The dog was a great dane black lab mix. 4 years old. 200 pounds. Since last night he wouldnt sit or lay down and was panting/drooling very heavy. Today he was just laying down panting heavy and was staring into space. He got up and went outside very shakily then he just sat and layed down. He was staring into space panting heavy and couldnt move. He wouldnt react to anything and his breathing got very quiet. He was rushed to the vet but his breathing was very low and quiet. What could have caused this?