How do I know if my cat has any heart issues?

What would I see in my cat to suspect that he or she has heart disease? Your cat might be lethargic. They may sleep 24 out of the 24 hours instead of 23. Sometimes they don't show any symptoms at all, which is why a physical exam at least once a year is vital for your cat. But your cat may have shortness of breath or open-mouth breathing, which would be some symptoms you might notice.