An incompetent veterinarian. Dogs are usually euthanised when necessary with high doses of barbiturates. This is why it's called putting them to sleep. That's exactly what it does. Sleep, coma, then death. It is never, in any way, supposed to be a painful experience for the dog. Just easy and peaceful. The vet screwed up.
Euthanasia is when they put a pet "to sleep." Permanently, like in killing it. Anesthesia is to put it to sleep so they can do surgery. I already answered your other question and you jumped down my throat. Did you have your pet put down or was he having surgery?
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Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them