Which watch for heart monitoring?
I'd like to get my Mother a used Apple watch for it's heart monitoring function. Far as I can tell, there hasn't been any new heart monitoring functions or sensors added to the newer watches, so I should be able to get by with an older model. We're not concerned with getting any monitoring function like blood oxygen level, only heart monitoring.
How far back in models can I go when looking for this used watch and it still have all the heart monitoring functions the newest watches have? As far as I know Apple hasn't added any new heart monitoring sensors for a few models now.