The Atrioverter
abstract & commentary
Synopsis: The implantable atrial defibrillator can recognize atrial fibrillation with high specificity and can safely and effectively deliver low-energy defibrillation shocks.
Source: Wellens HJJ, et al. For the METRIX Investigators. Circulation 1998;98:1651-1656.
Wellens and colleagues from 19 international centers report the first long-term clinical data with the use of the internal atrial cardioverter or atrioverter. The atrioverter (In-Control, Redmond, WA) system used in this study consists of an implantable atrial defibrillator, shocking coils that are positioned in the right atrium and distal coronary sinus, and a bipolar ventricular pacing lead. The atrial defibrillator is a battery-powered, pectorally implanted device about the size of a single-chamber ventricular defibrillator. The atrioverter sensing circuit detects atrial fibrillation using proprietary diagnostic algorithms and delivers R-wave synchronous defibrillator shocks at preset energy levels. Post-shock ventricular pacing is available but the device does not have the capability for atrial pacing. The maximum shock energy for the two models used was three joules and six joules, respectively.
One hundred nineteen patients with recurrent atrial fibrillation were screened for possible atrioverter implantation, and 51 patients eventually underwent the procedure. All had a history of recurrent atrial fibrillation. Patients who had a prosthetic heart valve, Wolff-Parkinson-White syndrome, a recent MI, a left ventricular ejection fraction less than 40%, a history of left atrial thromboembolic events, congestive heart failure, or high atrial defibrillation thresholds were excluded from participation. The 51 patients who eventually received an implant included 40 men and 11 women with a mean age of 58 years. They had previously received an average of 3.9 antiarrhythmic drugs. The mean (± SD) ejection fraction was 58 ± 11%. The mean left atrial size was 4.4 ± 0.8 cm. Seventy-one percent of the patients were New York Heart Association functional class I.
At implantation, atrial fibrillation was induced on multiple occasions and the atrial defibrillation threshold was determined. All patients had to have an atrial defibrillation threshold of 240V or less. Although this feature was not used during the outpatient clinical part of the study, automatic mode operation was also tested during the implant procedure. Patients were seen at one, three, and six months after implantation, and subsequently at six-month intervals. Atrial defibrillation was tested periodically. The average duration of follow-up in this series was 8.6 ± 4.5 months.
During follow-up, 41 patients had 227 episodes of spontaneous atrial fibrillation. The device terminated 96% of these episodes. One episode converted spontaneously before complete device therapy delivery and 10 episodes in nine patients did not respond to the delivered shocks. Early recurrence of atrial fibrillation was seen after initially successful treatment of 62 episodes. If these episodes of early recurrence are considered clinical failures, the overall efficacy of the device for all conversion attempts drops to 86%. Atrial defibrillation thresholds showed no major changes during follow-up. Forty-eight of 51 patients were treated chronically with anti-arrhythmic medications to control shock frequency.
Significant complications during follow-up included two cases of subclavian vein thrombosis, one late pericardial effusion secondary to perforation by the right atrial appendage, one device infection, one atrial lead dislocation, and three acute increases in atrial defibrillation threshold. One other patient had a right ventricular lead dislodgement that required reoperation. One patient had continued frequent episodes of atrial fibrillation and underwent his bundle ablation with permanent ventricular pacing.
Wellens et al conclude that the implantable atrial defibrillator can recognize atrial fibrillation with high specificity and can safely and effectively deliver low-energy defibrillation shocks. However, many patients have atrial defibrillation thresholds that are higher than those required for use of the current device, and early recurrence of atrial fibrillation remains a significant problem. Technical complications related to the device implantation itself will require future improvements in lead design and implantation techniques.
Comment by John P. DiMarco, MD, PhD
This article represents the first long-term clinical experience with an implantable atrial defibrillator. The data presented in this report are a pilot experience that establishes the validity of the concept and seem promising enough to warrant further clinical investigations.
In the interest of safety, all of the episodes of atrial fibrillation included required the patient to seek medical attention so that the shocks could be delivered under observation. Subsequently, patients who have had a successful observed cardioversion have been allowed to activate the device on their own. Although the device is capable of fully automatic operation, most patients prefer to activate it themselves at an appropriate and convenient time during a symptomatic episode. This reflects a significant difference in design philosophy for devices that deal with arrhythmias that are not usually life-threatening compared to the philosophy for devices used to treat ventricular arrhythmias. Although quality-of-life data are not presented in this report, a major advantage reported by patients who have received an atrioverter is the feeling that they now have greater control over their condition.
The substantial complication rate in this trial reflects the fact that this device incorporates new lead designs and requires new techniques for implantation. As clinical experience accumulates and as lead designs are refined, we should expect these complications to substantially decrease.
A large number of issues, however, still must be addressed in future clinical trials. This was a highly select group of patients who were motivated because of recurrent, disabling symptoms to accept repeated self- administered shocks. Wellens et al stress that the use of sedation was variable from center to center but shocks at the level required to terminate atrial fibrillation do cause pain and may not be acceptable for all patients. Another important question is whether the atrial defibrillator will affect remodeling due to atrial fibrillation. As has been reported from several laboratories, atrial fibrillation changes the electrophysiologic properties of the atria. One of the hopes for the atrial defibrillator is that by rapidly terminating each episode of atrial fibrillation, this remodeling will be prevented and deterioration of atrial electrophysiologic parameters will be prevented. Other questions about the clinical use of an atrioverter will be addressed by improvements in the device. A number of strategies involving various types of atrial pacing for the prevention of atrial fibrillation or for termination of atrial flutter have been reported. The device used in this series does not allow atrial pacing at all, and ventricular pacing is available only as a backup after shock delivery. Future devices will no doubt include dual-chamber pacing with sophisticated antitachycardia-pacing algorithms. Although not reported in this series, production of ventricular fibrillation with the atrial shock is a theoretical probability. The atrioverter minimizes this risk by requiring that the atrial shock be synchronized to a QRS complex that occurs more than 500 msec after the preceding QRS complex to avoid the T wave of the preceding beat. This has been shown during testing to eliminate the risk of ventricular fibrillation. However, this concept needs to be tested further in patients with more advanced forms of heart disease who are likely to be more susceptible to induction of ventricular arrhythmias. Incorporation of this type of technology in a defibrillator with both atrial and ventricular defibrillation capability would eliminate at least some of this risk. Such devices are now in early clinical trials.
This paper establishes the concept that an implantable device can be used to convert atrial fibrillation to sinus rhythm. Further clinical studies will indicate the degree of patient acceptance, the effects of early defibrillation on atrial fibrillation occurrence, and the long-term safety of the device in other patient groups.
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