 Pacemakers can be attached to one or more than one of the heart chambers. After watching pacemakers work over time, doctors have found that the pacemakers that stimulate only one chamber of the heart sometimes lead to problems later. These problems may be changes in the size and shape of the heart. The heart cannot work as well when some of these changes happen. We need to learn more about these changes and how to prevent them. There has not been an easy way to do this. A new treatment called Cardiac Resynchronization Therapy CRT is associated with biventricular pacing where two chambers of the heart are stimulated simultaneously. Tissue Doppler Imaging,Tissue Synchronization Imaging and 3 dimensional echocardiography are new forms of technology that look at the heart while it works. They are similar to a moving x-ray that can watch the heart muscles moving. The movement can be measured. Doctors will check for changes that happen over time. This has not been studied in children before because this kind of is new to this group of patients. This technology is noninvasive which means it can be done from the outside of the body and is painless. 