Rapid, excessive panting dog

Question
I left my dog (2 year old border collie) tonight (in the air conditioning) for about 2 hours to go to dinner.  When I got home I noticed that he was panting very fast with his tongue hanging out of his mouth.  I gave him some water - he drank a little bit but he continued to pant like this for about 1/2 hour.  He was breathing so hard I thought he was going to have a heart attack - Any idea what this was and how I can help him if it happens again?

Answer
This is odd.  Is he on heartworm prevention?  Does he have a history of siezure activity?  Could he have been chasing something while you were gone? Was anything out of place in the house?  Was there a storm building in your area?Border collies are usually a little high strung, but for this to have happened then he returned to normal so quickly doesn't really seem to be something to worry about.  I would watch him closely to see if this happens again and carefully note what's going on around him when it does so you can give that information to your Vet.  If he is developing some kind of separation anxiety, you might give him some Benedryl before you leave him next time to calm him in your absence.