There are four main types of positive airway pressure machines used to treat sleep apnea: standard CPAP, auto-adjusting CPAP (APAP), bilevel (BiPAP), and adaptive servo-ventilation (ASV). Each delivers pressurized air through a mask to keep your airway open during sleep, but they differ in how they control that pressure and who they’re designed for.
Standard CPAP: Fixed Pressure
A standard CPAP machine delivers one continuous pressure all night long. Your sleep specialist determines the right pressure level during a sleep study, and the machine is set to that number. It stays constant whether you’re in light sleep, deep sleep, or briefly waking up. This is the most common starting point for obstructive sleep apnea treatment and the least expensive option.
The simplicity is both the strength and the limitation. Fixed pressure works well for many people, but it doesn’t adapt to real-time changes in your breathing. If your apnea severity fluctuates (because of sleeping position, alcohol, weight changes, or nasal congestion), the machine can’t respond. Some people also find that a constant pressure feels uncomfortable during exhalation, since you’re breathing out against the same force the machine uses to push air in.
APAP: Pressure That Adjusts Automatically
An auto-adjusting machine, often called APAP or AutoPAP, monitors your breathing throughout the night and raises or lowers pressure in response to what it detects. When the device senses airflow limitation, a partial blockage, or a full apnea event, it increases pressure. When your airway is stable, it drops back down.
This results in a meaningful practical difference: the average pressure you experience over the course of a night is significantly lower than what a fixed CPAP would deliver. Studies show APAP normalizes breathing just as effectively as fixed CPAP in terms of reducing apnea events and improving daytime sleepiness, quality of life, and cognitive function. The lower average pressure can make the experience more comfortable, especially for people who struggle with the feeling of high, constant airflow.
APAP also handles variability in your condition automatically. If you gain or lose weight, drink alcohol before bed, sleep on your back instead of your side, or move through different sleep stages, the machine recalibrates without needing a new sleep study or manual adjustment. For many people diagnosed with straightforward obstructive sleep apnea, APAP has become the default prescription.
BiPAP: Two Separate Pressure Levels
A bilevel machine (BiPAP or BPAP) delivers two distinct pressures: a higher one when you breathe in and a lower one when you breathe out. The inhalation pressure keeps your airway open, while the gap between the two pressures actively assists your breathing by making exhalation easier and supporting your lungs in moving more air.
Clinical guidelines recommend that the difference between the two pressures fall between 4 and 10 cm of water pressure for treating obstructive sleep apnea, with maximum inhalation pressure capped at 30 cm of water pressure for adults. That higher ceiling matters: some people need more pressure than a standard CPAP can comfortably provide, and the dual-pressure design makes those higher settings tolerable.
BiPAP is typically prescribed in specific situations. You might be moved to a bilevel machine if you’ve tried CPAP or APAP and couldn’t tolerate the pressure during exhalation, if you need very high pressures that feel suffocating on a single-pressure device, or if you have a condition beyond simple obstructive apnea. BiPAP is also used for people with COPD, hypoventilation syndromes, and other conditions where the lungs need active assistance moving air in and out. Insurance coverage for a bilevel machine generally requires documented evidence that a standard device wasn’t sufficient, often through a sleep study showing persistent oxygen drops below 88% for five or more cumulative minutes while using a CPAP.
ASV: For Central Sleep Apnea
Adaptive servo-ventilation is the most sophisticated type. Unlike the other machines, which primarily keep a physical airway open, ASV targets a different problem: your brain intermittently failing to send the signal to breathe. This is called central sleep apnea, and it involves a cyclical pattern where breathing gradually weakens, stops, then restarts with a burst of effort.
ASV works by tracking your breathing pattern over the previous several minutes and calculating a target ventilation level. When your breathing weakens or stops during an apnea event, the machine increases pressure support to maintain stable airflow. When you’re breathing normally, it backs off. The goal is to smooth out the unstable cycling pattern rather than simply splinting an airway open.
This type of machine is prescribed for central sleep apnea, complex sleep apnea (a combination of obstructive and central), and Cheyne-Stokes respiration, a specific breathing pattern commonly associated with heart failure. ASV is the most expensive category and requires a clear diagnosis showing that central events are the primary problem. It is not appropriate for all heart failure patients, so the prescribing criteria are more specific than for other PAP devices.
Comfort Features Across Machine Types
Regardless of which type you use, most modern machines include comfort features that make the experience more livable. The most important is expiratory pressure relief, a technology that briefly reduces pressure at the start of each breath out. ResMed’s version (called EPR) can drop pressure by up to 3 cm of water pressure during exhalation. Philips’ version (called C-Flex) works similarly, lowering expiratory pressure by at least 2 cm. These features are available on both fixed CPAP and APAP devices and can make a noticeable difference for people who feel like they’re fighting the machine when they exhale.
Humidification is the other major comfort feature. Most current machines have an integrated heated humidifier built directly into the unit, which adds moisture to the air before it reaches your mask. This reduces dryness in your nose, mouth, and throat. Standalone humidifiers that connect via a separate hose still exist and offer more customizable settings, but integrated units are far more common and convenient. If you travel frequently, be aware that running the heated humidifier on battery power can cut your battery life by 50% to 75%.
Noise and Portability
Modern CPAP machines are remarkably quiet. Most operate between 25 and 35 decibels, roughly the volume of a whisper. The difference between the quietest and loudest models in that range is barely perceptible, so noise is rarely a deciding factor between machine types.
Travel-specific CPAP machines are a subcategory worth knowing about. These are smaller, lighter versions designed to fit in a carry-on bag. They work on battery power, with most portable batteries lasting one to two nights per charge. Larger battery packs can extend that to three to five nights. The tradeoff is that travel machines typically lack built-in humidification, and you’ll need to turn off heated features to get usable battery life. They’re designed as supplements to your primary home machine, not replacements.
How the Right Type Gets Chosen
The type of machine you end up with depends on your diagnosis, your anatomy, and how your body responds to treatment. Most people with obstructive sleep apnea start with either a fixed CPAP or an APAP. If you tolerate the pressure well, that’s often where you stay. If you struggle with exhalation comfort despite pressure relief features, or if your pressures need to be unusually high, a BiPAP becomes the next step. ASV is reserved for the subset of patients whose central nervous system, not their throat tissue, is causing breathing interruptions during sleep.
Your sleep specialist will base the recommendation on your sleep study results, specifically the types of apnea events recorded and how severe they are. The process is sequential in most cases: you try the simplest effective option first, and move to more advanced devices only if needed. Insurance follows the same logic, requiring documentation that a simpler machine was tried or is inappropriate before covering a more complex one.