I'm very sorry that you lost your last bun in surgery, but, as Haley said, it is very uncommon for an experienced vet to lose a rabbit in surgery.
Think of it this way - Your bun will be in danger for only about an hour or two. That two measly hours equal to your bun living a happy, healthy life. Since most rabbits live longer if they're spayed/neutered, that two hours could equal three more years of enjoyment with your bun.
Make sure you get a really good examination before, and, if you have any concerns, don't hesitate to take her back again before she is spayed. It's better to have a healthy, spayed bun a month later than to have your bun pass away in surgery because she had an undetected heart problem.
She'll probably get through it fine, and a month from now you'll be thinking "What was I so worried about?"
I was terrified before my bun has his neuter, and all day while I was in school I kept checking the time to see when I would be getting home and when he would be having his surgery etc.
As the others said, bloodwork, and some good examinations would be a good idea. If your bun is found to have a heart problem, then it would be better to leave her unspayed if the surgery might be to stressful for her.
Good luck!
ray:
Rachel & Ollie.