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Hello. I'm so sorry to hear about your dog.
That dose is really not all that high where I would expect significant signs, let alone dying. The LD50 (dose where 50% die) is 200 mg/kg. > 20 mg/kg we can see moderate to severe signs, but they are usually self limiting and resolve with symptomatic care.
I would wonder whether she ingested more than one tablet or if she actually went into congestive heart failure or had a ruptured chordae in the heart that holds the valves in place. In one of those situations we could see death fairly quickly.
You did everything you were supposed to. I'm so sorry for the outcome. I'm sure the vet you had her at is also upset and second guessing what happened.