Flex is a comfort feature on Philips CPAP machines that briefly lowers air pressure when you breathe out, making it easier to exhale against the airflow. Standard CPAP delivers one constant pressure all night, which many people find uncomfortable because it feels like they’re pushing their breath out against a wall of air. Flex eases that sensation by dropping the pressure slightly at the start of each exhale, then ramping it back up to your prescribed level before your next inhale.
How Flex Works
A CPAP machine keeps your airway open by pushing a steady stream of pressurized air through your mask. The problem is that the same pressure that holds your airway open during inhaling can feel like resistance when you’re trying to exhale. Flex technology senses when you start breathing out and reduces the pressure for that portion of the breath cycle. The drop happens quickly at the beginning of exhalation, then the machine gradually returns to your full therapeutic pressure before you inhale again.
You can adjust Flex to one of three levels. Level 1 provides a small amount of pressure relief, and each higher number increases the drop. At the maximum setting of 3, the pressure can decrease by roughly 3 cmH2O from your set pressure during exhalation. The feature is designed to make breathing on CPAP feel more natural, closer to how you’d breathe without a mask on.
The Different Versions of Flex
Philips has used several variations of Flex across its machine lineup, and the names can be confusing. Here’s how they differ:
- C-Flex: The original version, found on standard fixed-pressure CPAP machines. It reduces pressure at the start of exhalation based on your breathing flow and your chosen setting (1, 2, or 3). The pressure relief tops out at about 3 cmH2O.
- C-Flex+: Works like C-Flex during exhalation but also maintains a fixed 2 cmH2O gap between your inhale and exhale pressures throughout the entire breath. This makes it behave more like a bilevel (BiPAP) machine, giving you consistently lower pressure on every exhale.
- A-Flex: Found on auto-adjusting CPAP machines. It lowers pressure when you exhale and smoothly increases pressure when you inhale, so you barely notice the transitions. Because auto machines are already adjusting pressure throughout the night, A-Flex works alongside that algorithm to keep breathing feeling as natural as possible.
On newer Philips machines like the DreamStation 2, the feature is simply labeled “Flex” in the settings menu, with options of Off, 1, 2, or 3.
ResMed’s Equivalent: EPR
If you’ve used or researched ResMed machines, their version of this feature is called EPR (Expiratory Pressure Relief). It works on the same principle: reducing pressure during exhalation to improve comfort. EPR also offers three levels, with level 3 dropping pressure by about 3 cmH2O. The two technologies are functionally similar, so if you’re switching between brands, know that Flex and EPR are doing essentially the same thing.
Does Flex Affect Treatment Quality?
This is where it gets important. Flex improves comfort, but at higher settings it can reduce treatment effectiveness if your prescribed pressure was set just high enough to keep your airway open. A bench study published in the Journal of Clinical Sleep Medicine found that C-Flex+ at level 3 and EPR at level 3 both dropped the actual delivered pressure by roughly 2 to 3 cmH2O below the set value. In those tests, the machines failed to fully normalize breathing because the effective pressure during exhalation wasn’t high enough to prevent airway collapse.
The practical takeaway: if your pressure was carefully titrated in a sleep lab without pressure relief turned on, activating Flex at a high setting could mean you’re getting slightly less effective therapy. Auto-adjusting machines can compensate for this by raising their overall pressure to make up the difference. Fixed-pressure machines can’t do that on their own, so your sleep specialist may need to set your pressure a bit higher if you’re using Flex at level 2 or 3.
At lower settings (level 1), the pressure drop is small enough that it’s unlikely to cause problems for most people.
Does It Actually Help People Stick With Therapy?
People who use pressure relief consistently rate it as more comfortable than standard CPAP on comfort scales. That said, clinical trials have produced mixed results on whether it actually gets people to use their machines more. A randomized trial of 100 patients across multiple sites in the U.S. and Germany found no difference in hours of CPAP use between those using C-Flex and those using standard CPAP at 30, 90, and 180 days. A smaller German crossover study found the same thing at seven weeks.
Comfort and adherence aren’t always the same thing. Many factors beyond pressure comfort determine whether someone sticks with CPAP, including mask fit, nasal congestion, and noise. Still, if exhaling against pressure is specifically what bothers you, Flex addresses that complaint directly and is worth trying.
How to Change Your Flex Setting
On most Philips machines, your sleep provider sets the initial Flex level, but you can adjust it yourself. On the DreamStation 2, the Flex option appears in the therapy settings on the touchscreen. You can cycle through Off, 1, 2, and 3 to find what feels right. Some older DreamStation models use a dial or button navigation to reach the same setting.
If you’re not sure whether Flex is enabled on your machine, check your device’s settings menu or look at the summary screen that displays when you turn it on. Machines that have Flex enabled will typically show the current level. If you don’t see the option at all, your provider may have locked it, and you’d need to contact them to have it turned on.
A good starting approach is to begin at level 1 or 2 and increase only if exhaling still feels difficult. If you go straight to level 3 and your AHI (the number of breathing events per hour shown on your machine’s data) starts creeping up, that’s a sign the pressure relief may be reducing your therapy’s effectiveness. Dialing it back down or having your provider increase the base pressure can solve that.