About the INTRINSIC RV Study: The Inhibition of Unnecessary RV Pacing with AV Search Hysteresis in ICDs (INTRINSIC RV) tested the hypothesis that outcomes of ICD patients programmed DDDR with AV search hysteresis (AVSH) 60-130 are not inferior to those ICD patients programmed VVI-40 with respect to all-cause mortality and heart failure hospitalizations. Patients with standard indications for an ICD underwent a VITALITY AVT® ICD (Guidant Corporation, St. Paul, Minnesota) implant. Then, patients were programmed to DDDR AVSH 60-130 bpm with consistent pacing parameters in all patients. At 1 week, the percent of RV pacing was assessed using ICD counters. Patients who were RV paced < 20% were randomized to one of two standardized programming arms: DDDR AVSH 60-130 or VVI-40. Programming to detect ventricular arrhythmias and their treatments were left to the physician. Patients with ? 20% RV pacing were placed in an observational arm in which ICD programming was left to the treating physician. Patients were followed for 12 months. Enrollment began in July 2003 and follow-up was completed in October 2005. Thirty-two patients (6.4%) in the DDDR AVSH arm and 46 (9.5%) in the VVI arm died or were hospitalized for heart failure (p < 0.001 for non-inferiority) with a trend toward superiority in the DDDR AVSH arm (p = 0.072). All-cause mortality alone favored DDDR AVSH, with 18 deaths (3.6%) versus 25 (5.1%) in the VVI group, although that result was not statistically significant (p = 0.23). Fifty-nine patients (12.5%) in the observational group died or were hospitalized for heart failure. In summary, the notion that dual-chamber programming poses an inherent safety risk is incorrect as those patients randomized to dual-chamber rate-responsive programming with AV search hysteresis experienced lower rates of mortality and heart failure hospitalizations. As hypothesized, DDDR AVSH programming was not inferior to VVI programming. How many patients were studied in the trial? A total of 1,530 patients were enrolled in INTRINSIC RV, with 988 randomized into either the DDDR AVSH or VVI-40 arm, and 473 patients in the observational arm. Is either type of pacing preferred in your institution (VVI vs. DDDR)? We assess what is most appropriate based on patient needs. We do not focus on eliminating ventricular pacing, but focus on appropriate amounts of right ventricular pacing (less than 20% in most cases). Any trend toward VVI programming based on the DAVID study should now be questioned. In my institution, DDDR with AVSH is used routinely as it provides the most flexibility for patient needs and is as good if not better than VVI programming in terms of death and heart failure hospitalizations based on the INTRINSIC RV trial. AVSH allows for appropriate RV pacing in most patients (about 80% of patients). It guarantees back-up pacing if the AV interval gets too long without allowing AV block. Additionally, this type of programming allows enhancement of tachycardia discrimination, atrial-based pacing, and better understanding the causes for ICD shocks if they occur. What are the benefits of DDDR pacing? DDDR pacing allows for atrial-based pacing. Atrial support pacing may improve outcomes in patients with sinus node dysfunction, can decrease the incidence of atrial arrhythmias, will provide rate support in patients with chronotropic incompetence and allow optimization of necessary medical therapies for heart failure, especially beta-blocker therapy in which target doses may not be reached due to the risk of chronotropic incompetence or bradycardia. In the long term, it appears to be cost effective and better for patient management to implant a dual-chamber ICD rather than a single-chamber ICD.1 Describe any disadvantages or possible complications to VVI and/or DDDR pacing. When using a single-chamber device, programming flexibility and rhythm detection are sacrificed, which could potentially lead to inappropriate therapy including shocks, difficulty in the prediction of the cause for ICD activation and loss of atrial-based pacing for chronotropic incompetence. A patient may need additional surgery, with its inherent risks, to upgrade from a single-chamber to a dual-chamber device if determined that the patient needs therapy not offered by single-chamber devices (i.e., atrial pacing). What did the study conclude? We concluded that the notion that dual-chamber programming poses an inherent safety risk is not correct. Additionally, with AV search hysteresis properly programmed, outcomes with dual-chamber programming were as good, if not better, than with single-chamber programming. What does this mean for single- and/or dual-chamber pacing? The INTRINSIC RV Study supports the notion that dual-chamber programming and dual-chamber ICDs are not inferior to single-chamber programming and single-chamber ICDs. Will further studies be conducted? We continue to analyze the data, and many interesting findings are arising. We see that in all patients programmed to DDDR, the group who had the least amount of right ventricular pacing did not do the best with respect to the primary endpoint, whereas event rates appeared lowest in patients with RV pacing between 10 - 19%. Similar to the DAVID trial, increasing percentages of right ventricular pacing over 20% were associated with worsening outcomes. It is also possible that there is a balance between amounts of right ventricular pacing and the AV interval. More research is needed in this area. Since about 20 percent of patients did not meet criteria for INTRINSIC RV, we would like to study the effect of DDDR AVSH in this population as well.