normal AHI with elevated flow limitation. This is the pattern that catches many users off guard. Your AHI is under 5 (clinically "normal"), but your flow limitation is elevated. This combination is common in a condition called upper airway resistance syndrome.

Upper Airway Resistance Syndrome (UARS)

UARS is a condition where the airway narrows repeatedly during sleep without ever fully collapsing. Because there are no full apneas or significant hypopneas, the AHI stays low. But the repeated partial restrictions trigger micro-arousals that fragment sleep and produce symptoms identical to obstructive sleep apnea: daytime fatigue, unrefreshing sleep, morning headaches, and difficulty concentrating.

Research suggests UARS is more common in women and in younger, leaner patients who don't fit the typical sleep apnea profile. It is also more commonly identified through in-lab sleep studies that measure respiratory effort-related arousals (RERAs), which home sleep tests typically do not detect.

Your CPAP's flow limitation data is one of the few ways to see this pattern at home. If your AHI is consistently low but your flow limitation is consistently elevated, that is worth discussing with a sleep specialist.

What Can Affect Flow Limitation

Several factors influence how much flow limitation you experience on a given night:

Sleep position. Sleeping on your back (supine) tends to increase airway narrowing due to gravity pulling soft tissue toward the back of the throat. Side sleeping typically reduces flow limitation. If your chart shows higher values in the first and last hours of the night, you may be falling asleep and waking up on your back while spending the middle of the night on your side.

Pressure settings. Your CPAP pressure is designed to hold your airway open. If your minimum pressure is set too low, the machine may not provide enough support during the early part of the night before the auto-algorithm ramps up. If your EPR (Expiratory Pressure Relief) setting is too high, the reduced pressure during exhalation may allow the airway to narrow.

Alcohol and sedatives. Both relax the muscles around your airway, making it more likely to narrow. If you notice higher flow limitation on nights after drinking, this is likely the reason.

Nasal congestion. When your nose is partially blocked, the machine has to push harder to deliver airflow. This can affect the breathing waveform and increase measured flow limitation.

Weight changes. Even modest weight gain can increase tissue around the airway, making partial narrowing more likely.

What You Can Do

Flow limitation data is informational. It helps you and your provider understand what is happening with your airway during sleep. Here are productive next steps based on what your data shows.

If your flow limitation is occasionally elevated (average 0.2 - 0.3): This is common and usually not a concern by itself. Monitor it over time. If the trend worsens, bring it up at your next appointment.

If your flow limitation is consistently elevated (average above 0.3): Generate a PDF report from CPAP Clarity and bring it to your sleep specialist. The chart and summary stats give your provider concrete data to work with. They may consider adjusting your pressure range or EPR setting.

If you have low AHI but high flow limitation and persistent fatigue: This is the UARS pattern. Ask your sleep specialist whether a pressure adjustment or an in-lab study to evaluate RERAs would be appropriate.

In all cases: Do not adjust your own pressure settings based on flow limitation data alone. Pressure changes should always be discussed with your prescribing provider, who can evaluate the full clinical picture.

When to Talk to Your Doctor

Bring your flow limitation data to your next appointment if any of these apply:

• Your average flow limitation is consistently above 0.3 across multiple nights
• Your AHI is below 5 but you still feel fatigued, unrefreshed, or unable to concentrate during the day
• You see a pattern of high flow limitation (above 0.5) during specific parts of the night
• Your flow limitation has been trending upward over weeks or months
• You have tried positional changes and nasal treatments without improvement

Your CPAP data is a conversation starter, not a diagnosis. Flow limitation measurements from a CPAP machine are not the same as clinical respiratory effort measurements from a polysomnogram. But they provide a useful signal that can help your provider make more informed decisions about your care.

The Signal Your AHI Misses

Flow limitation is one of the most underused pieces of data in CPAP therapy. Most users never see it because the tools they use don't show it. But for the subset of patients whose AHI looks fine while their sleep quality suffers, it can be the difference between thinking "my therapy is working" and understanding why it might not be working well enough.

Import your SD card data into CPAP Clarity to see your flow limitation chart, or check the guide to reading your CPAP data to learn what all the metrics on your dashboard mean.