Comparing expiratory relief in different CPAP machines

August 17, 2016 | By Peter Stanza | Filed in: CPAP Advice.

The most subtle differences in CPAP machines can often mean the difference between being successful with therapy, and being the proud owner of a very expensive paperweight. One of the most widely implemented comfort features on modern CPAP machines is a form of expiratory relief. This is where the pressure delivered by the CPAP machine is reduced in order to reduce the effort for expiration. Despite most manufacturers having some form of this on their machines, the delivery is often slightly different. This impacts how the machine feels to breathe with, and the overall comfort of the machine.

Each manufacturer seems to have their own take on this function. Also, they all seems to have their own name for it; be it C-Flex or A-Flex (Philips Respironics), EPR (ResMed), EZEX (Transcend), SmartFlex (DeVilbiss), Reslex (3B Medical), Z-Breathe (Human Design Medical), EUT (Curative Medical), or softPAP (Weinmann). They all effectively mean the same thing, the machine drops the pressure slightly during expiration.

However, the means by which they do this differ slightly.

Points of difference for the different versions include:

Whether the expiratory pressure relief is triggered by the patients breath, or is predicted
Whether the expiratory relief occurs full-time, or only during the ramp period
The depth of the pressure drop, and whether it can be changed
How abruptly (quickly) and smoothly the pressure drops and increases occur (and whether this is adjustable)

Some of these things are very much user-dependent, and one person may prefer a different manufacturers take on these settings compared to another.

Patient-triggered versus predicted breath expiratory pressure relief

One of the major differences between the two major CPAP manufacturers is how the drop in pressure during expiration is triggered. The ResMed EPR system effectively uses the patients expiratory breath to trigger the EPR. Thus, the pressure does not start to drop for expiration until the patient starts to breathe out. Respironics on the other hand utilizes an algorithm that predicts when the patient will take another breath, starting to drop the pressure in anticipation of this next expiratory phase. The same story exists for inspiration where the ResMed user needs to trigger the rise in pressure, whereas the Respironics machine will start to increase the pressure when the inspiratory phase is predicted to take place.

Now, the Respironics machine seems to be the more advanced of the machines here, however it isn’t perfect for everyone. Many patients have reported feeling ‘rushed’ to take another breath with the Respironics machines as the machines predictably drop the pressure ready for the expiratory stage. For these patients, a ResMed (or similar machine with patient-triggered pressure drops) machine with EPR may be a better option as it will react to your

This is purely a personal preference, and you are best off trying both versions before making a decision.

Whether the expiratory relief occurs full-time, or during the ramp period only

Dropping the pressure during expiration is first and foremost a comfort feature. It is there to make the CPAP pressure more comfortable and tolerable while you try to drift off to sleep. Therefore, it is my opinion that it should not be active whilst you are asleep, and should be disabled during this time.

Some manufacturers offer a setting where the pressure relief will only be active during the ramp feature. With the addition of automatic ramps (available on most top line CPAPs these days) this means that the pressure relief will only be active when you are effectively awake. I strongly recommend looking for a machine that allows this, as full-time expiratory pressure relief can adversely affect your therapy by increasing the chance of having apneas, hyponeas or sub-criterion respiratory events that affect your sleep.

In addition, some machines automatically turn off the expiratory relief in the presence of obstructive events. This means that the full prescription pressure is delivered during this time. I can confirm that both Devilbiss and ResMed machines have this feature.

The depth of expiratory pressure relief

Most devices will allow you to change the degree of pressure relief between 0 and 3. Generally speaking, this is the equivalent of a drop of between 0 and 3 cmH2O.

How abrupt the transitions from inspiration to expiration (and vice versa) are

How abruptly the pressure changes occur can effect how the machine feels to breathe with. If the pressure changes from the inspiratory pressure to the expiratory pressure too quickly or slowly, the it can feel unnatural. A nice, even, sine-like wave is ideal as it matches the natural breathing oscillations better.

The relative ‘smoothness’ of these transitions is somewhat limited by the means by which the expiratory pressure relief is triggered (I.e. patient-triggered or predicted). Expiratory pressure relief that is triggered by expiration needs to play catch-up, and decreases the pressure more abrubtly (generally). Predictive style machines can start to decrease the pressure sooner, and therefore can allow for a smoother transition between inspiration and expiration. Both styles will generally allow for a smooth increase in pressure during the inspiration phase.

How do the different manufacturers stack up?

Sometimes trying to get detailed information out of a CPAP manufacturer about a product is like trying to get blood from a stone.

