I am sorry but I don't understand your question.
When your reduce PS on a bilevel machine then IPAP gets reduced.
As everyone has point out PS gets added to EPAP to give you IPAP whereas EPR gets taken from IPAP to give you EPAP.
The end result in both situations is X amount of difference between inhale and exhale.
If your bilevel machine is in auto mode and if you decide to reduce PS and if the max IPAP is 25 cm it is still going to auto adjust within the established parameters if the machine deems such auto adjustment necessary.
Reducing PS to 3 would actually allow the EPAP to come up more than it can with PS of 6......if it deems it necessary.
EPAP does the bulk of the work preventing the airway from collapsing anyway.
With PS of 6 IF (big if) your machine went to 25 IPAP then your max EPAP is going to be 19.....with PS of 3 then the max EPAP is going to be 22. In theory... there are probably some situations where 22 EPAP might be better for holding the airway open than 19 EPAP might be.
Remember the regular BiLevel auto machines won't/can't do anything for central apneas anyway. They only auto adjust for obstructive apneas/hyponeas. They do absolutely nothing when a central apnea occurs.
I don't know why your sleep tech opted for 6 PS. I don't know if you had enough centrals during your sleep study to warrant addressing them and while increased PS is what is done to force a breath when a person simply isn't breathing, I once asked a sleep tech how much PS is actually needed to do what is needed to force a breath to deal with a central and he told me that he felt 8 cm minimum PS and ideally 10 cm PS.
Unfortunately when we start going over 6 cm PS (and for some people even less) then we run the risk for actually triggering the very thing we are wanting to avoid and make things a LOT worse in terms of the number of centrals as well as frequency of centrals. With the regular bilevel machine and a high PS we can potentially end up causing a truckload of centrals that could potentially cause desats and be a problem. It's why I cringe when I see people using the ST model bilevel with high PS with each and every breath to treat complex sleep apnea. PS of 10 is pretty much going to trigger a truckload of centrals when we have that much PS with each and every breath. It's going to trigger a large number of events that it is supposed to be "fixing". I feel it is better to have the ASV only do the higher PS needed to deal with a central when needed as opposed to doing it with each and every breath.
PS of 6 on a regular bilevel machine isn't adequate to deal with centrals...it just isn't unless someone is using ASV bilevel where the PS can auto adjust upwards as needed.
Now higher PS can potentially be used to increase oxygenation and as long as it doesn't cause the carbon dioxide washout which causes the centrals then it probably is fine to use. So a lot depends on why a higher PS is chosen and does that higher PS cause a problem or not.
Now here's the other monkey wrench that confuses a lot of people in the whole bilevel causes centrals in some people thing....regular bilevel is usually the first thing tried when people have treatment emergent central apnea and sometimes it works as long as the cause of the centrals popping up isn't from the wash out thing. The wash out thing isn't the only cause for central apneas. Heck, sometimes regular cpap actually helps with central apnea that was seen on the diagnostic sleep study....rare for it to help but not totally impossible.. That's why most insurance companies require all the trials with other machines before they will pay for the high dollar ASV machine.
I don't understand all this panic about central apnea.
1...it's normal to have an occasional central for one thing
2...centrals are only a problem when they are numerous enough to create a problem either with desats or in the case of sleep onset centrals the person has so many of them they keep bouncing out of sleep and don't get the sleep they need
3...if present in numbers that are a problem centrals are easily dealt with ....regular cpap/bilevel sometimes will fix the problem...rare but not impossible and if it doesn't then there is ASV which will very nicely auto adjust to deal with both obstructive apnea AND central apneas if needed.
4...and remember awake/arousal centrals don't count other than when we have a lot of them it means we aren't sleeping so great which of course is unwanted but when that is what we see then the centrals are a symptom of a problem and not the cause of the problem. The cause of the problem has to be addressed.