When the heart is too weak to adequately pump blood, fluid accumulates in the lungs, causing a condition known as congestive heart failure (CHF). Your dog may have CHF if he coughs frequently, has trouble breathing, seems restless at night, or tires easily after mild exercise. It's most common in old, overweight dogs. There's no cure for CHF, but it can be managed for a time with diet, medication, and rest. Weight loss helps, too.