I know this is an odd question but I'll try to make this as short as possible. I'm a 27 year old male at 5'10 135 lbs and fairly healthy. So in 2014 when I was 22, I had a bad experience with marijuana that caused a series of continuous panic attacks that eventually caused me to develop IST which was later diagnosed by a stress test around a year later around mid 2015. After my diagnosis the doc told me that the heart rhythm was normal however that my sinus node was sensitive to exertion. So ever since I've been trying to deal with this condition and it seems manageable as long as I try to avoid my fight or flight response from happening. My anxiety has gotten better and I don't really get panic attacks that often anymore. I have all the symptoms of IST except for the "over 100 bpm resting heart rate" symptom. When I sleep my heart rate gets down to about 45 bpm. When I'm sitting down or casually standing or walking it gets anywhere to 50-65 bpm. The only time where it gets higher then that is if I'm actually exerting myself, then it can easily skyrocket to over 100 bpm as if I'm running on a treadmill or something. Now when my heart rate is this low I don't have any symptoms you would think that I would have such as dizziness, shortness of breath, fainting, or any of that. I know athletes are known for having low heart rates due to the heart muscle strengthening from physical exercise but I'm not exactly an athlete so why is my heart rate that low? Also one more thing I forgot to mention is that I did have an echocardiogram early 2015 and the doctor said, there were no defects or structural deformities so my heart is physically normal. Is it possible that this condition has been actually strengthen my heart or somehow this condition may have affected my sinus node when resting? I've been thinking about this for a while and I can't seem to find an explanation as to why this is happening and my doctor told me there was nothing to be concerned about as long as I don't have symptoms. I mean think about it, when you do cardio you pretty much run, jog or use the treadmill. With me all I have to do is casually walk my dogs around my block one time and my heart rate is already equivalent to someone who has done these activities. I do less exertion to get the same heart rate as a person who runs a marathon. It's just a mystery at this point.