I could not disagree with the previous poster more.
Giving a vet many visits to find the right diagnosis is a terrible idea.
Once you find the RIGHT vet with the RIGHT experience in your dogs problem diagnoses is immediate. In our case it just came to late to make a big difference in Duke's longevity.
Duke is 10 years old and will pass away in the next few days so this is a cautionairy tale.
The first piece of the puzzle was the heart murmer. He always had it but each vet that noticed it pronounced it to be a mild murmer.
It all started with the cough/yak. First only while playing ball. He started to have trouble holding the ball in his mouth. He'd drop it and give a good Yak.
Next came a deeper more persistant yak/clearing of the throat.
Vet's focuses on the yaking and throat which turns out not to be the big issue.
Six months ago a new vet blamed it on his heart which had become enlarged to the point of pressing on the lower part of his airway. At first this seems counterintuative but as his illness progressed proved to be correct.
As of today he probably has a day or two left before heart failure.
The murmer led to an enlarged heart which is now struggeling to circulate fully.
This is a terrible way to watch Duke die. He is only 10 and up until a few days ago was incredibly vibrant. Played ball constantly. He is the smartest dog I have ever known (we have way too many yorkies!) and basically runs the household as he has an impecable internal clock. Everything must occur at the exact scheduled time or he will let it be known the things are off schedule.
Had we paid more attention to the heart than the throat meds might have started sooner and given him more time. Now all we can do is wait for the final bad day to arrive.
Heart Murmer + yaking just might have nothing to do with the collapsing trachea problem. Especially if the yaking started in adulthood.
Xray the heart. And do it every couple months and find a vet who has seen this problem before so they don't under medicate.
http://www.edhamilton.com/public/Duke.JPG