Heart block is an abnormal heart rhythm that usually results in a slow heart rate. It is caused by a problem in the heart’s electrical system, also called the conduction system. When a child has heart block, the electrical impulse is delayed or blocked completely as it travels from the heart’s upper chambers (the atria) to the heart’s lower chambers (the ventricles). There are three different types of heart block called first degree, second degree, and third degree heart block.

Murmurs are sounds made by blood circulating through the heart's chambers or valves, or through blood vessels near the heart. Heart murmurs may be caused by a number of factors or diseases, including the following: • defective heart valves • holes in the heart walls • surgical repair of congenital (present at birth) heart defects • fever • anemia (a decrease in the red cells in the blood)

According to the latest statistics from The United Network for Organ Sharing (UNOS), 280 children were waiting for a heart transplant in the United States on Sept. 30, 2000, including: • 124 in the newborn to 5 years age group • 43 in the 6 to 10 years age group Nationally, children account for almost 6 percent of the 4,121 people who are on the heart transplant waiting list. Approximately 360 children receive heart transplants each year. The first step in the heart transplant

"Cardiomyopathy" itself is a very general term referring to any condition (and there are many) importantly affecting the heart muscle itself while "hypertrophic cardiomyopathy" refers to a specific and genetic condition which usually shows a familial pattern. The most characteristic feature of HCM is a hypertrophied left ventricle (asymmetric thickening of the wall usually most prominently involving the ventricular septum) without abnormal enlargement of the ventricular cavities.

In this condition, for unknown reasons, the left side of the heart does not develop properly while the baby is in the mother's womb. The parts of the heart that are usually affected are the mitral valve, the left ventricle, the aortic valve, and the aorta. In the normal heart, red blood returning from the lungs, flows from the heart's left upper chamber called the left atrium through the mitral valve to the left ventricle where it is pumped through the aortic valve and out to the body.