I've been seeing a lot of FUD about this in the Tech Press, worries that users will use the Apple Watch 4 instead of calling 911 when they think they're having a heart attack. They say the "Apple Watch saved my life" stories will make people believe the Apple Watch 4 is equivalent to a full-blown EKG when it's not. Apple does have a lot of warnings to dissuade users from coming tot hat conclusion, but we all know that Ts & Cs are for pressing "Next" - "Next" - Next"...
I'm sure we're getting closer to Arthur C. Clarke's CORA (from "Rendezvous with Rama") which monitors the heart's electrical activity and can contact your cardiologist when it detects a serious cardiac event, as well as make suggestions to "take it easy for a few minutes" during moments of lessor stress.