Heart Disease is the second most common cause of death in dogs and cats. This disease is usually caused by valve disease or muscle disease. Usually but not always heart disease is causes a heart murmur that can be heard on physical examination.
In some cases, with certain breeds of dogs (boxers and Dobermans) there is an arrhythmia associated with bad heart muscle (cardiomyopathy) that cannot be determined on a physical examination. In the past ECG studies were used to hopefully determine the presence of cardiomyopathy. Cardiomyopathy is common in certain breeds of dogs and many cats. It is now best discovered or diagnosed with a blood test called a ProBNP test or a Troponin test. These tests detect stressed heart muscle cells.
Ultrasound is usually the Gold Standard for determination of cardiac function and will be recommended should a heart murmur be detected in your pet. Heart disease can usually be managed with medications.