I would go with a prescription food recommended by your veterinarian.
http://www.hillspet.com/products/pd-canine-hd-canine-cardiac-health-canned.html (sodium 0.07% per 13 oz can)
http://www.vetstreet.com/royal-canin-veterinary-diet-canine-early-cardiac-ec-22-dry#overview (sodium 0.5% per 1000 kcal)
If you soak the kibble in water in the fridg overnight it has the consistency of wet food.
PS: Wysong also makes a cardiac prescription food you may like.
You need to consider the serving size indicated when trying to evaluate the correct sodium amounts and such. For example: On a box of crackers it will state the sodium amount per serving. In tiny print it will say a serving size is 5 crackers!
