EPR is supposed to be a comfort feature. Many people, particularly new users, find it difficult to exhale fully against xPAP pressure. EPR reduces the pressure on exhalation and increases it when you start to inhale. EPR can be set to 1, 2, or 3, and the number represents the decrease in pressure measured in cmH2O. On the Resmed machines, the mask pressure graph gives you a good look at exactly how EPR works with the Resmed "EasyBreathe" algorithm. Here's a screenshot from my husband's Resmed data:
His EPR = 2, and the spikes in the Mask Pressure graph are 2cm high. The stuff between the two red lines is one breath---in other words, one inhalation/exhalation cycle. The green line is drawn at the the boundary between his inhalation and his exhalation. As my husband is inhaling, the pressure increases from his current "EPAP" of about 6.2cm to an "IPAP" of about 8.2cm. The peak mask pressure (i.e. full IPAP) occurs right at the end of his exhalation. The pressure rather quickly drops back to "EPAP" of about 6.2 during the first half of his exhalation. The pressure starts to increase once his inhalation is detected.
At least that's the theory. In practice, some people find that if the EPR setting is either too high or too low, the "EasyBreathe" algorithm can seem to be out of sync with their breathing. People will say things like the machine is rushing them to inhale (or exhale) before they want to or the machine won't let them finish their full inhalation (or exhalation). Fiddling with the EPR setting is usually enough to get people over the hump of feeling like the machine's EasyBreath is out of sync with their breathing. And it's also important to remember that not everybody likes or wants to use EPR. If none of the EPR settings are comfortable, you can always try turning it off.
Now it's also important to understand some other things about EPR: If you look at that snippet of my husband's data, you'll notice that even though his min pressure is set to 8cm, most of the time, the mask pressure is actually around 6cm. That's because the pressure setting is for the "IPAP" setting on a Resmed Autoset. And Pressure-EPR is the "EPAP" setting. The EasyBreathe algorithm has the pressure at the full pressure setting only at the critical "end of inhalation" timing in the respiratory cycle.
The upshot of this is that some people find that if they want to use EPR, they need to bump their min pressure setting up by about the same as the setting they use for EPR.
Now there's one other potential problem with EPR being set to 3: In a few people, setting EPR = 3 can lead to too much CO2 being blown off during exhalation, and that can depress the respiratory drive, which leads to the brain forgetting to send the signal to "inhale" and you get a CA scored, and in the worst case scenario, this can set up a string of centrals being scored as the body goes through so-called CO2 overshoot/undershoot cycles. In other words, for a small number of xPAPers, setting EPR = 3 can lead to strings of CAs. That's why Pugsy often recommends turning EPR down when new xPAPers post data with a bunch of CAs: It's an easy experiment to try and if it fixes the problem, then that's one less thing to worry about. If it doesn't fix the problem then we know there's something more significant going on, and that's when looking at the data more closely and examining the diagnostic sleep test along with any titration study that was done becomes critical.
