[ladyjane]

Here is a link I just found that explains this:

01 Tracheal Collapse - VeterinaryPartner.com - a VIN company!

From the site:

What Animals are Affected?

The victim is almost always a toy breed dog, with poodles, Yorkshire terriers, and Pomeranians most commonly affected. The disease usually becomes problematic in middle age but can occur at any age. The cartilage defect that leads to the flattened C rings seems to be hereditary.

Many dogs with collapsed tracheas do not show symptoms, however, until a second problem complicates matters. Factors that bring out symptoms might include:

Obesity
Anesthesia involving the placement of an endotracheal tube
Development of kennel cough or other respiratory infection
Increased respiratory irritants in the air (cigarette smoke, dust, etc.)
Heart enlargement (the heart can get so big that it presses on the trachea)
If a secondary factor such as one of those listed above should occur and make a previously incidental collapsed trachea problematic, oftentimes removal of the secondary factor (weight loss program, getting an air filter, etc.) may clear up the symptoms of the collapsed trachea.