The best is magnesium aspartate
The role of magnesium in normal heart function is tremendous. Profound magnesium deficiency causes muscles to underfunction, malfunction or not function at all. Several hundred of your body’s most important biochemical reactions depend on this mineral. "The synthesis of all proteins, vital cell nuclear materials such as nucleic acids and nucleotides, lipids, and carbohydrates require ionized magnesium (Mg ++)." (Williams, SR Nutrition and Diet Therapy, Seventh Ed, St. Louis: Mosby, 1993, pp 230-233) Even most, ah, "healthy" adults fail to get the US RDA of magnesium, which ranges from 280 to 400 mg for adults. These figures are elemental weights: just the corn, not the can. Most magnesium supplements are compounds of magnesium with something else. The weight of the "something else" is often obscured in dosage recommendations. That is why Melvyn Werbach, M.D. cites studies that advocate daily dosages of 2,000 mg of magnesium per day for CHF. (Textbook of Nutritional Medicine. Tarzana, CA:Third Line Press, 1999, pp 273 and 275.) The elemental quantity is significantly lower than that. Green vegetables and whole grains contain quite a bit of magnesium. Pinto beans, almonds, and especially figs are oustanding food sources.
Of the oral supplements, magnesium aspartate or magnesium orotate may have the best chance of getting into cardiac muscle cells. These forms of magnesium are rarely found on store shelves. Your doctor may be able to have them compounded for you by a cooperative pharmacist, or you might find them with an internet search. Intravenous administration of magnesium may be necessary in more serious cases of congestive heart failure. Have a test ordered to check serum magnesium. Most doctors don’t. It is even better to check myocardial magnesium (Textbook, p 275). This is because the amount of magnesium in the heart muscle cells may be considerably lower than in the blood.