Can I use AppleWatch features without it being connected to BlueTooth or WiFi 24/7 (no cellular either)?

I am specifically interested in the heart rate monitor and ECG/EKG feature. Does the watch HAVE to be connected in order for those features to work? Or can I still use those features in real-time—without being connected—and then, at the end of the day, connect to BT or WiFi in order to download any info from the day that I want to keep?

Basically I'm looking for a wearable fitness tracker with a reliable heart rate monitor (to track irregular heart beats) but I don't want all the radiation of wearing it on my body all day.

Thanks!