Either the reps don’t know, or they argue that they don’t want to give up trade secrets. Therefore, this post will be a work in progress and will be updated in kind as I learn more.


ResMed’s expiratory relief is called… wait for it…. Expiratory Pressure Relief or EPR for short. I guess they were able to coin this term, as they were the first manufacturer to implement it.

As far as I can gather (correct me if I’m wrong), ResMed’s EPR Easy-Breathe has changed little since it was released on their S8 platform. It has four settings; off, 1, 2, and 3 that correspond to the amount of pressure relief given. ResMed’s EPR algorithm is triggered by your expiration and ends at the start of inspiration.

As the ResMed machines expiratory relief is ‘breath triggered’, the flow changes from the ResMed machine resemble crashing ocean waves. Gentle increases in pressure with inspiration, followed by a steep drop in the pressure when expiration is detected. Changing the EPR inhale setting to ‘fast’, however, means the machine will increase the pressure more quickly to the treatment pressure during the inspiration phase.

Interestingly, the EPR in ResMed’s machines will reduce automatically if any flow limitation is detected through the night. This means that the full treatment pressure is given in these periods, so as to reduce the likelihood of a respiratory event.

More information on ResMed’s EPR can be found onResMed EPR Fact Sheet.

Philips Respironics

Philips Respironics have a few names for their expiratory pressure relief system. These include C-Flex (the original version available on fixed pressure machines), A-Flex (automatic machines), and C-Flex + (their updated version). The main difference between the C-Flex and the C-Flex + is that the C-Flex + devices have ‘smoother edges’ at the changeover from inspiration to expiration; making the changes in pressure a little more tolerable. Respironics offers four levels of Flex, ranging from 0 to 3, and the C-Flex + can be turned on and off (if you like it a bit rough around the edges wink wink)


Devilbiss have perhaps the most adjustments available to their version of expiratory relief, called SmartFLEX. Devilbiss allow four levels (0, 1, 2, and 3 cmH2O) of expiratory relief. But, they also allow 6 levels of SmartFLEX rounding, for both inspiration and expiration. These range from 0 (sharp edges) to 5 (super smooth transitions) allowing you, the CPAP user to decide what you are most comfortable with.

I contacted DeVilbiss, and they got back to me with the following answers. The SmartFlex offered by DeVilbiss ‘is not predictive and relies on patient triggering to activate the increase/decrease of pressure.’ In addition, ‘the algorithm is programmed to disable the SmartFlex function in the presence of OSA events to ensure the patient receives the full prescription pressure’.


Transcend’s version is called EZEX. At this stage this is as much information as I have.


A representative from Apex was kind enough to answer my enquiry regarding this issue. According the to representative, Apex machines pressure relief is triggered by the patient beginning to expire. Somewhat similar to the ResMed system. This is as much information as I have at this stage.

Other manufacturers

At the time of writing this particular post, the other manufacturers including Weinmann, Curative Medical, and BMC did not have an answer for me. The post will be updated as I have more information on the topic.


Indeed, the method of pressure reduction is very much your own preference. Some people prefer larger drops in pressure, and some don’t like the feeling of being rushed by a ‘predictive breath’ device. It’s worthwhile trying different types in order to decide which one you are most comfortable with.

Another thing worth considering when looking at buying a CPAP machine.


3 comments on “Comparing expiratory relief in different CPAP machines

  1. Steve says:

    Thanks, a very helpful article which explains expiratory relief between the various CPAP machines. It probably helps me understand what I have experienced in trying to get the right machine for me. The resmed a10, I felt I was more conscious of breathing, more I had to push myself to breath (so felt less relaxed) and sometimes feeling starved of air. Whereas the dreamstation more gentle and smooth and felt more comfortable in breathing but sometimes feeling, I had to breath to catch up to the machine which felt a bit strange. I suppose in the end its personal preference. I did try the weihmann prism the air hit you hard and expiration even at 3 it was not smooth – it made the mask jump.
    I am glad your article isn’t to technical and helpful for newbies like me to understand. Thanks

  2. michael masterson says:

    resmed EPR in the s9 and later has ‘easy breath’ which provides a gentle ramp up in pressure, as opposed to a fast hit and drop. you can see it if you examine the mask pressure trace in the data.

    it shows up as ‘fast’ or ‘medium’ in the epr inhale in cpap mode. always on in autoset mode.

    with the vpaps, it shows up as ‘easy breath’ in vpap S mode, and is always on in vpap vauto mode.