Accepted Answer

Hi again,
Sorry for the delay in my reply. Yes it very well could be a collapsed trachea although usually this is going to be worse with exercise or excitement. The way collapsed trachea is diagnosed is by imaging the chest and neck while the patient is coughing to look for extreme narrowing of the trachea.
Other possibilities to consider would be kennel cough especially if he has been around other dogs recently. A tracheitis or laryngitis (or inflammation of the trachea or larynx) can also cause gagging and if he has recently been chewing on rawhides or other bones/toys that can be irritating to the larynx that could have irritated his throat.
Unfortunately heart disease can also cause coughing like you are describing and I would suggest having your vet check him for signs of an enlarged heart - recheck for a heart murmur and take chest xrays (ideally a collapsing trachea could be ruled out at the same time) I would also suggest testing for heartworm especially if he has not been on a preventative or if you live in an endemic area.
Bronchitis is rare but possible in dogs and some antitussives (cough medicines) can help control cough if this is the case.
Pneumonia from bacteria (or fungus depending on what area you live) is another potential cause of cough and again chest xrays would help your vet determine if this is the case.
Finally a tumor of the chest or airway is possible although this would be extremely rare in a dog his age. And again chest xrays would help to rule this out.
I would recommend keeping him quiet as best you can and providing ample humidification in the form of warm mist humidification (mist from a hot shower works well for this every few hours). I would not give him any medications until a diagnosis is made as some cough medications can worsen a pneumonia (ie a cough is sometimes protective for the body).
If you do walk him on a leash I would recommend a harness instead of a collar to reduce stress on his airway as collapsing trachea occurs do to weakening of the tracheal cartilage and compression on the trachea can worsen this.
I have attached some information on collapsed trachea for you to read below:

http://www.veterinarypartner.com/Content.plx?P=A&A=1527

Please let me know if I can be of further assistance or if you have further questions.
I'll be anxious to know what happens.

Sincerely,
XX. XXXXX XXXXXX

Expert:	Dr. Duncan
Pos. Feedback:	100.0 %
Accepts:
Answered:	2/8/2009

Veterinarian

DVM from UF, Internship in Surgery and Internal Med; ER vet; 11 years of veterinary experience