Are you using, kibble, canned, or both?

The Medi Cal Preventive is made by Royal Canin, it is to prevent the blockages, has nothing to do with the heart condition. It isnot a very good food, it contains grains and corn which is just a filler. You can use Google to find lots of info on it.

A cat prone to blockages needs lots of fluids, and an acidifier.

See if you can add a canned food like Weruva, Tiki Cat, or Fussie Cat. These are grain free foods, with formulas in a gel, not a gravy, so it helps hold the fluid in the cat a bit longer. Mix it with the dry kibble and see if this makes any improvement.