Seems highly unlikely that your heart rate is actually decreasing, much less that much -- so it must be your HRM is faulty. Are you using a wrist based or chest strap or other? Wrist based are more finicky -- especially for running -- so that may be it.

Thank you for your reply. This does not happen when I run slower but longer (10km in 42 to 45 minutes). My heart rate does rise if I stop for a few minutes and resume. Can one’s heart rate vary that much ?

I was having slight drops in my Heart rate, mine are less than a minute and occur during running and cycling (no idea if they occur during swimming). First I thought that it was just a blip in my data from the strap, but I have seen it consistently across multiple straps and devices. I scheduled a 24 hour holter monitor with my doctor, and as soon as I did that I found my problem. A vagal response to burping. I do not see it coughing, swallowing or any other time, just a burp.

Depending on what you training is, you may want to consider a Holter.

If you have an image o the HR or a link to your strava there could be clues there. Some artifacts are obvious even there.

Do you used a wrist-based HR monitor? I have found the HR is extremely erratic, and I always use a chest strap, which is much more consistent. I wouldn't expect such a drastic drop in HR over a workout as you are referring to. Sometimes my HR will drop by a few beats during an easy bike or run, but not by more than 3-4 bpm at the same intensity.

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Thank your for your responses. I have a TomTom Cardio spark 3 wrist band. I have (manually) also estimated my heart rate and it seemed acurate.
In the attachment one can see what I mean. My pace is constant between 3:55 to 4:00 min/km

How do you check your heart rate manually while running?

I think you will likely get more consistent HR info with an HRM strap that is compatible with your TomTom like a Wahoo Tickr -- especially when running.