US carriers, especially flying domestic, generally exempt and do not count medical equipment as your carry-on. Check with your airline/carrier and their online sites for specific rules, always helpful to print & take a copy along as gate/ground agents are often clueless
Flying to/from/thru London & Canada could be issues but you should be fine - portable Cpap and BiPap machines should be exempt, make it your personal item. Ask for ground supervisor if necessary. Depending on the type of aircraft or equipment being flown, storage space could be challenged on a ERJ or CRJ and forcing it to be gate-checked.
On cruisecritic forum, almost all reported no problems flying with CPAP as extra carry-on since it's a piece of medical equipment.