Implantable atrial defibrillator: quo vadis?

Atrial Fibrillation

Atrial fibrillation will present the most significant arrhythmia management challenge for clinicians in the new millennium, particularly as the percentage of elderly patients and longevity increase worldwide. The clinical manifestations of the arrhythmia are wide ranging: paroxysmal to permanent modes of occurrence and asymptomatic to severely symptomatic presentations. Perhaps most important, the major risks of atrial fibrillation are stroke and death. Current therapies remain heavily focused on pharmacologic efforts to reduce the severity of primary symptoms and to prevent stroke and other thromboembolic complications by means of anticoagulation. It has not yet been proven that prevention of atrial fibrillation will prolong survival, however. Nonpharmacologic therapies remain under intense basic and clinical investigation as promising means to improve outcome further for patients suffering from atrial fibrillation.

Clinical experience with implantable atrial and combined atrioventricular defibrillators

The high prevalence of atrial fibrillation (AF) and its clinical complications, the poor efficacy of medical therapy for preventing recurrences, and dissatisfaction with alternative modes of therapy stimulated interest in implantable atrial and combined atrioventricular defibrillators. In a multicenter study, the safety and efficacy of a stand alone implantable atrial defibrillator, the Metrix system, were evaluated. The device was implanted in 51 patients with highly symptomatic episodes of AF refractory to pharmacological treatment. During a follow-up of 9 months, 96% of 227 spontaneous AF episodes were successfully converted to sinus rhythm in 41 patients. In 62 episodes (27%), several shocks and/or additional drug treatment were required to maintain stable sinus rhythm because of early recurrences of AF. A total of 3719 shocks were delivered and no induction of ventricular proarrhythmia or inaccurately synchronized shocks occurred. The AF detection algorithm exhibited a 100% specificity for the recognition of sinus rhythm and a 92.3% sensitivity for the detection of AF. The combined atrioventricular defibrillator, Jewel AF 7250, was evaluated in a multicenter, randomized, cross-over trial. The primary study objectives included: overall safety as determined by complications-free survival at 6 months, efficacy of tiered atrial pacing and defibrillation therapies for termination of spontaneous atrial tachycardias (AT) and AF, and relative sensitivity of a new dual-chamber detection algorithm. The device was implanted in 211 patients with either a history of ventricular tachyarrhythmias (VT/VF) alone or with a history of both AT/AF and VT/VF. During a mean follow-up of 4.5 months, it has been shown that the Jewel AF is safe and effective in treating atrial and ventricular tachyarrhythmias. Pace termination of 85% of AT episodes were achieved with painless delivery of antitachycardia pacing; additional 35% of AT episodes were terminated by high frequency burst pacing.

CONCLUSIONS

The stand alone implantable atrial defibrillator may be safe and clinically useful in selected patients for the treatment of highly symptomatic, drug resistant recurrences of AF. The combined atrioventricular defibrillator may be particularly indicated in patients presenting with both a history of atrial and ventricular tachyarrhythmias.

Marked reduction in atrial defibrillation thresholds with repeated internal cardioversion

OBJECTIVES

This study was performed to assess the atrial defibrillation threshold in patients with recurrent atrial fibrillation (AF) using repeated internal cardioversion.

BACKGROUND

Previous studies in patients with chronic AF undergoing internal cardioversion have shown this method to be effective and safe. However, current energy requirements might preclude patients with longer-lasting AF from being eligible for an implantable atrial defibrillator.

METHODS

Internal shocks were delivered via defibrillation electrodes placed in the right atrium (cathode) and the coronary sinus (anode) or the right atrium (cathode) and the left pulmonary artery. After cardioversion, patients were orally treated with sotalol (mean 189 +/- 63 mg/day). Eighty consecutive patients with chronic AF (mean duration 291 +/- 237 days) underwent internal cardioversion, and sinus rhythm was restored in 74 patients. Eighteen patients underwent repeated internal cardioversion using the same electrode position and shock configuration after recurrence of AF (mean duration 34 +/- 25 days).

RESULTS

In these 18 patients, the overall mean defibrillation threshold was 6.67 +/- 3.09 J for the first cardioversion and 3.83 +/- 2.62 J for the second (p = 0.003). Mean lead impedance was 55.6 +/- 5.1 ohms and 57.1 +/- 3.7 ohms, respectively (not significant). For sedation, 6.7 +/- 2.9 mg and 3.9 +/- 2.2 mg midazolam were administered intravenously (p = 0.003), and the pain score (0 = not felt, 10 = intolerable) was 5.1 +/- 1.9 and 2.7 +/- 1.8 (p = 0.001). Uni- and multivariate analyses revealed only the duration of AF before cardioversion to be of relevance, lasting 175 +/- 113 days before the first and 34 +/- 25 days before the second cardioversion in these 18 patients (p = 0.002).

CONCLUSIONS

If the duration of AF is reduced, a significant reduction in defibrillation energy requirements for internal cardioversion ensues. This might extend the group of patients eligible for an implantable atrial defibrillator despite relatively high initial defibrillation thresholds.

