Expert: Brandy - 1/20/2007

Question
My cat loves to lie on my side with his head close to mine, I pet him and he enjoys it immensely. He purrs really loudly and then at one point - about after 3 minutes of extensive purring, he starts to cough. At this point I usually just put my hand lightly on his back in order to calm him down, but after a few seconds he jumps down and continues to cough for another 30 seconds. Does anyone know why he can't purr longer than 3 minutes without starting to cough? I always feel bad for him because first he's enjoying himself and being affectionate and then he has a coughing episode. Is there something I can do?
Thanks for any advice.
FYI - Some Additional Details:
some more details:
My cat had a recent check-up and does not have cancer.
It's been going on for years.
He's 8 years old.
After coughing he doesn't throw up any hairballs
Again, thanks for any thoughts.

Answer
One of my cats does the same thing, but only when the purring was extra loud - like yours. The problem is that we don't know exactly HOW a cat purrs. Cancer is usually not the cause of coughing in cats (as far as I know, and I'm not a vet), though sometimes it can be blamed on weight. I think that the cat just becomes extremely relaxed, and like someone with snoring issues or sleep apnea, the area around the throat becomes too relaxed.