I had a reaction to a flu shot a few years ago - heart rate went over 190 for over 10 minutes recorded on Apple Watch and I verified with manual heart rate measurement. Just felt a little funny is all. Went to cardiologist many months later as had never been to one before just in case. She laughed because it was no big deal - I didn't feel faint, I didn't have shortness of breath. She said next year go get your flu shot again and don't worry about the reaction if it occurs. She did run tests and even some more because I wanted to be 1000% sure. Bottom line she said I'll never need to see a cardiologist again for the rest of my life and push your heart even more than I do now get it up over 150 and even 170 with intense exercise. One thing I did learn, males should get an under $100 'heart CT scan with calcification score' to see heart attach risk and as a baseline for future scans. My score was zero which was another reason she said no worries for me thankfully and also echo stress test too. She also said primary care and ER docs get all concerned about a lot of heart issues that are really no big deal. So I see these stories of high heart rates and minor possible arrhythmias and think it is overblown. We all get an extra beat here and there and don't usually feel them at all. If resting you have over 120 bpm most of the time, sure there is something wrong in the electrical pathway but it isn't like you will just drop dead like from a massive heart attack - spend the $100 and get a heart ct scan with calcification done if you are male.