I don't know if they are related, but I have had arrhythmias, heart issues as well as MG. I haven't had to have a bypass.

It would be logical that mestinon would have an influence on heart rate, but it would be more likely to slow it down than to speed it up. It increases the amount of acetylcholine that is available. Acetylcholine stimulates smooth and skeletal muscle but it actually inhibits cardiac muscle.