My RB Mandy had SAS-Sub Aortic Stenosis. She was a Lab/Golden mix. She was diagnosed at three months. She had a grade 5-6 murmur. The Vets at Ohio State U. put her on atenolol. The surgeries don't prove to prolong the dogs life and they felt she was so bad she would not survive the surgery. She lived just past five years old. Infection took her life in the form of Pyrometra. All we could try was antibiotics as she would not have been able to handle the other treatments. We didn't have her spayed due to the higher risk with that surgery.
Zara, if you're interested, just ask for more info and good luck.
Here's a link about SAS...
http://www.marvistavet.com/html/subaortic_stenosis.html