Welcome to 2017 and Watch OS 4. For Apple Watch Series 1 and above, the new Apple Heart Rate app will notify you of a sudden increase in heart rate.
Let's define "sudden" and "increase"
The Heart Rate app from Apple monitors your pulse and takes note when you appear to be "at rest" for "a while". Once it appears you are not moving, a "resting heartbeat" is established. From this point on (until you get up, or move significantly) your heartbeat should remain at rest. If your resting heartbeat "jumps" above a threshold (configurable from 100 to 150 bpm) a standard alert is generated.
That's almost great. I had hoped the "alert" could be customized. I had hoped my Apple Watch would start haptic vibrating or my phone would go crazy if a Heart Rate app alert occurs. Enough to wake me up. It does.. but.. so does getting an email, a text, etc.
There's the problem. In the democratic WatchOS API world, all alerts are created equal. After the drive home I look at my watch face. Above Mickey is a tiny red dot on the watch face (indicating some alerts are waiting for me). I pull down from the top. I then scroll through 10 alerts. Alert 1 is an email. Alert 2 is 500 free coins from Slotomania, Alert 7 says my heart jumped about 90 minutes ago.
Like the feature that allows our "Favorite" contacts to "break through" to you in Do Not Disturb mode, it would be a great feature if could do the same with alerts.
If, like me, you are wanting to be notified of an irregular heart beat, this is as good as it gets for now. Apple is working with Stanford on a research study to detect AFib using an Apple Watch in the near future. Once the FDA allows it, we can expect much more from our wearables.