Heart failure is a progressive disease that decreases the pumping action of the heart. This may cause a backup of fluid in the heart and may result in heart beat changes. When there are changes in the heart beat sometimes an implantable heart device is used to control the rate and rhythm of the heart beat. In certain heart failure cases, when the two lower chambers of the heart no longer beat in a coordinated manner, cardiac resynchronization therapy may be prescribed.

People who have a dangerously fast heart beat, or whose heart is at risk of stopping beating, may be in need of an electronic device called an implantable cardioverter defibrillator (ICD).

Atrial tachyarrhythmia (AT) is the name for rapid beats in the upper chambers of the heart. People with AT may experience symptoms such as heart palpitations (a racing or pounding feeling in the chest), shortness of breath, dizziness, fatigue or weakness.

The purpose of this study is to characterize the impact of cardiac resynchronization therapy versus cardiac resynchronization therapy with atrial therapies on the incidence, duration and termination of atrial tachyarrhythmias.

Inclusion Criteria:

• Subjects with atrial fibrillation
• Subjects with a heart beat that is too fast and who have been determined by a doctor to require an implantable cardioverter defibrillator (ICD) i.

Exclusion Criteria:

• Subjects with a natural heart rate less than 30bpm