If you've been on CPAP for a while and started wondering whether BiPAP might be relevant to your situation — or if you've just been prescribed BiPAP and want to understand what's different — this article covers the key distinctions between the two device types and how their therapy data looks when you analyse it.
The Core Difference: One Pressure vs. Two
CPAP(Continuous Positive Airway Pressure) delivers a single, continuous pressure throughout the entire breathing cycle. Whether you're inhaling or exhaling, the pressure stays the same. On auto-adjusting CPAP (APAP), the pressure varies based on detected events — but it still delivers one pressure level at any given moment.
BiPAP (Bilevel Positive Airway Pressure) delivers two separate pressure levels:
IPAP — Inspiratory Positive Airway Pressure
The higher pressure level, active during inhalation. Supports the breathing effort as air flows in.
EPAP — Expiratory Positive Airway Pressure
The lower pressure level, active during exhalation. Keeps the airway open while reducing the effort needed to breathe out against the machine.
The gap between IPAP and EPAP — called pressure support (PS) — is set by the prescribing clinician. This pressure difference actively assists the breathing effort during inhalation.
Why Two Pressures Instead of One?
For most people with obstructive sleep apnea, CPAP at an appropriate pressure is effective at maintaining airway patency. BiPAP is typically prescribed in situations where:
- High pressure is required and exhaling against it is uncomfortable or difficult — the lower EPAP makes exhalation easier
- There is a component of hypoventilation — conditions where breathing volume (tidal volume) needs support, not just airway patency
- Central or complex sleep apnea patterns are present — though specific device selection in these cases is complex and highly individualised
- Respiratory muscle weakness or certain neuromuscular conditions — where breathing assistance is needed beyond just keeping the airway open
- CPAP intolerance — when patients cannot tolerate single fixed pressure despite adequate time and fitting adjustments
The decision about which device is appropriate for a given person is made by a sleep physician based on diagnostic study results, clinical history, and response to therapy.
What the Therapy Data Looks Like: CPAP vs. BiPAP
When you review therapy data from each device type, the pressure reports look different.
CPAP / APAP reports show
- A single pressure line (or pressure distribution on APAP)
- 90th or 95th percentile pressure
- AHI with event subtypes
- Leak rate
BiPAP reports show
- IPAP and EPAP values (or ranges, on auto-BiPAP)
- Pressure support (IPAP − EPAP)
- Tidal volume and minute ventilation (advanced models)
- Leak rate
- AHI with event subtypes
One practical note: BiPAP devices — particularly those in the S/T (spontaneous/timed) category — may not export the same EDF data fields as APAP devices. If you're loading BiPAP data into AirwayLab or OSCAR, the available metrics may differ from what you'd see with a ResMed AirSense.
How Flow Patterns Differ
On a CPAP waveform, the inspiratory flow shape reflects the patient's own respiratory effort modulated by the constant applied pressure. Tools like the Glasgow Index and FL Score analyse these shapes to characterise breath quality — flatness, limitation patterns, and shape irregularities.
On BiPAP, the pressure support component actively augments inhalation. The resulting flow shapes look different: inhalation is typically faster and more assisted. Flow limitation analysis tools designed for CPAP waveforms interpret BiPAP waveforms with this context in mind.
Common Questions
“My AHI is fine on CPAP — why would I switch to BiPAP?”
AHI measures event count. It doesn't fully capture comfort, breathing effort, or ventilation adequacy. Whether AHI alone is sufficient to evaluate therapy outcome is a clinical determination. Your sleep physician considers the full picture.
“Can I request BiPAP if I find CPAP uncomfortable?”
Discomfort with CPAP is worth discussing with your sleep clinician — there are fitting, humidity, and pressure adjustment approaches that sometimes resolve tolerance issues without changing device type. Your clinician can advise which approach fits your situation.
“Is BiPAP more expensive?”
Generally yes — BiPAP hardware and supplies typically cost more than equivalent CPAP. Insurance coverage varies. Your equipment provider and clinician can walk through the options.
“Does AirwayLab work with BiPAP data?”
AirwayLab currently reads ResMed EDF format. Some ResMed BiPAP models write compatible EDF files. The available metrics depend on what your device records. Check the community forum for device-specific compatibility notes.
What to Ask Your Clinician
If you're wondering whether your current device type is the right fit for your situation, useful questions include:
- “Based on my recent data, does my event and pressure pattern suggest my current device is well-matched to my needs?”
- “Are there any aspects of my data — central events, pressure levels, flow patterns — that would warrant a review of my device type?”
- “If I'm finding exhalation difficult at my current pressure, is bilevel worth exploring?”
These questions give your clinician the opening to review your data in context. They are in the best position to compare your diagnostic study, your current data trends, and your symptoms together.
Further Reading
BiPAP Data Analysis: AirCurve 10 SD Card Guide — how to load and interpret BiPAP SD card data from a ResMed AirCurve.
BiPAP vs CPAP Data: What Changes in Your Reports — a closer look at how the data fields differ between device modes.
What Is Flow Limitation on CPAP? — the breath-shape analysis that goes beyond AHI.
AHI Normal but Still Tired? Understanding the Gap — why a normal AHI doesn't always mean therapy is complete.
A note on device decisions
AirwayLab is a data-visualisation tool, not a medical device. This article describes how CPAP and BiPAP devices work and how their therapy data differs — it is not a recommendation for any particular device or treatment. Device selection and prescription decisions belong with your sleep physician. Your clinician can help interpret these findings in context.