How to Analyze BiPAP Data From Your SD Card

A CPAP machine holds one pressure. A bilevel machine delivers two, and records them separately. The NHLBI describes a bilevel PAP machine as one that delivers different pressure depending on whether you are breathing in or out [S1]. Three terms come up again and again:

• IPAP is the higher pressure while you breathe in. In plain terms, it is the push the machine adds to each breath in.
• EPAP is the lower pressure while you breathe out. In plain terms, it is the pressure left in your airway as you exhale, so breathing out is against less pressure than on the way in.
• Pressure support is IPAP minus EPAP. With an IPAP of 14 and an EPAP of 10 cmH2O, pressure support is 4 cmH2O. In plain terms, it is how much extra help each breath in gets.

Some modes add more. An ST (Spontaneous-Timed) machine records backup breath triggers, which show when the machine started a breath rather than you. ASV and CS models also record target ventilation and minute ventilation, which describe the amount of air per minute the machine is aiming for and the amount you actually breathe. The BiPAP glossary entry and CPAP vs BiPAP cover how the two machine types differ.

On a ResMed AirCurve 10 or 11 (VAuto, ST, ASV and CS models), the card has a DATALOG folder with one folder of EDF files per night. For bilevel machines those files hold the flow waveform, the IPAP and EPAP pressure channels, pressure support data and the respiratory events. A file called STR.edf in the card root is the session summary, and AirwayLab reads the settings context for each night from it. Identification.tgt names the device model.

The step-by-step for finding the slot and removing the card is in the AirCurve SD card walkthrough and the AirCurve 10 guide. Copying the files is read-only, so the card goes back in the machine unchanged.

BMC Luna 2 and RESmart machines store their data differently, in numbered files named after the device serial number rather than in EDF files. They record the therapy mode (CPAP, AutoCPAP, S, S/T or AutoS), pressure settings, mask type and leak rates. For modes other than CPAP, some displayed settings are currently approximated rather than read from the card. The BMC Luna guide has the details.

Once the card is open in AirwayLab, a bilevel night reads in four passes. None of them needs a medical background.

1. Start with events. AHI is the number of apneas and hypopneas per hour, so it counts breathing pauses and shallow breaths. On ASV and CS models the breakdown also includes central events, where the effort to breathe stops briefly.
2. Look at flow limitation. Flow limitation means breaths where the airway narrows without being counted as an event. AirwayLab scores it with the Glasgow Index, which checks each breath for nine shape traits and sums them into a night score from 0 to 9, where lower is a more typical shape. Bilevel users often see flow limitation patterns that differ from CPAP users, and the NED engine, which looks at how effort and airflow relate, is particularly relevant for bilevel data. A high score is a pattern in the data, not a conclusion. Your clinician can help interpret these findings in context.
3. Check the pressure channels. AirwayLab estimates the delivered IPAP and EPAP from the pressure signal. On a VAuto machine, which adjusts EPAP automatically, the record shows how the machine moved through the night rather than only the minimum and maximum you set.
4. Read leak next. Leak is air escaping around the mask rather than reaching you, so the leak trend shows how well the seal held over the night. Look at it beside the flow limitation score for the same night, because AirwayLab shows both for every night.

When a flag looks odd, open the flow waveform behind it. AirwayLab shows the flow waveform, so you can see the breaths that produced the score.

AirwayLab shows the event breakdown, flow limitation scores, delivered IPAP and EPAP, pressure support, breathing regularity and leak trends for each night. That gives you and your clinician more to look at than AHI alone.

It does not tell you what your pressure settings should be, and it does not replace your sleep team. Pressure settings are set by your clinician, so bring any questions to your clinician. AirwayLab describes what the machine recorded, nothing more.