Bloat or Torsion - the mother of all emergencies.
** Get them to a veterinarian immediately! **
There can be irreversable damage in less than an hour.
Alert your vet that you're on your way - Don't wait in-line - this is no time for polite manners -
push to the front of the line - explain your emergency - you need an X-ray NOW.
Your dog's life depends on you.
Symptoms can be subtle - one or more of these -.

Please visit these sites.
This is one of those things that you must be informed about because this knowledge may save your dog's life.
Mar Vista animal medical center
Dog Owners Guide
Elektrik Borzoi
Great Dane Links magazine

You know your own dog the best and you know when things aren't quite right.
If you notice any of the signs in your dog, call your vet or take him to an emergency clinic as soon as possible.