APAP vs CPAP vs BiPAP: Machine Modes and the Data Each Produces

Your CPAP machine isn't just one kind of device. "CPAP" has become a catch-all word, but the airflow your machine delivers — and the data it records — depends on which mode it runs. This guide explains the four main PAP (positive airway pressure) modes, which real-world models use them, and why two people with the same diagnosis can end up with very different reports.

Knowing your mode is the first step to reading your numbers correctly. A "95th-percentile pressure" means something very different on a fixed machine than on an auto-adjusting one, and a bilevel report has columns a basic CPAP report never shows.

CPAP — fixed pressure

A true CPAP (Continuous Positive Airway Pressure) machine delivers a single, constant pressure all night long, set by your provider after a titration study. If your prescription says 9 cmH₂O (centimeters of water, the standard pressure unit), the machine blows 9 on every breath — inhale and exhale, 2 a.m. or 6 a.m., REM or deep sleep.

This is the simplest mode, and its data reflects that simplicity:

• Pressure is essentially a flat line. There's no range to interpret.
• The report focuses on AHI (apnea-hypopnea index, events per hour), leak, and usage hours.
• A "95th-percentile pressure" reading is uninformative here, because pressure barely varies.

Many machines that are capable of auto mode are deliberately set to fixed CPAP. If you have a comfort feature like ResMed's EPR or Philips' Flex turned on, your machine briefly drops pressure on exhale — that's exhale relief, not bilevel, and the prescribed pressure is still fixed. (See ResMed EPR, Ramp & Climate Control for how to verify those settings in your data.)

Fixed CPAP reliably treats obstructive events — the airway collapses, and constant pressure splints it open. It does not directly correct the unstable breathing drive behind central (clear-airway) apneas; more on that below.

APAP/AutoSet — auto-adjusting within a range

APAP (Automatic Positive Airway Pressure), branded AutoSet by ResMed, doesn't pick one number. Your provider sets a minimum and maximum pressure, and the machine moves continuously within that window, raising pressure when it detects flow limitation, snoring, or an apnea, and easing off when your airway is stable.

APAP is the most common mode for new patients because it adapts to the things that change night to night — sleep position, alcohol, congestion, weight changes. It's also the mode that produces the richest pressure data:

• A minimum and maximum pressure setting that defines the operating range.
• Median (50th-percentile) and 95th-percentile (P95) pressure — the pressure your machine met or stayed below 95% of the night.
• A pressure trace that visibly rises and falls across the night.

The P95 matters far more on APAP than on fixed CPAP, precisely because pressure varies. On an auto machine, P95 tells you the pressure your therapy actually needed most of the night — useful context for a conversation with your provider about whether your range is set well. Our deep-dive on 95th-percentile vs median pressure walks through how to read those two numbers together.

If your auto-CPAP is constantly pushing toward its maximum, or your P95 sits far above your median, that's a data pattern worth discussing — not a cue to change settings yourself. Use your data to have an informed conversation with your provider.

BiPAP/bilevel — separate IPAP/EPAP with pressure support

BiPAP (Bilevel Positive Airway Pressure), also called bilevel, splits pressure into two distinct levels:

• IPAP — inspiratory (breathing-in) pressure, the higher number.
• EPAP — expiratory (breathing-out) pressure, the lower number.

The gap between them is pressure support (IPAP minus EPAP). This makes breathing out against the machine far more comfortable and actively assists each breath, which is why bilevel is prescribed when someone needs high pressures, struggles to exhale against CPAP, or has conditions where breathing assistance helps (such as some neuromuscular or hypoventilation cases).

Bilevel data is genuinely richer than CPAP or APAP data. Instead of one pressure column, your report shows:

ResMed's AirCurve line covers bilevel (VAuto for auto-adjusting bilevel, ST for spontaneous/timed). On the Löwenstein side, the prisma30ST is a BiLevel S/T device. Reading these reports takes a bit more practice — our AirCurve bilevel and ASV data guide breaks down what each column means.

ASV — adaptive servo for central/CSR

ASV (Adaptive Servo-Ventilation) is the most sophisticated mode. Built for central sleep apnea and Cheyne-Stokes respiration (CSR) — a waxing-and-waning breathing pattern often linked to heart conditions — ASV does something the other modes can't: it watches your breathing in real time and delivers a varying amount of pressure support, breath by breath, to smooth out the peaks and fill in the pauses (the "servo" part). When you breathe too much, it backs off; when you stop, it gives a backup breath.

