In a commercial airline, I realize that the cabin is pressureizd, but I know
that it is not kept at a sea level pressure. What effective "altitude" is
the passenger cabin kept at? Is it the same for all air liners?

The reason I am asking is that I have to use a CPAP machine to breath while
I sleep, (Continuous positive air pressure) for sleep apnea. I am
contemplating a trip to Hawaii where I will have to sleep during the flight.
Issues about whether I can power my equipment asside, I would need to know
what my effective altitude is to know how to set up my CPAP machine.

Thanks
Stephen