Heart murmurs are abnormal sounds, heard while listening to the heart with a stethoscope. The normal heart sounds or beats (lub-dub) are caused by vibration occurring during the colsure of 4 valves within the heart. Murmurs are typically caused by a change in the blood flow through the heart or outflow vessels that causes turbulence. There are many causes and most can be catagorized into congenital/pathologic, physiologic, or benign/innocent. Murmurs are typically graded by intensity on a scale of 1 to 6, with 1 being mild and 6 being severe.

Congenital/pathologic murmurs are caused by physical abnormalities present at birth. Examples are a narrow outflow vessel, abnormal communication between vessels or chambers, and faulty valves. Many congenital heart problems do not cause clinical signs (especially in very young puppies), but some display serious symptoms (stunting, failure to thrive, exercise intolerance, etc...). Congenital murmurs also appear to be more common in purebred dogs.

Physiologic murmurs occur as a result of an abnormal condition unrelated to the heart. This condition causes an increased heart rate or increased blood flow velocity resulting in an audible murmur. Causes can be fever, pregnancy, anemia, and hyperthyrodisim. The murmur should disappear when the condition resolves.

Benign/innocent murmurs typically occur in puppies and are usually caused by an alerted flow in blood exiting the heart chambers. The intensity of these murmurs can increase with exercise or excitement. These murmurs are usually low grade (grade 1-2), show no clinical signs, and often disappear spontaneously in weeks to months.