My husband is going to the emergency room.

[albacore]

79. Had AVR replacement two years ago...

... and I'm still kicking. I had lots of other surgeries - I beat up my body pretty good - and this was the worst. I say that not to scare you, but to prepare you.
It's one of those bungee-jump things. NOT a roller-coaster... that's too smooth. The ups are really high, and the downs are really down.
Luckily, the best hotshots in the business, from the nurses and doctors to the most modern equipment is there to take care of him. The surgeons are megalomaniacs who will NOT let their patients die! The nurses have seen it all before, and are totally cool with the setbacks and are ready for them.
Think of it this way. Without surgery i had 2 months to live.... probably the same with your husband. Now I bike 20 miles with no sweat....about 70 miles a week, I can do 60 minutes at the gym going full bore, and I feel great. (Oh... I'm 75.)
That's where your husband will be in 6 months. Never mind what they tell you...that shit hurts. And he'll be tired. But he'll get better and better. Not day by day...but week by week he'll get his strength back. And maybe more strength than he's had in years. My little pig valve will outlive me.
Be worried... that's fair. But don't despair. He's joining the "zipper on the chest" fraternity. There's lots of us.
Lots of good wishes going out to him - and you.