Wow, I am sorry for what you are going through and truly hope everything turns out okay.
I lost my dear mother this past summer to complications from leukemia. She had the foresight 4 years ago to meet with a family-planning attorney and get everything in writing. She had Power of Attorney paperwork for both financial matters and health care decisions (naming me as having POA if and when she reached a point where she could not handle decision making for herself, so I was able to manage her bank accounts, write checks for bills, etc.) She had an advanced directive that stated explicity what her wishes were as far as medical care (i.e. whether or not she would have a feeding tube placed, whether or not she would allow CPR if her heart or breathing stopped, etc.).
My brother and I were already grown, so naturally there wasn't an issue as far as any kind of guardianship, but I would think that you should have something like that in writing in case something happens to both you and your husband. If something happens to just you, and she is not yet 18, I would think she would just stay with your husband (assuming he is also her father).
It's a grim task and not such a fun thing to have to think about or deal with, but everyone (especially those with children) should have documented plans for just in case the unthinkable happens. I would recommend meeting with an attorney who specializes in these matters and let them guide you through the process. Good luck, I truly hope the decisions you make now never have to be followed trough with.
p.s. Just read some of the other responses - admittedly, I don't know much about atrial fib, but hopefully it will not be an issue for you. But I would still consult with an attorney because life is unpredictable and the scary truth is that something could happen to either you or your husband, or both, that has nothing to do with the atrial fib. I hope this helps...