I'm glad you asked. This is one of my favourite topics to bend people's ears about. EPR stands for Expiratory Pressure Relief. It is a function that ResMed introduced in their S8 and later models and it is the the same as C-Flex on Respironics machines, which they have had since the M series (although I'm sure both companies would swear blind they're totally different). The idea is to drop the pressure slightly on expiration in order to make the pressure more tolerable. Sounds reasonable but it is, in fact, a huge problem and should be avoided in almost every instance. "Why?", I hear you ask. I'll try not to get into too much detail explaining this.
The first problem is that EPR (and C-FLEX) lower your effective CPAP pressure. For example, if you need a CPAP 12 to breathe properly, but have EPR set on 3, you will only be getting an effective CPAP pressure of somewhere between 9 and 10, and because this is lower than is required you will obstruct. What will often happen next is that the patient will complain of persistent symptoms (because their OSA is not being adequately treated) and their download will show a higher than desirable AHI, so the pressure will be turned up making it less tolerable, increasing leak problems and leading to overall lower compliance with therapy. I have had to deal with this on more than one occasion.
The second problem is that by raising and lowering the pressure as you breathe, EPR actually increases the amount of air that you are breathing. With standard CPAP the pressure is constant, and the movement of air in and out of you lungs is done purely by your own respiratory muscles, so the amount of air you breathe is the normal amount that you should be breathing. All CPAP does is hold your airway open so that you can breathe normally. EPR works like low level BiPAP. As you breathe in the pressure increases and as you breathe out it decreases, which means that more air is moving in and out of your lungs than normal - the EPR is slightly augmenting your respiratory effort. This may sound all well and good, but there is a reason that you breathe the amount you do. Too little is a problem, we all know that, but too much can be a problem too. The extra breathing work done by EPR can be enough to hyperventilate you, sending your CO2 level too low which, in turn, causes central events. Again, I have recorded evidence of this happening.
Where I work, we only ever allow our patients to use EPR or C-Flex if they have had a sleep study with it and we can verify that it is not causing any harm. Otherwise we do not use it at all. I can think of fewer than 5 people who have actually had some benefit from using EPR/C-Flex in all my years of being a sleep tech. The people who sell the machines and the reps for the companies who make them will extol the virtues of EPR/C-Flex and tell you that it is perfectly safe. IT IS NOT. One of the engineers who designed the system admitted as much to another tech I work with.
So, to sum up, do not use EPR/C-Flex unless you have had a sleep study with it to make sure it's OK. It's not worth the risk. On ResMed machines EPR can be set to ramp only which is much less risky. Otherwise, if you think you really need it, get a study done while using it. As always, consult with your doctor, but you will probably find that they know nothing about this, as most of them don't in my experience.
