55 years old here. Runner for about 30 years now, still training 'hard' (55-65mpw with 1-2 workouts a week). This past Saturday I had a serious problem with my heart. Noodling around the house, started feeling something in my chest - kind of like maybe how your lungs would feel after running a very hard workout in super cold conditions - burning sensation? Anyway, it got worse and I just couldn't get comfortable. Finally sat down in an easy chair and checked blood pressure / HR and the HR was bouncing all over. From 45 (my normal resting HR) up to 88. BP was very low for me (90/60 when it's normally 128/75). After about 15 minutes of this everything went back to normal. Was this afib? Heart attacK? I hadn't run at all that day though I did do an 11.5 mile hike in the mountains in the morning. I don't think I was dehydrated or low blood sugar as I'd eaten and drank plenty.