Try your best to calm down
I know the words "heart failure" are very scary, but it's truly not as bad as it sounds. Failure is too harsh a word.
I take care of many (very elderly) people in a nursing home who have had CHF for years and years.
It is controlled by medications and I've never seen anyone in pain due to the CHF.
The vet sounds fantastic! I'd bet your baby's in very good hands!!
You'll be in my thoughts! Good luck!