Atrial fibrillation (AF) is the most common sustained arrhythmia and is associated with substantial morbidity and mortality. Present treatment strategies are aimed at termination of AF and prevention of AF recurrence using antiarrhythmic drugs or heart rate control drugs. Drugs are not always well tolerated, so atrial pacing as a strategy for prevention of atrial tachyarrhythmias is being explored.

The AT501 pacemaker has both "prevention" and "treatment" algorithms for atrial tachyarrhythmias. The investigators wish to determine whether these special features, over the long term, decrease the amount of time the person experiences AF.

There are some clinical and experimental data to suggest that atrial overdrive pacing should prevent AF. In our pacemaker population with tachy-brady syndrome, and in the PA3 study population, we observed that AF frequently clusters and may recur early following an episode of AF.

We are conducting a randomized trial of both prevention algorithms and antitachycardia pacing (ATP) therapies for the treatment of atrial tachyarrhythmias and thereby prevention of AF over the longterm.

Patients with a history of paroxysmal AF who received an AT501 pacemaker for the treatment of bradycardia will be randomized to having both the prevention and therapy algorithms "ON", both "OFF" or having only the therapy algorithms "ON". They will be followed every 3 months for 1 year, then every 6 months for an 2 additional years. Recurrence and frequency of AF will be determined over time based on data retrieved from the device at each follow-up visit.

Inclusion Criteria:

• Reason for pacing: symptomatic bradycardia
• Paroxysmal AF (>5 min duration, >3 episodes) post implant of AT501 pacemaker
• On stable antiarrhythmic drugs
• Life expectancy >3 years

Exclusion Criteria:

• Life expectancy <3 years
• Unable to give informed consent
• Unable to come for followup
• Chronic AF