I'm an anesthesiologist. I put people to sleep for cardiac surgery. My hospital does around 400-500 hearts a year... and we don't kill any dogs.
So maybe I'm not up to date, or things are/were different in research hospitals.
My personal info was based on stories told by my mother, in about the '60s, when she was a special duty RN at the University of Michigan hospital, often handling cardiac recovery.
My favorite was the one where the UofMich hospital cafeteria, which had been purely open seating, established separate rooms for the staff to eat after an incident where patients' families overheard, and were traumatized by, a cardiac surgeon's response to a question. Asked how his operations the previous day had gone (referring to his experimental and/or practice surgery on a collie and another dog), he said "The blonde lived but the old bitch died."
The kids and adopted dogs story was from my wife. The surgeon in question was Dr. Albert Starr in (at least) the '60s through '80s. He was at St. Vincent's and also flew, with his team, to operate at a number of other west coast hospitals, university and otherwise.