noisie lungs in puppy

Question
My 4 month old king Charles spaniel has fluid of some sort in her lungs. I've taken her to the vet several times since shes been 2 1/2 months old they've given her amoxicilin and it doesn't seem to help relieve the hacking up reflex any. And nothing comes up when she hacks. the vet says her heart is fine and they don't know what is causing it. Would it hurt her if we tried to give her some Expectorant, what would be an doseage dose for a puppy only weighing 3.2 lbs. what should we do.

Answer
I would not recommend giving her an expectorant without knowing the cause of the problem.  Has she had any chest x-rays?  Due to her young age, I would be concerned about congenital problems (conditions she may have been born with).  

Is the "hack" a cough, gag, sneeze?  Small breed and toy dogs are prone to developing a collapsing trachea or reverse sneeze syndrome.  However, these conditions would be uncommon in a 4 month old.  

I recommend x-rays first to evaluate her heart, lungs, trachea, esophagus.  This can help determine if her respiratory tract or her digestive tract may be causing the problem.  If the x-rays are normal, the next diagnostic test would be bronchoscopy (performed at veterinary specialist centers and universities).  I also recommend a recheck with your veterinarian so he knows the treatment of antibiotics wasn't successful.  This information would be very helpful and can help guide you to the next step.