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It depends on how slow the heart rate is and what the cause of it is to know if it is life threatening; but I would be inclined to think that it is not unless it is excessively slow. Do you have the name of the medication? I could help explain that to you if you have the name. Also, what breed is your dog?
I wouldn't worry about it being life threatening - generally it is not, and given the fact they sent you home, it sounds like she is stable.