Hemodynamic and cardiorespiratory function following internal atrial defibrillation for chronic atrial fibrillation

Internal atrial defibrillation (IAD) is able to restore sinus rhythm in patients with chronic atrial fibrillation (AF) and failed external electrical and/or pharmacologic cardioversion. To assess whether cardiorespiratory and hemodynamic function improve after IAD, 35 patients were prospectively investigated during constant workload exercise by spiroergometry and Doppler echocardiography before IAD, and 1 day and 1 month after IAD. Oxygen uptake kinetics, ventilation, left atrial mechanical function, and pulmonary artery pressure were determined simultaneously at rest and during steady state. During the serial follow-up, 20 patients maintained sinus rhythm. The time interval for achieving the steady state (146 +/- 53 vs 132 +/- 42 seconds; p = 0.5) and the oxygen deficit (645 +/- 190 vs 670 +/- 174 ml; p = 0.7) were not different before and 1 day after IAD, but decreased significantly after 1 month (98 +/- 16 seconds, p = 0.01 and 487 +/- 72 ml, p = 0.02). Exercise pulmonary artery systolic pressures were 38 +/- 13 mm Hg before IAD, increased significantly to 46 +/- 11 mm Hg on day 1 (p = 0.03), and decreased below baseline values at 1 month to 31 +/- 12 mm Hg (p = 0.07). Peak A-wave velocities increased from 0.51 +/- 0.1 m/s after 1 day to 0.67 +/- 0.2 m/s after 1 month (p = 0.03). Restoration of sinus rhythm in patients with AF resistant to external electrical and/or pharmacologic cardioversion improves hemodynamic and cardiorespiratory function at daily activity exercise levels.

The atrial defibrillator: a stand-alone device or part of a combined dual-chamber system?

Atrial fibrillation (AF) is an extremely common arrhythmia seen in clinical practice. Because of the limited efficacy of traditional therapeutic strategies to restore and maintain normal sinus rhythm, several nonpharmacologic options have evolved. The promising results achieved with internal atrial defibrillation have facilitated the development of an implantable atrial defibrilator. Preliminary results obtained from an initial study on a small number of highly selected patients with refractory AF suggest that atrial defibrillation can be performed effectively and safely with adequate patient tolerance by using a stand-alone device. The extension of this therapy will depend on the results of well-designed prospective studies comparing this new therapeutic option with traditional methods. Several acute studies have shown that internal conversion of AF is feasible at low energies with current endocardial transvenous lead configurations primarily designed for ventricular defibrillation, but long-term efficacy has, to date, only been demonstrated with atrial implantable defibrillator lead systems. As AF is a frequent arrhythmia in implantable cardioverter defibrillator (ICD) recipients, it would seem desirable to incorporate the capability for atrial defibrillation into an ICD. Clinical studies have shown that an atrial defibrillator, as part of a combined dual-chamber ICD system, may not require a potentially complicated switching network for establishing different electrode configurations for atrial and ventricular tachyarrhythmia. The efficacy in atrial cardioversion of such a combined, less complex device seems to be as high as reported for a pure atrial defibrillator, but generally at somewhat higher energy requirements. The results of further investigations will show whether a dual-chamber cardioverter defibrillator would be of clinical relevance in patients with ventricular and supraventricular tachyarrhythmia.

A new defibrillator discrimination algorithm utilizing electrogram morphology analysis

Inappropriate therapies delivered by implantable cardioverter defibrillators (ICDs) for supraventricular arrhythmias remain a common problem, particularly in the event of rapidly conducted atrial fibrillation or marked sinus tachycardia. The ability to differentiate between ventricular tachycardia and supraventricular arrhythmias is the major goal of discrimination algorithms. Therefore, we developed a new algorithm, SimDis, utilizing morphological features of the shocking electrograms. This algorithm was developed from electrogram data obtained from 36 patients undergoing ICD implantation. An independent test set was evaluated in 25 patients. Recordings were made in sinus rhythm, sinus tachycardia, and following the induction of ventricular tachycardia and atrial fibrillation. The arrhythmia complex is defined as wide if the duration is at least 30% greater than the template in sinus rhythm. For narrow complexes, four maximum and minimum values were measured to form a 4-element feature vector, which was compared with a representative feature vector during normal sinus rhythm. For each rhythm, any wide complex was classified as ventricular tachycardia. For narrow complexes, the second step of the algorithm compared the electrogram with the template, computing similarity and dissimilarity values. These values were then mapped to determine if they fell within a previously established discrimination boundary. On the independent test set, the SimDis algorithm correctly classified 100% of ventricular tachycardias (27/27), 98% of sinus tachycardias (54/55), and 100% of episodes of atrial fibrillation (37/37). We conclude that the SimDis algorithm yields high sensitivity (100%) and specificity (99%) for arrhythmia discrimination, using the computational capabilities of an ICD system.

Long-term outcome of patients with drug-refractory atrial flutter and fibrillation after single- and dual-site right atrial pacing for arrhythmia prevention

OBJECTIVES

An initial crossover study comparing dual- and single-site right atrial pacing was performed followed by a long-term efficacy and safety evaluation of dual-site right atrial pacing in patients with drug-refractory atrial fibrillation (AF). Also examined was the efficacy of two single-site right atrial pacing modes (high right atrium and coronary sinus ostium) and the long-term need for cardioversion, antithrombotic and antiarrhythmic drug therapies during dual-site atrial pacing.

