Theoretical Setting for a Bi-Level Machine Question

[Crimson Nape]

This is based on my curiosity only.  My understanding is a Bi-Level PAP addresses EPAP and IPAP as separate entities, only being connected by a delta value, Pressure Support(PS).  

My questions are:
1. Can you set the EPAP to a fixed value and allow the IPAP to vary only? (I don't recall ever seeing a setting like this.)
2. If so, what conditions or indicators would warrant such a configuration?

in advance.

[Sleeprider]

The Philips BiPAP auto has settings for minimum EPAP, maximum IPAP and can automatically vary PS in a set range. The Resmed VPAP auto has min EPAP, max IPAP and single PS. None of these can perform the fixed EPAP trick while varying IPAP. The ASV machines are capable of fixed EPAP with variable IPAP and PS.

As far as what purpose there might be in the capability to fix EPAP and vary IPAP, the machines it is used in (ASV) speaks for itself. Variable EPAP is desireable in an auto ASV to treat the obstructive component of complex apnea; however someone treating straight central might benefit from a fixed EPAP with the variable IPAP being to induce breathing. The advancement of ASV was really the variable EPAP capability, so this was apparently thought to be more valuable than a fixed value. What surprises me is how few doctors ever prescribe auto ASV vs fixed, which is the older technology.