It says that a fan is a constant volume machine so for a given cpap (gage) pressure the machine should pump the same volume flow rate at all altitudes. The formulas say that the mass flow rate is proportional to the density so as the altitude increases the absolute pressure and mass flow rate will decrease. Someone said that the percent of oxygen in the air is the same at any altitude which makes sense but there will be less oxygen in the air at higher elevations because the air is less dense so there will be less air and oxygen per liter of air that you breath. If you increase the cpap pressure at higher altitudes the absolute pressure of the cpap air will increase so you will get more oxygen. I did not figure out how much the cpap pressure needs to be increased to compensate for altitude but they did in one of the posts I think.
I haven't tried to figure the whole altitude thing out yet but hopefully this link will help. The post may be confusing so look at the link.