Rikky,
I tried doing epsom salt baths a while back for detoxing and also got the racing heart symptom you mentioned. I stopped doing them because it freaked me out. Note that this symptom never occurs when I detox through other means such as the sauna.
I just looked it up and it seems like the magnesium absorption could affect the heart:
"Magnesium and calcium need to work together at very precise ratios to ensure your heart functions properly. If you have a cardiac history, talk to your doctor before using magnesium therapies including epsom salt baths. "
/answers.yahoo.com/question/index?qid=20110224205556AAzPVHg