In the DDD pacing mode, the ventricular sensing channel waits for a conducted beat until the programmed AV interval (AV delay) times out. This typically results in a high percentage of ventricular pacing. Programming longer AV delays can increase the frequency of intrinsic conduction. However, long AV delays may introduce complications such as non-physiologic PR intervals, pacemaker-mediated tachycardia, and artificially limited maximum tracking rates. Attempts to manage AV delays to promote conduction are even more complex for ICDs. ADI Mode: Product Development In 2003, ELA Medical, now SORIN GROUP CRM, introduced in Europe a dual chamber pacemaker where the ventricular lead tests for AV block instead of AV conduction. When the patient's intrinsic AV conduction is normal, the pacemaker paces in the AAI mode. If the test criteria for AV block are met, the pacemaker switches to DDD pacing to preserve cardiac output. Switching to DDD mode at the onset of AV block allows physicians to select appropriate AV delay settings optimized for maximum cardiac output. SORIN named its version of ADI mode pacing AAIsafeR. In 2004, Medtronic introduced in Europe its version of an ADI algorithm for both implantable defibrillators and pacemakers. Medtronic's ADI mode is called Managed Ventricular Pacing (MVP). In 2005, SORIN GROUP CRM released a second generation ADI algorithm, AAIsafeR2, incorporating new therapeutic and diagnostic features to better manage pacemaker patients. This feature is offered in the Symphony DR pacemaker and was unlocked and made available retroactively upon FDA approval in May 2005. A third generation of SORIN's ADI algorithm is included in the Ovatio DR implantable defibrillator, which is currently awaiting FDA approval in the U.S. ADI Mode: Operation of SORIN and Medtronic Algorithms SORIN GROUP CRM's algorithm uses five different criteria to test for either blocked P waves (consecutive P waves without an intervening ventricular contraction) or long PR intervals: 1. 3rd degree AV Block: Two consecutive blocked P waves. 2. 2nd degree AV Block: Three blocked P waves within 12 atrial cycles. 3. 1st degree AV Block: Seven consecutive atrial cycles, where the PR interval exceeds a programmed value. 4. Ventricular Pause: In order to prevent inappropriate cycling to DDD mode during atrial arrhythmias, the pacemaker switches to DDD mode only if a ventricular pause occurs, (programmable up to 4 seconds). 5. Ventricular safety pacing: SORIN's new Ovatio DR defibrillator will employ a fifth test based on ventricular safety pacing criteria. If ventricular sensing occurs within the ventricular safety pacing window after two consecutive atrial events, the device switches to DDD. (Pending FDA approval.) By way of comparison, Medtronic's MVP mode uses a single test criterion. When a P wave is blocked, the pacemaker synchronizes a ventricular pace on the next P wave. It then looks for one conducted beat after the next two P waves. This test is sensitive to 3:2 or higher AV block. SORIN GROUP CRM and Medtronic also differ in the way they check for the resumption of intact conduction. SORIN paces for 100 cycles in the DDD mode, then switches to AAI mode and uses the 5 AV block test criteria to look for intact conduction. Medtronic paces for 60 seconds, then switches to AAI for one atrial cycle to look for intact conduction.7 If conduction is not found, SORIN and Medtronic devices both revert to DDD mode. SORIN retests for conduction after 100 cycles. Medtronic retests after two minutes. Patients could become symptomatic because of frequent dropped beats caused by retesting for conduction. To limit this, SORIN deactivates the 100-cycle test until 8 am the next day, after 45 switches to DDD mode in one day or 15 switches to DDD in each of three consecutive days. SORIN also deactivates the 100-cycle test after the third switch to DDD during any exercise period. Testing for AV conduction resumes at the end of exercise. Medtronic doubles the DDD mode pacing interval after each failed test, up to a maximum of 16 hours, then tests every 16 hours thereafter. Sensitivity to the onset of AV block and also to the resumption of AV conduction is critical to the clinical performance of ADI mode operation. Lack of sensitivity to the latter elevates the incidence of unnecessary RV pacing. This is perhaps less of an issue with ICD patients, because of their low incidence of AV block. It is a very important consideration in pacemaker patients, though. Clinical Results Published clinical results for pacemaker patients are encouraging but limited, considering the potential value of ADI mode to the cardiac rhythm management patient population. At the Heart Rhythm Society (HRS) meeting 2005, data was presented on both SORIN's AAIsafeR and Medtronic's MVP modes. MVP mode reduced average RV pacing to 6.7% in the SND population and 28.6% in the AV block population.6 SORIN's results were for a mixture of patients with either the first or second generation AAIsafeR mode.7 The SORIN study group was enlarged for an updated presentation at Europace in June 2005.8 In the updated study, 77% of the population had either no switches or transitional switches to DDD mode. The average percent RV pacing for these two groups was 0.2%, with a maximum of 3%. The 23% of the population with long-lasting switches averaged 43% RV pacing. The SORIN study has since been repeated to remove patients with the first generation ADI mode; the findings are to be presented at the annual meeting of the Heart Rhythm Society in May 2006. The results are expected to corroborate the conclusion that chronic AV block requiring a high percentage of RV pacing is a rare occurrence, even in a non-selected pacemaker population. The advent of ADI mode pacing opens an entire new area of dual chamber clinical exploration. Have the benefits of restoring AV synchrony been masked by the disadvantages of ventricular dysynchrony associated with right ventricular apical pacing? The DAVID Trial9 was designed to test for the benefits of AV synchronous pacing in the ICD population. Future clinical trials are designed to test the benefit of ADI pacing compared to back-up VVI pacing. These trials should confirm the benefit of the ADI pacing mode compared to no pacing, and confirm the absence of a deleterious effect of this pacing mode on cardiac morbidity and heart failure hospitalizations. Diagnostic Information The new pacing modes offer additional diagnostic information to assist in managing the patient. Medtronic's MVP mode diagnostics consist of a pacing states histogram that presents the percentage of ventricular pacing between clinic visits. It is necessary to consider the amount of time the patient spent in mode switch during atrial arrhythmias to fully interpret this histogram. The Cardiac Compass also reports daily percent RV pacing. The SORIN GROUP CRM cardiac rhythm management devices include a full suite of diagnostic tools for AV conduction, including up to seven stored intracardiac EGMs with marker channel. Complete statistics on conduction time, types of block, and long-term daily trending are automatically stored. Pacing mode data is separated into DDD, AAI and mode switch percentages.