My dog Moochie attacked a cat...

Yes, it is quite possible the cat had a heart attack. My daughters cat who was 12 years old recently had one and passed away. My sympathies about the cat, Moochie must have really scared the poor thing.

I think yes , the cat get a heart attack because he got pain. This is normal in the nature of the animals, was it a old cat? Your dog is a lil bigger than the cat? ;D Cya later

I do not know dog will fight with cat, i though cat will always run away from dog. Sometimes when dogs play with each other, they can be really rough, and they don't know they have actually hurt someone.

Awww that's so sad!! I'm sorry your dog did that. Sounds like you need to keep him on a leash at all times so he can't get another cat ever again. That's terrible but don't blame yourself for you didn't cause it to happen. Maybe if you train it with a leash, in due time he'll get the understanding that he's not to do that to another cat but maybe make friends. We can only hope, can't we?