Perhaps someone more knowlegeable can pitch in?
More advanced ECGs can measure the impedance of the chest (which changes as you breath). This is more complicated to implement, but much more reliable.
In either case, it's not something you'd be able to measure without several stickers on your chest, with wires running to some central box. I think the CPAP would be less obtrusive. If you don't need a full fledged CPAP, you could use something like nasal capnography, that just measures exhaled CO2.