The purpose of this study is to learn what effect rapid atrial pacing (in patients with dual chamber pacemakers) will have on the electrocardiogram including the QT Interval. The investigators are also interested in the differences caused by genes.

This study requires a total of two study days as well as two follow up visits. During study day one, baseline blood will be drawn, a baseline electrocardiogram will be performed, and continuous QT Interval monitoring will be performed. The pacemaker rate will be increased to 90bpm at different time intervals and additional blood will be drawn to measure the effects of these rates changes.

On study day two, baseline blood will be drawn, a urine specimen will be obtained, an electrocardiogram will be performed, and continuous QT Interval monitoring will be performed. Following the acquisition of this information, the pacemaker rate will be decreased to 80bpm. We will obtain blood specimens, a urine specimen and an electrocardiogram at 30 minutes post rate change and again at 60 minutes post rate change.

Two days following the rate decrease to 80bpm, the patient will return to clinic to have their rate decreased to 70bpm. In two more days, the patient will return for a final rate change to 60bpm.

Inclusion Criteria:

• Must be at least 21 years old
• Must have a dual chamber pacemaker implanted more than three months ago
• Must be followed by the Vanderbilt University Medical Center's Arrhythmia Device Clinic
• The indication for the pacemaker must not be related to coronary artery disease (CAD) or congestive heart failure (CHF)

Exclusion Criteria:

• Cannot be pacemaker dependent
• History of ventricular arrhythmias and/or implantable defibrillators (ICDs)
• History of paroxysmal, persistent, or permanent atrial fibrillation (AF)
• Congenital long QT syndrome
• History of coronary artery disease
• History of, or currently receiving, treatment for congestive heart failure
• Unable to tolerate dual-chamber pacing (DDD) at 90 bpm due to palpitations, chest pain, shortness of breath, lightheadedness, dizziness, or presyncope
• Pregnant women
• Any patient without specific documentation of an echocardiogram negative for CHF (systolic or diastolic dysfunction)within five years prior to study enrollment
• Any patient without specific documentation of an objective diagnostic test to rule out CAD within five years prior to study enrollment

Cardiology Clinic