Excerpt from Ask the Vet About Dogs: Easy Answers to Commonly Asked Questions
As you may remember learning in high school biology, the heart has four chambers—two atria and two ventricles—and the valves between each part of the heart control the flow of blood within the heart and to the lungs and the rest of the body. A heart murmur is simply vibration of the heart caused by abnormal blood flow in the heart. Heart murmurs are classified by the volume and strength of the vibration, on a scale of six. A grade one out of six murmur (1/6) is barely audible through a stethoscope. A grade three murmur can be easily heard, while grade four murmurs (4/6) can both be heard with a stethoscope and faintly felt if you place your hands on the dog’s chest. Grade six murmurs (6/6) are very loud and can be heard without the aid of a stethoscope if you simply place your ear near the dog’s chest; you can feel the strong vibration with your hands.
There are many, many causes of heart murmurs. Some are due to congenital defects: malformations of the heart and its blood vessels. Anemia can cause a murmur, as can a fever, and puppies sometimes have unexplained low-grade murmurs that disappear as they mature. Inflammation (endocarditis) or permanent scarring (endocardiosis) of the valves of the heart can cause a murmur. Many older dogs develop these conditions as the result of dental disease, a source of blood-borne bacteria that infects the valves.
Cardiomyopathy, abnormal thickening or thinning of the muscular walls of the heart, can cause a murmur, though its causes are not yet well understood. Heartworm disease can also cause a heart murmur, not only because the worms obstruct the flow of blood but also because the heart becomes enlarged as it tries to continue pumping blood around them.
X-rays of the heart provide your veterinarian with good information about what might be causing a murmur. Echocardiography (cardiac ultrasound) is available for dogs and can be used to measure all the anatomic parts of the heart, as well as the speed of blood flow through each of them. Veterinary cardiology is a well-advanced specialty, and the causes of most canine heart murmurs can be successfully treated, although your dog may require medication and close monitoring for the rest of her life.