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If you just want the heart monitor for irregular heart rhythm, the smallest and lightest watch which will track that is Apple Watch SE 40mm. Note it will not offer Blood Oxygen or ECG. If you'd like to also have those features on your Apple Watch, you'd want Apple Watch Series 8 in Aluminum 41mm. If you are concerned about your heart health, I'd really recommend moving up to the Series 8, which isn't that much more expensive. Then you'll need to determine if you want a GPS+ Cellular model or if you'd be happy with a GPS only model. The difference means you'd have to have your paired iPhone with you in order for many features to work on a GPS only model. Final consideration for any of the current watches is if you own an iPhone which is capable of running iOS 16. You MUST have a modern enough iPhone running iOS 16.

Perhaps this compare the models link will help you determine what meets your needs best --> Apple Watch - Compare Models - Apple

Hi Watch4A-fib,

Thanks for posting here in Apple Support Communities. That's a great question. We'd recommend that you connect with the Apple Online Store here: Apple Store Online

Use the option to 'Ask a Specialist' at the top of the page.

Have great day!