My Pug Keeps Chasing His Tail Until He Gets Totally Exhausted What Is Wrong With Him? He Has Never Done This Before And I Am Afraid He Will Have A Heartattack. He Barks At The Same Time And Whimpers Too. Can You Help?

Dogs do not have true heart attacks in the way people do.  However being a pug they can have respiratory distress if they become extremely agitated.  Check his gum color during these episodes to make sure it is pink.  If it looks pale, white, or purple make him stop and settle down.
The whimpering and barking indicate something is bothering him.  If he is also scooting them it is almost certain his anal glands are full and may be impacted.  Constipation can also cause discomfort.  A wound or trauma to the tail may be the culprit.  Dermal parasites like fleas can cause irritation, itching, and lead to strange or self-mutilating behavior.
Have your dog examined by a veterinarian to determine the cause of his discomfort.

I think you have a very good question.  I have been a pub owner for 45 plus years and I love this breed.  I believe that you will find the dog has an allergy.  The vet can give a shot to help with this, or medication.  I don't believe that the running will cause a heart attack unless your pet is in poor health or too heavy.  The problem could be fleas, but I feel like you have already checked him for this problem.  Good luck...

The whimpering indicates distress.  The most likely problem is impacted anal glands.  These need emptying out by a veterinary nurse.  Get her to show you how, so you can do it at home in future.  This is an unpleasant and smelly job, but will give poor pugsy much relief.

He is bored, and this is a new game for him. He could sure use some time to spend some energy, or maybe you can get him some toys to play with. He isn't going to have a heart attack if he isn't that old. Hope this helps, good luck.