IMPLANTABLE ATRIAL DEFIBRILLATOR

Atrial fibrillation (AF) is a frequent and costly health care problem representing the most common arrhythmia resulting in hospital admissio n. Total mortality and cardiovascular mortality are significantly incr eased in patients with AF compared to controls. In addition to symptom s of palpitations, patients with AF have an increased risk of stroke a nd may also develop decreased exercise tolerance and left ventricular dysfunction. All of these problems may be reversed with restoration an d maintenance of sinus rhythm. External electrical cardioversion has b een a remarkably effective and safe method for termination of this arr hythmia. Originally described by Lown et al. in 1963, it has been a we ll accepted mode of acute therapy. However, this technique requires ge neral anesthesia or heavy sedation. Internal atrial defibrillation has been evaluated as an alternative approach to the external technique f or over 2 decades. Recent studies have shown that low-energy internal atrial defibrillation using biphasic shocks is an effective and safe m eans in restoring sinus rhythm in patients with AF and should be consi dered especially in patients in whom external cardioversion attempts h ave failed. Implantable Atrial Defibrillator: Recently, a stand alone IAD, the Metrix(TM) System (model 3000 and 3020),has entered clinical investigation. Atrial defibrillation is accomplished by a shock delive red between electrodes in the right atrium and the coronary sinus. The right atrium lead has an active fixation in the right atrium. The cor onary sinus lead has a natural spiral configuration for retention in t he coronary sinus, and can be straightened with a stylet. Both leads a re 7 French in diameter and the defibrillation coils are each 6 cm in length. The electrodes may be placed using separate leads, or very soo n by using a single bipolar lead. A separate bipolar right ventricular lead is used for R wave synchronization and post shock pacing. The Me trix(TM) defibrillator can be used to induce AF by using R wave synchr onous shocks and can store intracardiac electrograms (EGMs) for up to 2 minutes from the most recent 6 AF episodes. The device can be progra mmed into one of the following operating modes: fully automatic, patie nt activated, monitor mode, bradycardia pacing only, and off. As AF is not life-threatening, in the automatic mode the device is only interm ittently active in detecting and treating AF, and this ''sleep wake-up '' cycle interval is programmable. The device employs extensive proces sing both for detection and R wave synchronization. In April 1996, the phase I Metrix(TM) multicenter clinical trial was started. As of May 1997, a total of 51 Metrix(TM) systems had been implanted as part of t he phase I multicenter clinical trial. Preliminary data suggest that b oth defibrillation thresholds and electrograms are stable over time (i mplant to 3 months). Detection accuracy has been excellent (100% speci ficity, 92.3% sensitivity) and there have been no errors of R wave sel ection for synchronization. No proarrhythmias have resulted from over 3700 shocks delivered. The device is effective in electrically convert ing 96 % of the spontaneous episodes of AF. In 27% of episodes several shocks were required because of early recurrence of AF. In 5 patients . the atrial defibrillator was removed: 2 infections, 1 cardiac tampon ade, 1 permanent loss of telemetry, 1 patient required His-Bundle abla tion because of frequent episodes of drug refractory AF with rapid ven tricular response. Initial clinical experience under controlled condit ions with the Metrix(TM) system suggests that the implantable atrial d efibrillator may offer a therapeutic alternative for a subgroup of pat ients with drug refractory, symptomatic, long lasting, and infrequent episodes of AF. Further efforts must be undertaken to reduce the patie nt discomfort associated with internal atrial defibrillation in an att empt to make this new therapy acceptable to a larger patient populatio n with AE Combined Atrioventricular Defibrillator Recently, a new dual -chamber defibrillator, the 7250 Jewel(R) AF AMD, has entered clinical evaluation. Concern has been raised whether or not a stand alone impl antable atrial defibrillator is safe enough or should provide ventricu lar backup defibrillation in the rare case of shock induced ventricula r proarrhythmia. The availability of a dual-chamber defibrillator has reactivated the discussion about the safety of a stand alone implantab le atrial defibrillator. The most important new features of the 7250 J ewel(R) AF AMD system include: dual-chamber pacing, a new dual-chamber detection criterion for rejection of supraventricular tachycardias, d etection and treatment modalities of atrial arrhythmias. prevention st rategies for atrial arrhythmias. Initial clinical experience with the 7250 Jewel(R) AF AMD device that combines both detection and treatment in the atrium as well as in the ventricle indicates a significant imp rovement in the management of patients with both suproventricular and ventricular tachyarrhythmias.