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I am so sorry to hear what happened to Moo. My sincere condolences. My name is Rebecca. I am a veterinarian and will do my best to help.
If he had a heart murmur, it still could have been a heart problem, like the young athletes that drop dead playing sports. An undetected heart problem can cause sudden death.
Another cause of sudden death in cats is a blood clot to the brain or lungs. Heartworm, although rare in cats, is another cause of sudden death.
Only an autopsy could tell you for sure why he died. My deep sympathy for your loss. Let me know if I can help with or answer anything else.
How are you doing? Did this help at all, or at least answer your questions?
I am so sorry for your loss.