What triggers a high or low heart rate alert on my Apple Watch?

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Your Apple Watch can tune in to your heart and send notifications when something out of the ordinary occurs. For example, your Apple Watch can notify you if your heart rate remains above a chosen threshold or below a chosen threshold after you’ve been inactive for at least 10 minutes.

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copied from: Heart Health - Apple Support

Setting up the threshold is described here:

Heart health notifications on your Apple Watch - Apple Support

I guess, since the Apple Watch is no medical device, this is all the information you can get from Apple.

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Since Apple Watch takes these background readings based on your activity, the time between these measurements will vary. Apple Watch also calculates a daily resting rate and walking average by correlating background heart rate readings with accelerometer data when sufficient background readings are available.(1) 

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(1) Some anomalies may appear in the displayed data, resulting in occasional heart rate measurements that are abnormally high or low.

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copied from: Monitor your heart rate with Apple Watch - Apple Support