ASV is appropriate only for specific diagnoses and is not interchangeable with CPAP or BiPAP. (Importantly, ASV has known contraindications in certain heart-failure patients — this is strictly a clinician's decision.) ResMed's AirCurve ASV and Löwenstein's prismaCR are examples.

Because support changes constantly, ASV data is the most dynamic of all: variable pressure support, a backup respiratory rate, and detailed central-event and periodic-breathing tracking. If you're seeing Cheyne-Stokes patterns on any machine, our explainer on periodic breathing and Cheyne-Stokes on your CPAP chart is a good companion read.

A note on central apneas

If central (clear-airway) events climb after you start CPAP, that can signal treatment-emergent central sleep apnea (TECSA), also called complex sleep apnea, which appears in roughly 5–15% of PAP titrations. The reassuring part: it resolves on its own in about 60–80% of cases within weeks to a few months of continued CPAP under clinician guidance. Persistent cases — especially with returning symptoms, heavy daytime sleepiness, deep oxygen drops, or new medications (like opioids) or heart/kidney conditions — warrant evaluation, and the answer may be ASV or BiPAP. The one thing it is never: a reason to raise your own pressure to "chase" central events. Higher pressure doesn't fix central apneas and can sometimes provoke them. See central apneas showing up on CPAP for the full picture.

What data each mode logs

The same diagnosis can produce wildly different reports depending on mode. Here's the at-a-glance map:

Every mode also logs the universal trio — AHI, leak rate, and usage hours — but the way to read them shifts. A few cross-cutting reminders:

• Device-reported AHI is an estimate. Your machine has no EEG and can't score arousals, so its AHI differs from a lab-scored study. Treat it as a trend, not a lab result. (More in how to read your CPAP data.)
• Leak invalidates everything else. ResMed reports excess leak (threshold ~24 L/min at the 95th percentile); Philips reports total leak — different baselines. A large leak can make your machine under-report AHI, so check leak first. See CPAP leak rate: what's acceptable.
• One night is noise; weeks are signal. Don't react to a single bad night — look at the trend over time.

The treatment goal for most adults is a residual AHI below 5 events per hour (the AASM's definition of an "optimal" titration). Some clinicians aim lower when it's comfortably achievable, but goals are individualized — your provider sets yours.

Whatever mode you're on, SomniCharts visualizes how your pressure moved through the night against your events — so you can actually see an APAP ramping toward its ceiling or an ASV smoothing a Cheyne-Stokes cycle. It imports data from ResMed, Philips Respironics (including the encrypted DreamStation 2), and Löwenstein prisma, and explains it in plain language automatically — no desktop software, no per-vendor learning curve.

Mapping real models to modes

Most modern machines are mode-flexible — the same hardware can run fixed or auto, and many bilevel units cover several sub-modes. Here's how the major brands line up:

The takeaway: AirSense = APAP/CPAP, AirCurve = BiPAP/ASV. If you have an AirCurve, you're on a bilevel or ASV machine and should expect IPAP/EPAP columns. If you have an AirSense, you're on fixed or auto and your pressure data hinges on whether auto is enabled.

Switching brands? Your mode and pressure history are worth carrying with you — see switching CPAP brands while keeping your data history. And for the bigger picture on what each machine records, head back to the CPAP Machines & Devices pillar.

Quick FAQ

Is APAP better than CPAP? Not universally. APAP adapts to night-to-night changes and produces richer data, but a well-titrated fixed CPAP works perfectly for many people. The "better" mode is the one your provider prescribes for your physiology.

What's the difference between BiPAP and ASV? BiPAP delivers two fixed (or auto-ranging) pressure levels for inhale and exhale. ASV goes further, varying pressure support breath by breath to stabilize central and Cheyne-Stokes breathing — it's a treatment for central, not just obstructive, sleep apnea.

Why does my report show a pressure range instead of one number? You're on APAP (or auto-bilevel). The range is your min/max setting, and the 95th-percentile figure tells you the pressure your therapy actually reached most of the night.