Inclusion Criteria:

• Indications for the device
• Availability for follow-up at an approved Field Following center, at the protocol defined intervals
• Willing and capable of undergoing a device implant and participating in all testing associated with this clinical investigation
• Prescribed to stable optimal pharmacologic therapy for HF
• Age 18 or above, or of legal age to give informed consent specific to national law
• Able to provide documented evidence of one or more episodes of AF/AT within 12 months of implantation NOTE: Guidant recommends anticoagulation therapy per physician discretion.

Exclusion Criteria:

• Right bundle branch block morphology
• Life expectancy of less than six months due to other medical conditions
• Expectation of a heart transplant during the period of the study
• Patients with or who are likely to receive a mechanical tricuspid valve during the course of the study
• Have a preexisting unipolar pacemaker that will not be explanted/abandoned
• Have an atrial tachyarrhythmia that is permanent (i.e., does not terminate spontaneously and cannot be terminated with medical intervention) within 180 days prior to enrollment
• Have a known hypersensitivity to dexamethasone acetate
• Enrolled in any other study, including drug investigation
• Women that are pregnant or planning to become pregnant
• A Cerebral Vascular Event/ Transient Ischemic Attack within 12 months of implantation
• During the four weeks prior to implantation, a patient experiences an episode of AF >= 48 hours in duration and was not anticoagulated for at an adequate therapeutic level (INR >= 2.0) for the 4 weeks prior to enrollment