METHODS

Thirty consecutive patients with drug-refractory symptomatic AF and documented primary or drug-induced bradycardia were implanted with a dual chamber rate-responsive pacemaker and two atrial leads. Single-site atrial pacing was performed at the high right atrium or the coronary sinus ostium. Continuous atrial pacing was maintained.

RESULTS

Mean arrhythmia-free intervals increased from 9+/-10 days in the control period preceding implant to 143+/-110 days (p < 0.0001) in single-site right atrial pacing and 195+/-96 days in dual-site right atrial pacing (p < 0.005 versus single-site pacing and p < 0.0001 versus control). Dual-site right atrial pacing significantly increased the proportion of patients free of AF recurrence (89%) as compared to single-site right atrial pacing (62%, p = 0.02). High right atrial pacing and coronary sinus ostial pacing had similar efficacy for AF prevention. Effective rhythm control was achieved in 86% of patients during dual right atrial pacing. Seventy-eight percent of patients at 1 year and 56% at 3 years remained free of symptomatic AF. The need for cardioversion was reduced after pacemaker implant (p < 0.05) and antithrombotic therapy was reduced (p < 0.06) without any thromboembolic event. Coronary sinus ostial lead dislodgement was not observed after discharge.

CONCLUSIONS

Atrial pacing in combination with antiarrhythmic drugs eliminates or markedly reduces recurrent AF. Prevention of AF is enhanced by dual-site right atrial pacing. High right atrial and coronary sinus ostial pacing do not differ in efficacy. Dual-site right atrial pacing is safe, achieves long-term rhythm control in most patients, decreases the need for cardioversion, and antithrombotic therapy can be selectively reduced.

[Atrial defibrillator]

Atrial fibrillation (AF) is a frequent and costly health care problem representing the most common arrhythmia resulting in hospital admission. Total mortality and cardiovascular mortality are significantly increased in patients with AF compared to controls. In addition to symptoms of palpitations patients with AF have an increased risk of stroke and may also develop decreased exercise tolerance and left ventricular dysfunction. All of these problems may be reversed with restoration and maintenance of sinus rhythm. External electrical cardioversion has been a remarkably effective and safe method for termination of this arrhythmia. Originally described by Lown et al. in 1963, it has been a well accepted mode of acute therapy. However, this technique requires general anesthesia or heavy sedation. Internal atrial defibrillation has been evaluated as an alternative approach to the external technique for over 2 decades. Recent studies have shown that low-energy internal atrial defibrillation using biphasic shocks is an effective and safe means in restoring sinus rhythm in patients with AF and should be considered especially in patients in whom external cardioversion attempts have failed. IMPLANTABLE ATRIAL DEFIBRILLATOR: Recently, a stand alone IAD, the Metrix System (model 3000 and 3020), has entered clinical investigation. Atrial defibrillation is accomplished by a shock delivered between electrodes in the right atrium and the coronary sinus. The right atrium lead has an active fixation in the right atrium. The coronary sinus lead has a natural spiral configuration for retention in the coronary sinus, and can be straightened with a stylet. Both leads are 7 French in diameter and the defibrillation coils are each 6 cm in length. The electrodes may be placed using separate leads, or very soon by using a single bipolar lead. A separate bipolar right ventricular lead is used for R wave synchronization and post shock pacing. The Metrix defibrillator can be used to induce AF by using R wave synchronous shocks and can store intracardiac electrograms (EGMs) for up to 2 minutes from the most recent 6 AF episodes. The device can be programmed into one of the following operating modes: fully automatic, patient activated, monitor mode, bradycardia pacing only, and off. As AF is not life-threatening, in the automatic mode the device is only intermittently active in detecting and treating AF, and this "sleep wake-up" cycle interval is programmable. The device employs extensive processing both for detection and R wave synchronization. In April 1996, the phase I Metrix multicenter clinical trial was started. As of May 1997, a total of 51 Metrix systems had been implanted as part of the phase I multicenter clinical trial. Preliminary data suggest that both defibrillation thresholds and electrograms are stable over time (implant to 3 months). Detection accuracy has been excellent (100% specificity, 92.3% sensitivity) and there have been no errors of R wave selection for synchronization. No proarrhythmias have resulted from over 3700 shocks delivered. The device is effective in electrically converting 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 tamponade, 1 permanent loss of telemetry, 1 patient required His-Bundle ablation because of frequent episodes of drug refractory AF with rapid ventricular response. Initial clinical experience under controlled conditions with the Metrix system suggests that the implantable atrial defibrillator may offer a therapeutic alternative for a subgroup of patients with drug refractory, symptomatic, long lasting, and infrequent episodes of AF. Further efforts must be undertaken to reduce the patient discomfort associated with internal atrial defibrillation in an attempt to make this new therapy acceptable to a larger patient population with AF. (ABSTRACT TRUNCATED)