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Hello, this is Dr. Jess. I am very sorry to hear about your dog! I wonder if he could have had a tumor on his heard that finally grew enough into a vessel or the heart chamber to cause a rapid bleed into the pericardium. I have seen this happen in 2 dogs in the past few years, where a dog was completely normal, and then collapsed and died, and upon necropsy we found pericardial effusion as the result of a heart tumor.