Every now and then a question is posted about rapid heart rate and heart pounding. I have it myself, on and off. For me it turned out to be multiple reasons, alternating or happening together. This makes it very hard to find out what's going on. The way I try to figure out which reason it is, is trying to look for other symptoms.

Apart from the salt loading, Mg and K, all the other remedies first raise the heart rate and then quickly lower it. This can feel a bit scary the first time when you're not aware. All this happens within half an hour to an hour of taking the remedy/workaround.

All remedies rely on plenty of roughage in the food and water.

My reasons for rapid heart rate and heart pounding are:

1. Bromide. The iodine I take displaces bromide and if it isn't removed from my blood it settles, for instance, in the heart. 
   Additional symptoms: zits, dark thoughts, fatigue (because the bromide will also settle in the thyroid), all mucous membranes itch (especially nose and eyes).
   Remedy: salt loading, but also large amounts of SSKI help. Both will deliver a halogen (resp CL^- and I^-) to replace the Br^-.
2. Copper. I am copper toxic so any detox causes my body to dump copper from storage into my blood. This is unbound copper that the body can't use but does cause damage.
   Additional symptoms: anxiety, anger, racing thoughts.
   Remedy: Molybdenum helps within 15 minutes. Vitamin C makes the unbound copper harmless, but this doesn't work as fast.
3. B1 (Thiamine) deficiency. I supplement with plenty of Zn (because of the copper toxicity) and they are antagonists. 
   Additional symptoms: not pronounced but fatigue and edema.
   Remedy: B1 of course. Although I find that benfotiamin helps even better. Take B2 to activate the B1.
4. High histamine. you get high histamin when your body can't break down or elimenate it quickly enough. Lots of nutrients are needed to break this down. 
   Additional symptoms: itching in eyes and nose (as with bromide), nausea, insomnia, alert (not like bromide which makes me feel tired).
   Quick workaround: anti histamine
   Remedy: I've tried all kinds of things. Iodine works best I find. Especially Lugols. But I haven't really figured this one out yet.
5. Mg and or K deficiency: Magnesium and potassium both soothe the heart. A deficiency in Mg causes a deficiency in K.
   Additional symptoms: cramps in muscles.
   Remedy: Sometimes the body finds it hard to absorbe Mg. You know this to be the case when you get diarrhea from only small amounts of Mg. Or you itch when using magnesium oil on your skin. Try magnesium phosphoricum (homeopatic remedy), this helps to get the Mg into the cells.
6. Iron (Fe) deficiency: Lots of people, but especially women, are deficienct in iron. I am too, always have been. I've been on iron pills when I was young. I'm always very white skinned. But the insides of my eyelids are nice and red so lately I thought I was ok. Turned out I wasn't. 
   Additional symptoms: fatigue, hair loss, chapped lips, feeling cold. Very vague which is the reason it took me a long time to think of this one.
   Remedy: Iron can be really hard on the gut. it is also an extreme oxidiser, so caution is advised. I find that taking my iron amino acid chelate with colostrum on an empty stomach is well tolerated. Colostrum contains lactoferrin which binds to Fe, getting it safely where it needs to go. There are also lactoferrin supplements. But I find the additional benefits from colostrum pleasant.

Hopefully this helps someone. Please add to it if you know any other reason!