Heart murmurs are graded 1 through 4, with 1 being the slightest and 4 being the worst. What grade is his/her murmur?
Did you get your puppy from a breeder? If so, you are within your rights to return this puppy as a puppy with a murmur is not considered fit for sale. They would HAVE to either return your money or give you a healthy puppy instead. I have seen instances like this where the breeder will refund the money and, if the murmur is not a serious one, as long as the buyer is amenable, they get to keep the puppy as well. It's just not ethical to take money for a puppy with a heart murmur.
If the murmur is not extremely serious, it will often resolve itself by the time the puppy is about six months old. Frequent vet checks to see how loud the "whoosh" is will tell whether or not it is in the process of resolving or not. If it is resolving, you could end up with a completely normal puppy within a few months. If it doesn't resolve, you'll have to keep an eye on the puppy to see how severely it is impacting his ability to have a normal life. If he gets tired very quickly, then you'll have to limit his interactive play with his humans accordingly. When he plays by himself, he will be his own barometer as to when he has had enough. He'll simply lay down until he feels his energy returning and then he'll resume his activities. Remember, though, an 8 week old puppy, even a very healthy one, is going to sleep A LOT. They are just like human babies. They basically eat, poop and sleep. They will play hard when they play and all of a sudden they will just drop like they ran out of gas and sleep until their little batteries are recharged. Don't mistake normally puppy sleeping for being tired because of the murmur.
At this point, it's really up to you. Depending on how much in love you are with this puppy already, you are, as I said, within your rights to return him to his breeder for a refund, or you can maybe work out a "give me back my money and I'll give the unsellable puppy a good home" deal, or you can ask the breeder to replace him with a healthy puppy. If you opt to keep him (however the money works out), just watch him and his activity will tell you how the murmur is affecting him.
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