Implantable cardioverter defibrillators (ICDs) reduce sudden arrhythmic death risk but when these devices are programmed DDD and pace in the right ventricle (RV), they can be associated with increased mortality and heart failure morbidity compared to an ICD programmed to back-up RV. An ideal ICD would provide effective treatment for life-threatening tachyarrhythmias, reduce unnecessary RV pacing and maintain AV synchrony. The Inhibition of Unnecessary RV Pacing with AV Search Hysteresis (AVSH) in ICDs (INTRINSIC RV) study will assess whether an ICD programmed to DDDR with AVSH is equal to an ICD programmed to VVI with regard to mortality, heart failure hospitalizations, and several predefined secondary enpoints. AVSH allows intrinsic AV conduction beyond the programmed AV delay to help minimize ventricular pacing. INTRINSIC RV, a multi-center, randomized, prospective trial will enroll >1,200 participants who receive a Guidant VITALITY AVT ICD. ICDs are programmed initially to DDDR AVSH 60-130. Then, after a week, if the %RV pacing <20%, patients are randomized to VVI-40 or DDDR 60-130 with AVSH. Those with RV pacing > or =20% are placed in an obvservational arm and programmed ad libitum by the treating physician. Patients are followed for one year. This large, randomized, controlled, clinical trial will address whether DDDR with AVSH programming is equivalent to VVI programming in an ICD with regard to mortality and heart failure hospitalization.