If you are going to use an EDS system, then you need to know how it is
meant to work (and from some of the comments on the thread clearly some
posters don't).

The EDS system gives a pulse of O2 at the beginning of the breathing cycle,
the length of the pulse depends on the ambient pressure. When you inhale,
the oxygen used by the lungs is taken from the air that ends up down there
any oxygen in the upper respiratory tract is unused and breathed out.

If you stop inhaling at the end of the pulse ('shallow breathing') then the
oxygen at the end of the pulse is wasted, it never gets down to the lungs.
The EDS box doesn't know that so you are under delivering useful oxygen.

Constant flow systems enrich all the air in the system, so 50% plus of the
oxygen you pay for is wasted.

You need to be aware of the need to breathe normally. I have used EDS with
cannula up to 24,000ft or so without issues. I have had an altitude
chamber run so I can recognise the symptoms of anoxia (yes, I know it can
creep up on you).