The following symptoms can help you determine if your dog is experiencing fluid in the lungs: Persistent cough, Rapid breathing, Gagging or wheezing, difficulty breathing, increased heart rate, and Lethargy/depression. In dogs, the most likely cause of fluid in the lungs is heart disease, followed by cancer and infections. Dogs who cough frequently have fluid in the lungs. A persistent cough in your dog should be evaluated by a veterinarian.