The puppy's owner Pauline Daley said the surgery to replace a missing wall between two atriums in Lottie's heart was breaking new ground in the canine world.
The operation, set to be carried out by the Royal Veterinary College, will cost £12,000 and is borrowing from techniques used in human babies.
Pauline said Lottie had shown no symptoms. It was only when getting her pet spayed that vets told her and her husband Paul that something was wrong that they knew how bad the situation really was.
"We were devastated," she said. "She's full of life, She's so playful – but the vets said they didn't know how she was functioning.
"Lottie went down to specialists in London and they confirmed she doesn't have a dividing wall between her two top atriums. It is incredibly rare. They get holes in the heart, but they had never seen such a large void before.
"Her heart on the right side is grossly enlarged, because it is under such immense strain. The blood is going in, and oxygenated blood and non-oxygenated blood is getting mixed up. You wouldn't know she has a problem, she's not showing any clinical signs yet. But vets said it won't be long until she is."
The surgery will give vets the chance to push the bar for this kind of work, paving the way for dogs with similar conditions.
"The vets have been talking to the Royal Brompton Hospital paediatrics ward, and are working alongside those who carry out the procedure in babies," Pauline said.
"It's about making a wall and patching any gaps with a Gore-tex material. It has a very high success rate in babies. The surgery gives a unique opportunity to do more research in open heart surgery in dogs. It will move things along leaps and bounds.
"We've been told she won't survive without the operation. People in a similar situation might feel like there is no chance, but there's lots of hope if you push for it and it's moved forward.
"I've been in shock since March, when we found out. I've been beside myself thinking we're going to lose the puppy. She's family to us. But with the surgery it's as if all of a sudden someone has told us our daughter is going to be fine."
Retired sales manager Pauline, 58, lives in Malpas, just north of Whitchurch, with Paul, 63. The couple have launched a campaign to help raise the £12,000 needed to saved Lottie's life.
Through the Lottie Daley Facebook page they are sharing updates about her condition, and have also started a crowdfunding page.
Local vets and kennels are helping with the fundraising efforts, and £780 has been raised through JustGiving.
"It's humbling," Pauline said. "It breaks my heart that we have to do it, but it makes me so gracious. I can't tell people how grateful we are."
To donate visit justgiving.com/crowdfunding/lottie-daley