EPR stands for Expiratory Pressure Relief, a comfort feature on ResMed CPAP machines that lowers air pressure when you breathe out. Standard CPAP delivers the same constant pressure on every inhale and exhale, which can make exhaling feel like blowing against a wall. EPR dips the pressure during exhalation so breathing feels more natural, then brings it back up to your prescribed level before your next inhale.
How EPR Works
Your CPAP machine monitors your breathing cycle in real time. The moment it detects you’ve started to exhale, it reduces the delivered pressure by a set amount. The drop is greatest right at the transition from inhaling to exhaling, then pressure gradually returns to your full prescribed level before the next breath begins. This mimics the way your body naturally experiences less airway resistance during exhalation compared to inhalation.
EPR is adjustable in three levels. Level 1 provides the smallest pressure drop, level 2 a moderate drop, and level 3 the maximum reduction of about 3 cmH2O. Setting EPR to 0 turns it off entirely, delivering standard constant-pressure CPAP. The exact pressure reduction at each level can vary slightly between device models, so the numbers don’t always correspond perfectly to a specific cmH2O value.
EPR vs. Similar Features on Other Machines
ResMed calls it EPR. Philips Respironics machines use a similar concept called C-Flex (on fixed-pressure devices) and A-Flex or P-Flex (on auto-adjusting models). All of these work on the same basic principle: detect exhalation and temporarily lower pressure. The way they shape the pressure curve differs slightly. C-Flex produces a sharper dip concentrated at the start of exhalation, while EPR creates a more gradual, predictable reduction across the entire exhale. In bench testing, both EPR at level 3 and P-Flex lowered the actual delivered pressure by about 2.6 cmH2O compared to standard CPAP, while C-Flex+ at its highest setting lowered it by roughly 1.8 cmH2O.
How EPR Differs From BiPAP
EPR can feel a bit like a bilevel (BiPAP) machine, but the two are fundamentally different. A BiPAP device uses two completely separate pressure settings: a higher one for inhalation (IPAP) and a lower one for exhalation (EPAP). The gap between those two pressures is typically at least 4 cmH2O and can be much larger, up to 25 cmH2O or more. That large pressure difference actively assists your breathing by boosting the volume of air you take in with each breath.
EPR, by contrast, only reduces pressure by a maximum of about 3 cmH2O and doesn’t boost your inhale at all. Your inhale stays at the same prescribed CPAP pressure. The pressure curve is also shaped differently: EPR produces a brief, rounded dip that quickly returns to baseline, while BiPAP holds a distinctly lower pressure throughout the entire exhalation phase. Think of EPR as a comfort tweak, not a different mode of therapy.
Does EPR Affect Treatment Effectiveness?
This is the trade-off worth understanding. Because EPR lowers the pressure you’re getting during part of your breathing cycle, it can reduce the effective pressure keeping your airway open. In a bench study published in the Journal of Clinical Sleep Medicine, EPR set to level 3 dropped therapeutic pressure by 3 cmH2O, and at that reduction, simulated apneas persisted. The study also found that the pressures reported by the machine’s own software were about 2.5 cmH2O higher than what was actually being delivered, meaning your device may tell you it’s delivering more pressure than your airway is actually receiving.
The practical takeaway: if you use EPR at its highest setting, your prescribed pressure may need to be set a few cmH2O higher than it would be on standard CPAP to achieve the same airway-opening effect. At lower EPR settings (1 or 2), the reduction is smaller and less likely to compromise therapy. If your sleep data shows a high number of residual events and you’re running EPR at 3, dialing it back is a reasonable step to discuss with your provider.
Comfort Benefits of Lower Expiratory Pressure
The main reason EPR exists is that constant high pressure during exhalation is one of the most common complaints among CPAP users. It can feel suffocating or unnatural, and it contributes to a side effect called aerophagia, where you swallow air during the night and wake up with bloating, gas, or stomach discomfort. Aerophagia affects at least 7 to 8 percent of CPAP users, and the risk increases with higher pressures. Each 1 cmH2O increase in average pressure raises aerophagia risk by about 13 percent.
Lowering expiratory pressure, whether through EPR, auto-adjusting CPAP, or switching to bilevel therapy, has been shown to reduce aerophagia symptoms. For people who struggle with the sensation of exhaling against pressure or who experience stomach-related side effects, EPR at a moderate setting can make therapy significantly more tolerable without meaningfully compromising its effectiveness.
When EPR Can Cause Problems
For most people with straightforward obstructive sleep apnea, EPR is safe and helpful. But in some cases, the rapid pressure changes during breathing transitions can push carbon dioxide levels low enough to trigger central apneas, which are pauses in breathing caused by the brain temporarily stopping its signal to breathe rather than by a physical airway blockage. This is called treatment-emergent central sleep apnea, and it’s more likely to occur at higher pressure settings or when pressure swings are large.
If your CPAP data consistently shows central apneas (as opposed to obstructive ones), EPR could be contributing. Reducing the EPR level or turning it off is one of the first adjustments typically made to see if central events resolve.
Choosing the Right EPR Setting
Most people start at EPR 1 or 2 and adjust based on comfort. If breathing out still feels labored, moving up a level is reasonable. If you notice your residual event count climbing on your CPAP data (through the machine’s app or an SD card reader like OSCAR), stepping back down is the logical response. Some machines also let you choose whether EPR is active all the time or only during the ramp-up period when you’re falling asleep.
There’s no single “best” setting. Someone on a low prescribed pressure of 6 or 7 cmH2O has less room to work with, since EPR 3 would reduce their effective expiratory pressure to just 3 or 4 cmH2O. Someone on a higher pressure of 12 or 14 cmH2O can generally tolerate EPR 3 without losing therapeutic effectiveness, though the prescribed pressure may need a slight bump to compensate. The goal is the lowest EPR level that makes therapy comfortable enough to use consistently, since the single biggest factor in CPAP effectiveness is actually wearing it every night.