What happened to me took me completely by surprise because in the previous 12 months of training, i was never able to reach 100% of my estimated max heart rate no matter how hard i pushed (my legs would always tire before my heart and actually my lungs tired before anything before my first long fast healed them at which point they became my strongest link). In this instance i had rested my knees from running for a month, doing other intense cardio during that time and with such rested running legs i was able for the first time to (accidentally) push to 102% of my estimated max... and by the time i realized it, i was too late.
I was running about a mile around a lake attempting to shatter my previous best time which i did, feeling great every step of the way until after i finished the race. I felt so good that i didn't imagine my heart rate might have been getting dangerously high.
Reaching 98% was never a problem before the injury... it never caused any strain, nor did i ever once feel any heart strain under any circumstances before the incident... but in any case once my heart is 100% healed, i don't believe i will ever let myself get above 95% again to play it safer.
As to the semi marathon a few days after a long water fast, i believe there may or may not be something going on physiologically that would make us unprepared for such a feat (temporary low platelets in our blood maybe?), but hopefully somebody else has more specific and sourced information on that. I break the exercise aspect of my fast gradually much the way i do with food.