Safety and Efficacy Study of RENEWAL 4 AVT

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