How to Read the CPAP Flow Rate Waveform (Flat-Top Breaths)

Most CPAP reports lead with a single number — your AHI. But buried in your data is a chart that tells a far richer story: the flow rate waveform, a breath-by-breath tracing of the air actually moving in and out of your airway. It's the most detailed view your machine records, and once you can read its shapes, you'll understand things AHI alone will never show you.

This guide is the deep end of Reading Your CPAP Data. If you're newer to the basics, start with How to Read Your CPAP Data and come back here when you're ready to read individual breaths.

What the flow rate waveform shows (breath-by-breath airflow)

The flow rate waveform is a continuous line that rises and falls with every breath you take. When you inhale, air flows toward your lungs and the line climbs above the center; when you exhale, the line dips below it. One full up-and-down cycle is one breath.

Unlike summary metrics — AHI, leak rate, 95th-percentile pressure — which compress your whole night into a handful of numbers, the flow rate waveform preserves the raw texture of your breathing. It's the closest thing your home machine produces to what a sleep lab sees on a polysomnogram's airflow channel.

What it lets you see directly:

• The shape of each breath — smooth and rounded, or flattened and strained.
• The rhythm of your breathing — steady, or rising and falling in waves.
• Brief disruptions — sudden gasps, irregular bursts, or pauses that may signal an arousal or an event.

Crucially, the waveform reveals problems that don't register in your event counts at all. You can have flow limitation — partial airway narrowing on inhale — with an AHI of zero. Your machine scored no apneas or hypopneas, yet your airway was working harder than it should every breath. The waveform is where that hidden effort becomes visible.

Reading breath shapes — rounded vs flat-top vs jagged

The single most useful skill here is reading the top of each inhalation. That curve, more than anything else, tells you how freely air is moving.

A rounded breath is what you want to see. Air accelerates smoothly to a gentle peak, then tapers off — no struggle, no ceiling.

A flat-topped breath is the telltale sign of flow limitation. As your soft tissues collapse inward, they impose a limit on how fast air can move. You keep trying to inhale harder, but the flow can't increase, so the top of the waveform flattens into a plateau instead of a smooth dome. Flow limitation is the mildest form of airway obstruction on the same spectrum that includes hypopneas and apneas — and it's the engine behind RERAs and conditions like UARS. For the full picture, see CPAP Flow Limitation: The Hidden Metric Beyond AHI and RERA and Flow Limitation: The Events That Don't Show in Your AHI.

Jagged breaths are noisier to interpret. Sometimes they reflect a genuine arousal or unstable breathing; sometimes they're an artifact of a large leak or mouth movement. Context matters — which is why reading the waveform alongside your leak and event data is essential.

The garden-hose analogy for flow limitation

Picture water running through a garden hose at full pressure. With the hose wide open, the flow is strong and steady — that's a rounded breath.

Now step on the hose partway. No matter how high you crank the tap, the water can't flow any faster past the pinch point; it hits a ceiling and holds there. That flattened ceiling is exactly what a flat-top breath is: your airway is the pinched hose, and the plateau is the speed limit it imposes on every inhale. You're working harder upstream — straining to breathe — but the airflow simply can't rise past the narrowing.

This is why flow limitation can quietly fragment your sleep even when your AHI looks perfect. The airway never fully closes (so no apnea is scored), but the constant extra effort can trigger micro-arousals that leave you tired despite "good numbers." If that sounds like you, Still Tired on CPAP With Good Numbers? digs into why.

Why the high-resolution (25Hz) data matters

The flow rate signal is recorded at high resolution — around 25 samples per second (25Hz). That means roughly 25 data points capture the shape of each fraction of a second of airflow.

This resolution is what makes breath-shape analysis possible at all. A coarse, low-frequency recording would smear a flat-top breath into something that looks rounded, hiding the very flow limitation you're trying to detect. At 25Hz, the subtle clipping at the top of a strained inhale is preserved faithfully — you can actually see the plateau.

A few practical implications:

• The detail is real, not cosmetic. The plateaus, notches, and irregularities you see are genuine features of your breathing, captured at medical-grade precision.
• It generates enormous amounts of data. A single night produces hundreds of thousands of flow-rate samples — far more than any person can review breath by breath (more on that below).
• Not every app exposes it. Manufacturer phone apps like ResMed's myAir don't show you the flow waveform at all; you generally need the SD-card data and a tool that reads it. (See How to Read Your ResMed myAir Score for what those apps hide.)

Spotting arousals and instability

Beyond single-breath shape, the waveform reveals patterns across breaths — and these tell you about sleep stability.

Arousals often show up as a sudden change in the rhythm: a few flat or shallow breaths, then an abrupt deep "recovery" breath or a burst of larger, irregular breaths as you partially wake. You may see this cluster right after a run of flow-limited breaths — the body straining, then arousing to reopen the airway. That arousal is exactly what a RERA captures.

Breathing instability appears as a waxing-and-waning rhythm — breaths that grow deeper, then shallower, then deeper again in repeating waves, sometimes with pauses at the low point. This crescendo-decrescendo pattern can point toward periodic breathing or, in some cases, central events. If you see it, read Periodic Breathing & Cheyne-Stokes on Your CPAP Chart and Central Apneas Showing Up on CPAP: Treatment-Emergent CSA Explained.

A grounding point on central events, because the waveform can surface them: CPAP is designed to splint the airway open and reliably treats obstructive apneas, but it does not directly correct the unstable breathing drive behind central (clear-airway) apneas. A rise in central events on CPAP can indicate treatment-emergent central sleep apnea (TECSA), which often resolves on its own within weeks to a few months of continued CPAP (reported spontaneous resolution roughly 60–80% of cases). Management is your clinician's call — watchful waiting, or, if it persists, options like BiPAP or ASV. Never raise your CPAP pressure on your own to chase central events — higher pressure doesn't fix them and can sometimes provoke them. Central vs Obstructive Apnea explains the distinction in plain terms.

One more interpretation caveat: a large leak can corrupt the waveform, throwing the line into jagged noise and causing the machine to under-report your AHI because it can't reliably "see" your breathing. Always sanity-check the flow waveform against your leak data — CPAP Leak Rate: What's Acceptable covers the thresholds (ResMed's excess-leak benchmark is 24 L/min at the 95th percentile).

Why you shouldn't have to eyeball thousands of breaths

Here's the honest problem with the flow rate waveform: it's the most revealing chart you have, and it's also the one almost nobody can practically use. A single night holds hundreds of thousands of samples and tens of thousands of breaths. Hunting through them manually for flat-tops and arousal clusters — the way power users do in OSCAR — is tedious, error-prone, and frankly intimidating. It's the exact reason most people open OSCAR once, see the wall of squiggles, and never go back. (OSCAR CPAP Software Guide is a fair primer if you want to try.)

This is the part SomniCharts is built to take off your plate. SomniCharts (through its SomniDoc analysis) interprets your flow rate waveform automatically — surfacing flow-limitation patterns and breathing instability in plain language, instead of asking you to scroll through every breath. It imports data from ResMed, Philips Respironics (including the encrypted DreamStation 2), and Löwenstein prisma machines, so the analysis works across vendors in one place.

And remember the recurring truth of all CPAP data: a single night is mostly noise. One night of flat-top breaths might be allergies or sleeping position; a trend of flow limitation over weeks is a signal worth bringing to your provider. Read your waveform for what it teaches you — then use what you learn to have an informed conversation with the person who manages your therapy.