Understanding Your CPAP Data: A Plain-Language Guide to AHI, Leaks, and Flow Limitations

Modern CPAP and BiPAP machines record data every night — pressure delivery, breathing events, airflow patterns, leak rates, and more. That data lives on an SD card inside your device. When you pull it out and run it through analysis software, you get a CPAP report.

Tools like OSCAR (the gold standard for local, open-source data analysis) and AirwayLab can both read that raw data and surface it in a readable format. They complement each other — OSCAR gives you deep, granular access to your raw data files; AirwayLab adds a browser-based interface for quickly visualising breathing patterns and reviewing your therapy over time. Your data never leaves your browser when you use AirwayLab. No account required, no server upload — it runs entirely on your device.

AHI stands for Apnea-Hypopnea Index. It counts the average number of breathing pauses per hour of sleep — either full apneas (breathing stops briefly) or hypopneas (breathing becomes significantly reduced).

For most people on PAP therapy, a lower AHI is better. But what your clinician targets for you specifically depends on your full clinical picture, not just this number. That's not a disclaimer — it's genuinely how sleep medicine works.

The more important point: AHI is the most-watched metric, but it doesn't tell the whole story. You can have a low AHI and still feel like your therapy isn't working. That's where the other numbers come in.

Your clinician can help you understand what your AHI trend means in context.

Flow limitation is what happens when your upper airway partially collapses — not enough to trigger an apnea or hypopnea that gets counted in your AHI, but enough to restrict airflow through part of a breath. Your device detects this as a flattening in the breathing waveform rather than a clean round curve.

Flow limitations matter because they don't always show up in AHI. If you consistently see them in your data — especially clustering during certain parts of the night — your clinician can help you understand what flow limitation patterns mean for your situation.

AirwayLab plots flow limitation data across the full night so you can see whether they appear in bursts, track with body position changes, or coincide with periods of high pressure demand.

RERAs — Respiratory Effort Related Arousals — are brief sleep disruptions caused by increasing respiratory effort. They don't fully meet the criteria for a hypopnea, but they can still fragment sleep and affect how rested you feel in the morning.

Not all devices score RERAs the same way, and some don't report them at all. If your device data includes RERA events, AirwayLab will surface them alongside your AHI breakdown. Your clinician can help you understand what RERA data means in context.

A mask that doesn't seal properly leaks air. That leak disrupts pressure delivery and can affect the data your machine records even when your AHI looks fine on paper.

Most devices report leak rate as a 95th or 99th percentile figure — meaning that for 95% (or 99%) of the night, your leak was at or below that value. What counts as an acceptable leak rate depends on your machine model and mask type. Your device manual and your clinician's guidance are the right reference points for your specific situation.

AirwayLab displays leak and event data together so you can see whether they correlate.

If you're on a fixed-pressure CPAP, you have one set pressure delivered all night. If you're on an APAP (auto-adjusting) or BiPAP, your device varies pressure in response to what it detects in your breathing.

Looking at your pressure data over time shows whether your machine is frequently reaching its upper limit — which can indicate the auto-titrating algorithm is working hard to keep your airway open. Your clinician can help interpret pressure trends in context.

AirwayLab is built to make your CPAP report readable — not to replace the clinical conversation. Upload your SD card data, and you get:

Everything runs in your browser. Your data is yours. The analysis is free, and always will be.

If you already use OSCAR, AirwayLab doesn't replace it — they're looking at the same data in different ways. Use whichever interface helps you see your therapy more clearly.

Understanding your CPAP data means being an informed participant in your care — not a self-diagnosing one. When you can look at a chart and describe specific patterns — like flow limitations clustering after midnight, or leak rate spiking on certain nights — you have a clearer picture of what your data shows.

That's the goal. Upload your data and explore what's there.