Aside from that, tracheal problems and heart problems can also cause coughing, as can a general virus.
Any time that I hear of an older dog coughing, I immediately think of a heart problem, especially if the dog is not on heartworm prevention.
You should be pretty concerned. I think that you should make an appointment with your vet as soon as you are able.
Let me know if you need more details or do not understand.