Heart murmurs are commonly present in smaller breeds of dogs that are middle-aged and older. Most often, a heart murmur will be detected by a veterinarian during a routine annual physical. Usually, the condition doesn't signify any kind of life-threatening problem—but it still shouldn't be ignored.
In my practice, I commonly offer second opinions for pet owners whose dogs have been diagnosed with a heart murmur. There are two mistakes I routinely encounter.
The first mistake is that the pet owner is told a bit prematurely that a heart murmur's nothing to worry about. It's true that a heart murmur doesn't usually signify a serious disease at the time of diagnosis, but it's also true that any time a heart murmur is heard, it signifies heart disease caused by abnormal leaky heart valves. Therefore, we shouldn't ignore the heart murmur but do a cardiac evaluation (EKG, cardiac ultrasound, etc.) to determine the severity of the heart disease and whether it should be treated with conventional medications.
The second mistake I see is that pets with heart murmurs are placed on strong cardiac medications, such as diuretics or ACE inhibitors. There's no need for pets with heart murmurs to take medications unless they show signs of heart failure on the cardiac evaluation. Doing so unnecessarily increases the cost of pet care for the owner and places the pet at risk for side effects such as kidney failures. Instead, consider treating your pet with cardiac supplements such as herbs, coenzyme Q-10, antioxidants, or fish oil, etc.) to provide nutritional support to the heart. As always, discuss treatment options with your vet.