Hello, I'm Dr Ralston, thanks for your question.
Bichons and Poodles both have an increased risk of heart disease over other types of dog. The most common is mitral valve degeration, the valve in the heart that separates the left atrium from the left ventricle. When that valve starts to break down because of old age, infection, or just plain genetics, it leaks. Blood flows backwards against the natrual beating of the heart. It is a leak. That leak makes the sound, or murmur that you hear.
Your Vet was right in starting medication based on the clinical signs of the pet : the coughing. But, the cough in heart disease is usually caused by one of two things:
1. fluid in the lungs - this is usually only when congestive heart failure is occuring and is very serious. Your Vet would have likely seen this on the x-ray
2. heart enlargement - the heart is right next to the trachea as it brings air to the lungs. When the heart gets big from the extra work it has to do because of the leak, or murmur, it pushes other things in the chest out of the way. Sometimes, it will compress the trachea. That's just like somebody pressing their hands on your trachea outside the body. It causes you to cough. It's like being choked, but on the inside.
The cough can sometimes respond to medication. If it doesn,t it might mean the medications should be increased.
You need to talk directly to your Veterinarian about that. You might want to schedule a recheck so they can look at her and listen to her lungs and heart to make the best judgement on which medication to increase.
Sometimes, if the heart muscle has greatly increased from the extra work, the cough will be a permanent change. Keeping the pet calm, and in cool temperture control will help reduce the cough.
In some cases if it is severe I have used some cough suppressants. But again, you will want to work very closely with your Vet because of the other medications she is already on.
JUST FOR YOUR EXTRA INFORMATION: I will include an extra link for you so that you can read more about heart disease in dogs. I think it is helpful. Here it is - (Just click here for a link that describes degenerative heart valve disease)
Of course if you have questions I am more than happy to keep answering them for you.