Congestive heart failure (CHF) causes difficulty breathing because of fluid accumulation in the lungs. It is an important and common clinical problem. Mitral valve regurgitation and dilated cardiomyopathy are common causes of CHF in dogs, which can develop as these conditions progress in severity. Because there currently is no cure for these heart diseases, the treatment focus has been on prolongation of the time to CHF development, relieving signs of fluid retention when CHF occurs, and supporting the function of a failing heart. Diuretic medications (“water pills”) cause increased urination after removal of fluid from the lungs and are the most effective treatment for CHF. Most dogs respond to diuretics initially, but over time, progression of heart disease and maladaptive mechanisms result in less urine production and return of CHF signs with potential for suffering, death, or euthanasia. Loss of responsiveness to diuretic medications may be multifactorial, and so identification of the underlying reason could guide medical interventions to restore urine production in response to diuretics, and improve quality of life for dogs. The investigators have identified several candidate blood and urine variables that correlate with urine production in dogs and may be useful to indicate responsiveness to diuretic medications. This study will evaluate these variables in dogs with progressive stages of naturally occurring heart disease to identify those that are poorly responsive to diuretic medications and determine underlying causes for improved patient outcomes.