A cough is produced in one of two mechanisms. The enlarged left atrium compresses nearby large airways, producing a cough reflex. As the disease progresses, enough blood becomes backed up to cause fluid to accumulate in the lung tissue, this also causes a cough. This stage of the disease is termed congestive heart failure. The signs of valvular disease include a heart murmur.
Heart murmurs are abnormal sounds heard through the stethoscope.
They are caused by the turbulent flow of blood, such as the flow that occur across a faulty valve. It is extremely common to find heart murmurs in older dogs due to mitral valve disease. Not all of these dogs have a cough or any other physical sign of illness. Some dogs live out a normal life span with the murmur, and never develop any other problems. It is also important to remember that an older, coughing patient with a heart murmur may have some other condition causing the cough, such as chronic bronchitis.