RetiredJim,
Thanks for the post.
Q:"How is the descision made in the case of aortic stenosis whether to operate and install a new valve or not?"
Mostly based on symptoms. Persons with heart failure, syncope, shortness of breath, cheat pain, plus some others are generally considered for surgery if the aortic valve is seriously stenosed.
In persons whom we are uncertain if they have symptoms, we will sometimes do an exercise test on a treadmill to try to demonstrate symptoms.
Please note that aortic stenosis is often a progressive disease -- many persons have had a murmur for 10-30 years before surgery is needed.
Check out the ACC guidelines:
http://www.acc.org/clinical/guidelines/valvular/jac5929fla16.htm#A6a
Best of luck.
I would definitely get a second and third opinion if necessary before having surgery.
Glenn