Dear Hideyo: I can't really add anything to what Nina said and said so well.
I am a bit surprised, however, to hear that the vet sent her home with you so soon after the surgery. Didn't you say she had only been awake for 100-15 min.?? I volunteer with an organization that does Spay/Neuter events for ferals once/month. The vets donate their time and we do them on a Sunday when most surgival facilities are not being used. The object is to do as many ferals as possible in the one day, but also to do it SAFELY, for the sake of the cats. We have teams of volunteers that work in concert with the vets/vet techs to handle the anesthesia, surgical prep, surgery (by vets only, of course), post op care and recovery. We normally do between 30-50 cats in a day and do not release any cats until they are well awake and have been observed for an extended period of time and we are pretty sure all is well. While it may seem that this sort of assembly line approach may not be the best, we haven't (knock on wood) lost one yet in several years. After surgery, we monitor the cats very closely with regard to their pulse/respiration and keep them warm and quiet. Most, typically, are held for at least an hour or two after their surgery, with those who are done earlier in the day it is longer. So, I guess I would question whether or not the vet who did Suzi's surgery had done everything that he/she should have to make sure Suzi was coming out of the anesthesia OK and that there had been no complications. Any kind of surgery is stressful for even a domesticated cat, and for ferals there is an added stress factor due to their being unaccustomed to handling. Whatever went wrong that caused Suzi to fail and not recover from the surgery, it was certainly not due to any fault or negligence on your part. You were trying the best you knew how to give her a better chance in life. Spaying/neutering is one of those things that needs to be done to cut down on the indiscriminate breeding behavior that is the main vector for the transmission of diseases like FIV, FeLV, etc. in feral populations. If more governmental bureaucracies would realize this is the single most important thing that can be done with limited resources to help put an end to unnecessary suffering and pet overpopulation both in domesticated and feral populations (instead of putting funds into testing and euthanasia for ferals), it would make a big difference. I know that regardless of anything that I or Nina or anyone else says, you will always feel badly about losing Suzi and wish you had not taken her to be spayed that day. I, too, would feel badly. Keep in mind that hindsight is always sharper and we are not given a crystal ball to see into the future when decisions must be made. Suzi needed to be spayed, if not that day, then another and you don't know that the outcome might have been the same at some other time. She may have been terminally stressed whenever the surgery was done. In the future, you might ask your homeopathic vet if he can recommend something that will helpstronger than you