Hi Bill,
My DH has always carried his on board as hand luggage. This is because most CPAP machines are sensitive to atmospspheric pressure and should not go in the hold. It should not count as one of your pieces of hand luggage as it is a medical aid. We have flown with many different airlines and they have been very accommodating. He has had the engineers connect the machine to the planes electrical supply so he could use it on a night flight but it was a bit of a pain so doesn't bother now.
Any more ? please ask away.