I noticed the same thing, rapid heart rate, sometimes palpitations, and as soon as I went from 18 mg to 12mg haven't had an issue since. That is what I would suggest. I started at 24 mg, then decided that I wanted to "step down" my nic. First went to 18 mg, and was still having issues, so I stepped down to 12 mg, didn't notice I was vaping more, and issues past. I've been vaping 12 mg for a couple of weeks with no issues. I will say one thing though, I still have a couple of 10 ml bottles with some 24 mg juice, and man it hits like a truck, I didn't really notice stepping down, but if it weren't for the heart issues I would probably go back to it.