LEARN · 7 MIN READ

Flow limitation explained

Flow limitation is a breath where air is still moving, but the airway is narrowed enough that the flow stops rising the way it should. On a waveform it looks like a breath with the top chopped off. It does not count toward your AHI, yet it is one of the main signals an auto CPAP uses to decide when to raise pressure, and it can leave you tired even when your AHI is low.

Create your free account Read on the Knowledge Hub

Three stages of upper-airway narrowing drawn as cross-sections, open, narrowing and narrowed, each above its airflow shape: rounded, mid-dip and flat-topped

What a normal breath looks like

Picture the airflow trace from your CPAP: a wave that rises as you inhale and falls as you exhale. A normal inhale is rounded, a bit like the top half of a sine wave. Flow builds smoothly, peaks around the middle of the breath, and eases off.

That rounded shape happens because the airway is open and offers steady resistance. The harder you pull, the more air comes in.

What a flow-limited breath looks like

When the upper airway is partially narrowed, something different happens. As you pull harder, the soft tissue is drawn inward and the airway narrows further. Flow rises at first, then hits a ceiling: pulling harder no longer brings in more air. The top of the inspiratory wave flattens into a plateau. In more pronounced cases it develops a dip in the middle or a sharp rise followed by a flat shelf.

That flattening is flow limitation. The breath is still a breath, so there is no apnea and often no hypopnea. But it is a breath taken through a straw, and the effort it costs can lighten your sleep or wake you briefly without you ever knowing.

Why it matters even though it is not in your AHI

The apnea-hypopnea index counts events that cross defined thresholds: airflow down 90 percent for an apnea, down a set percentage for a hypopnea. Flow limitation sits below those thresholds. A night full of flow-limited breathing can score an AHI of 1.

But flow limitation is associated with increased breathing effort, and increased effort is associated with arousals, the brief shifts toward wakefulness that fragment sleep. In the sleep-medicine literature, runs of flow-limited breaths that end in an arousal are called respiratory effort-related arousals, or RERAs, and they are counted in a broader index called the RDI (respiratory disturbance index). Your CPAP cannot see arousals, so it cannot score RERAs. It can, however, see the flattened breaths.

This is one of the explanations when someone has a residual AHI under 2 and still feels unrefreshed. It is not the only explanation, and it is a conversation for your clinician, but it is one worth having with the waveforms in hand.

How ResMed machines detect and use it

ResMed's AutoSet algorithm analyses the shape of each inspiratory flow curve and derives a flow limitation measure, reported on the SD card as a per-breath or per-interval index. The flatter the top of the breath, the higher the value.

The algorithm responds to three things when deciding to raise pressure: flow limitation, snoring and obstructive apnea. Flow limitation is the earliest and gentlest of the three, so in practice it drives much of the pressure movement on an auto machine during a typical night. When the machine sees a run of flattened breaths it nudges pressure up; when the breaths round out again it lets pressure drift down.

The AutoSet for Her mode uses a variant that responds more readily to flow limitation and raises pressure in smaller steps, on the basis that women with sleep apnea tend to have more flow limitation and fewer frank apneas.

This is why your pressure P95 can sit well above your minimum on a night with very few events. The machine was not chasing apneas; it was chasing flow limitation.

What drives flow limitation

Insufficient pressure. The commonest cause. A minimum pressure set low leaves the airway partially narrowed for stretches of the night before the machine responds.
Nasal congestion. A blocked nose restricts flow upstream of the airway and produces flattened curves even when the throat is fine.
Sleep position and stage. Supine sleep and REM sleep both relax the airway.
EPR. Lower pressure on exhale means the start of each inhale begins from a lower pressure, and some people see more flattening as a result. See EPR and Flex explained.
Mouth breathing and leak. Both distort the flow signal and can either mask or mimic limitation.
Alcohol and sedatives. Both reduce airway muscle tone.

Patterns worth noticing

A small amount of flow limitation on most nights is normal and does not need action. The patterns that tend to prompt a clinical conversation are:

Flow limitation that stays elevated most of the night while pressure sits at or near the minimum. The machine may be slow to respond, or the floor may be low.
Flow limitation rising with every EPR increase.
Clusters of flattened breaths followed by a sigh or a large recovery breath, which is what an arousal can look like on the flow trace.
Flow limitation that tracks your trigger journal: wine, late meals, pollen season.

How Apnealytics shows flow limitation

Flow limitation is a Pro feature in Apnealytics because it lives in the high-resolution data. The per-night breath-by-breath viewer shows the airflow trace with a crosshair and zoom, so you can step through individual breaths and see their shape. Where the SD card provides a flow limitation channel, it is drawn as a strip below the airflow so you can see the machine's own assessment line up with the flattened breaths.

Put the pressure trace beside it and the AutoSet logic becomes visible: flow limitation climbs, pressure follows a few breaths later, breaths round out, pressure eases. If pressure does not follow, you are probably looking at the ceiling of your range, or at central-looking events the machine has chosen not to chase.

The pressure insights page in Trends is where the nightly picture lives: pressure P95 inside the set range, with "What changed?" markers on settings changes so you can see whether a change in EPR or minimum pressure moved your flow limitation.

What to do with this

Nothing on your own. Flow limitation is a reason to talk to your clinician with specific evidence: a few screenshots of flattened breaths, the nights they appeared on, and your AHI, leak and pressure for context. The usual clinical responses, a higher minimum pressure, a change in EPR, a nasal review, are theirs to make.

What you can do yourself is notice. People who learn to recognise the flattened breath shape stop being puzzled by tired mornings with a perfect AHI, and start having a more productive conversation about why.

FAQ

Questions people ask

Help center →

What is flow limitation on a CPAP?

Flow limitation is a breath where the airway is partially narrowed, so airflow stops increasing even as you inhale harder. On the airflow waveform the top of the inhale flattens into a plateau instead of a rounded peak. It does not count as an apnea or hypopnea, but ResMed auto machines use it as a primary signal for raising pressure.

Is flow limitation included in AHI?

No. AHI counts apneas and hypopneas, which require airflow to drop by defined amounts for at least 10 seconds. Flow-limited breaths move too much air to qualify. In a sleep lab, flow limitation that ends in an arousal is scored as a RERA and counted in the RDI, but a CPAP machine cannot see arousals, so it reports flow limitation separately.

Can flow limitation make me tired with a low AHI?

It can contribute. Flow-limited breathing requires more effort, and increased breathing effort is associated with brief arousals that fragment sleep without producing scorable events. If your AHI is low and you still wake unrefreshed, flow limitation is one of several things worth reviewing with your clinician, along with usage hours, leak and other sleep factors.

How does ResMed AutoSet respond to flow limitation?

The AutoSet algorithm analyses the shape of each inspiratory flow curve and raises pressure when it detects a run of flattened breaths, then gradually lowers it when breathing normalises. Along with snoring and obstructive apneas, flow limitation is one of the three triggers for pressure increase, and in practice drives much of the pressure movement on a typical night.

Does EPR cause flow limitation?

It can increase it for some people. EPR lowers pressure on exhale, so each inhale begins from a lower pressure, and in some users the start of the breath shows more flattening as a result. Many people see no difference. Comparing your flow limitation and pressure data before and after an EPR change, with your clinician, is the way to find out whether it affects you.