Disease Information Sites
International Cat Care has some basic information about the different kinds of heart disease in cats and is a good starting place.
The Special Hearts: Cats with Heart Defects or Disease Facebook group has a lot of good information about heart disease in their Files section. You will need to be admitted to the group to view those files.
Tanya’s Comprehensive Guide to Feline Chronic Kidney Disease has excellent information about heart disease in cats. This page does spend time covering the specifics of heart disease in cats who also have kidney disease, but is still worth reading even if your cat does not also have kidney disease.
There are 2 different Facebook groups that are good for heart disease:
1. Cats with Multiple Medical Conditions (kidneys, heart, IBD, liver and more)
2. Special Hearts: Cats with Heart Defects or Disease
Cats Health and Behavior is a good, email-based support group through groups.io.