unexplained death of our cat

Question
Our healthy vibrant 12 1/2 year old female spayed cat was wheezing Sunday morning when I got up - no previous symptoms.  We took her to the vet right away.  She was given 3 shots (to clear airways, dry secretions, etc.).  Her breathing continued to get worse.  The shots had no effect.  Her temperature was below normal.  She had no other apparent symptoms.  She died within 15 minutes of so or arriving at the vet.  It was horrible, and we have no idea how this could have happened.  What could have caused this?

Answer
My sincerest sympathies for your loss.  Without being able to examine her it is hard to rule out the many possible causes of respiratory distress and death in cats, however what you have described is fitting for heartworm disease.  

Heartworm is passed by mosquitos and may infect your cat without any signs for quite some time.  The first sign can be when a group of the worms breaks off and travels in the bloodstream leading to pulmonary emboli, a very serious condition in their lungs.  By the respiratory symptoms and sudden death of your dear cat I think this is a possibility.

Again I am very sorry for your loss.