@Sean Au - Great point about the simple carbohydrates only providing quick fuel. Without any lasting supplies of muscle glycogen established through consumption of more complex carbohydrates, then working out will leave you lacking in energy.
I think that the fast, near painful heart beat could be related to dehydration, but more importantly related to an imbalance of electrolytes in the system, particularly potassium, calcium, and sodium. All key elements for muscle contraction (heart) and function of the entire body. I would look into a regular multivitamin supplement (pre or post work out) taken with food an hour before or an hour after working out (in your case it might be better before.
On a side note related to the multivitamin. A study was done examining the contents of portapotties. It was found that the majority of multivitamins consumed, even high quality ones, were not digested and simply passed through the system. For most multivitamins that you can purchase, the best way to make sure that your body absorbs the vitamin is to crush it before you consume it. Of course this makes it taste awful, but at least you are not throwing it away.
