High Expiratory Pressure Intolerance. I am suspicious that our instinct for “I must be able to breath well to do well” kicks in when we sense resistance as we try to breath out. Apparently in those with a small airway this sometimes leads to Upper Airway Resistance Syndrome (UARS).
The modern CPAP machine is made to be as small as possible. A small mask (or nose pillows). A six foot tube. And a tiny machine often with humidifier included. There is very little air space between you and the fan. You breath in, the fan must spin up to keep up. You breath out, your air stream meets the spun up fan and for a fraction of a second and I do believe a short term rise in pressure exists until the system and pressures work together to slow the fan down. And so perhaps your “I must be able to breath well to do well” instinct kicks your stress hormone level up a bit.
This issue came to mind as I worked to deal with another issue as documented below:
Tod on February 12th, 2014 2:15 pm 
So I learn a new term today that I have been thinking a lot about all week! Expiratory Pressure Intolerance (EPI).
And then I am reminded of another issue I have been thinking about all week! “Noise induced [sleep] fragmentation”.
I am thinking about these things because I am now enjoying a world more free from them.
It all started on a quest to limit the “feel” of the nasty “clear airway sensing pulses” from my CPAP machine. I really do believe that these rob me of sleep I could have had and do not contribute to my sleep or enough to my knowledge to justify their presence. I know that “arousal threshold” is usually related to respiratory effort but the fact that there is also “noise induced sleep fragmentation” attests that other sensory means can also contribute to fragmented sleep.
It occurred to me that if I added a bit of compressible air space (i.e. add air space to the circuit) the rise time of the pulses might well slow a bit and so be much less of the slap in the face that they seem to be to me. So I plumbed into the line a 2 liter soda bottle I had been using as water storage for a year or so. Yes this does limit the intrusive effects of the “clear airway sensing pulses” and I was happy to find that.
But I was even more amazed how much more natural it seemed to breath with the extra compressible air space in the circuit. I never really thought about it before but as you inspire the fan must ramp up a bit to compensate. Then when you rapidly shift to expire your outgoing breath meets the inertia of the fast moving fan. I have not measured it but I would guess there is a bit of a spike in pressure at that moment. When you breath into the airspace that I added it gives much more like the air gives when you breath out naturally. It is a very welcome natural feel. It is wonderful!!!
I liked it so much that I immediately plumbed in a second bottle! $20 in parts total.
When I did so I added some elbows. I found another thing I liked.
The “T”s used to plumb in the soda bottles also provide a place for sound and “vibration” to go and be dissipated. Frankly I never really knew how much I did not like the sound of the fan and all, which I can very definitely “hear” and feel with the air coming in through my nose, until the sound was not there. I added the elbows in the hope that some of the sound would be reflected back and more dissipated by having them in the way. There are now many visions of “CPAP mufflers” dancing in my head. I love the less sound and long for a truly quiet experience in my future.
 Danny J. Eckert, David P. White, Amy S. Jordan, Atul Malhotra, and Andrew Wellman “Defining Phenotypic Causes of Obstructive Sleep Apnea. Identification of Novel Therapeutic Targets”, American Journal of Respiratory and Critical Care Medicine, Vol. 188, No. 8 (2013), pp. 996-1004. doi: 10.1164/rccm.201303-0448OC
 Sairam Parthasarathy M.D., Emergence of Obstructive Sleep Apnea Phenotyping. From Weak to Strong! American Journal of Respitory and Critical Care Medicine VOL 188 2013
— critical closing pressure [Pcrit] – Arousal Threshold – ventilatory control Loop gain – and genioglossal Muscle responsiveness. Pcrit, Loop, Arousal, Muscle (PALM)–