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The way I understand it, breathing has an impact on heart rate variability, mostly because the chest cavity expands/contracts, which impacts the vagus nerve, which can then cause the HR to vary a bit. I don't know how reliable it is and if you can reconstruct every breath based on that data. I imagine shallower breaths would have less of an impact.

Perhaps someone more knowlegeable can pitch in?

Breathing in causes systolic pressure to drop slightly. The negative pressure in the chest cavity decreases the amount of blood that fills the heart prior to contracting ("preload"). The drop is small (<10mmHg), and generally your heart rate doesn't bother compensating for it to any significant extent. In theory you can detect breaths by looking at the R-R interval (the time between the beats), but it's a very subtle signal and in my experience it's useless in any real world setting.

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.

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