Magnesium is critical in cardio situations. It plays a key role in health since it is involved in so many enzymatic reactions in the body. When I did research on magnesium in the PubMed database (premier database for research), there were over 40,000 articles on the role of magnesium and cardio events.
I would suggest High Absorption Magnesium from Doctor's Best. UltraMag is another good product.
Vitamin E (natural not synthetic) is critical as well. Vitamin D levels should be checked. Other nutrients of note are selenium, vitamin C, b vitamins and others. Avoid processed foods and opt for nutrient dense foods that do not have pesticides herbicides and other chemicals, in other words choose organic.
In my opinion, I would use Miracle-Mineral-Supplement with caution (or not at all) since it is an oxidizer which causes stress on the body, with additional stress you will lower magnesium and other critical nutrients. If I was looking to kill pathogens, I would choose raw minced garlic. It has a wide range of microbe killing power and supports the cardiovascular system.
Check with your doctor to make sure the recommendations do not interfere with the medications he is on.