The success and safety of using low-energy internal cardioversion for AF led to the development of the implantable atrial defibrillator (IAD). The IAD could operate automatically in a manner similar to a ventricular defibrillator, but since AF is rarely an emergency, cardioversion could be delayed for hours, allowing for spontaneous termination of AF, an attempt at pharmacological conversion, and/or patient sedation. The typical candidate for an IAD is a patient with persistent and symptomatic AF occurring relatively infrequently despite treatment with antiarrhythmic medications. Patients with paroxysmal AF, which always spontaneously resolves, are not candidates because electrical cardioversion is not required. Two types of IADs have been developed: one is capable of atrial defibrillation only ("atrioverter"), whereas the other is capable of both atrial and ventricular defibrillation.
The Metrix system (InControl Inc., Redmond WA) was the first IAD tested in clinical trials on patients with persistent AF. It utilized defibrillation leads in the right atrial appendage and CS, and had a ventricular sensing/pacing lead in the right ventricle. In an international multicenter trial, the Metrix system with a maximum output of 6 J, was able to achieve sinus rhythm in 96% of patients (80). However, 27% had an ERAF within minutes of cardioversion, and approximately one-half of the patients required either iv antiarrhythmic drug therapy, repeat internal cardioversion, or external cardioversion. The overall success rate of the atrial defibrillator with and without antiarrhythmic drugs was reported as 86%, but some patients required as many as 8 shocks to treat episodes of recurrence (80). There were no reports of induced ventricular arrhythmias in this trial. The Metrix IAD recognized AF with a very high sensitivity and high specificity, and no inappropriate shocks were delivered. The internal cardioversion was generally well-tolerated by the patients, with only mild sedation required for most cases (80). Although the Metrix IAD has been shown to be effective and safe in appropriately selected patients, it will not be made commercially available in the United States because of the concern that inappropriate timing of an atrial defibrillation shock could induce VF.
Because of this concern, the alternative design of the combined atrial and ventricular defibrillator has gained broader acceptance. These systems usually use a lead system identicial to that of the simpler ventricular defibrillator—typical shocking surfaces including leads in the right ventricle and right atrium and the defibrillator canister in the left pectoral region. An additional CS defibrillation electrode can also be placed if desired to lower the atrial defibrillator threshold. The Jewel AF (model 7250, Medtronic Inc., Minneapolis, MN) is the first of this type of IAD to have undergone clinical testing (Fig. 5). It was originally approved by the FDA for patients with ventricular
tachycardia (VT) or VF who also have—or are at risk for—AF or other atrial tachyarrhythmias. In April of 2001, this device was approved for the treatment of patients with AF in the absence of ventricular tachyarrhythmias, the so-called "AF only" indication. Unlike a sole atrial defibrillator, the combined atrial and ventricular defibrillator can provide multiple levels of tiered and preventative therapy. As with a ventricular defibrillator, it can provide antitachycardia pacing (ATP) and low- and high-energy shocks for the treatment of VT and VF. If an inappropriately timed atrial defibrillation shock induces VF, then the device could rescue the patients from this potentially lethal complication. Three therapies for atrial tachyarrhythmias are available: ATP, 50-Hz-burst pacing, and cardioversion (shock). Although significant efficacy of overdrive pacing was expected in atrial flutter and other organized atrial tachyarrhythmias, 50Hz pacing terminated 18% of episodes classified by the device as AF (81). This finding may indicate that AF frequently begins with organized atrial tachyarrhythmias, which are susceptible to pace termination if delivered very early. The Jewel AF was reported to terminate organized atrial tachyarrhythmias with ATP or high-frequency-burst pace in over 85% of the detected atrial tachyarrhythmias (82). This may dramatically reduce the number of shocks a patient receives over time. First-shock cardioversion efficacy was 91%, although ERAF occurred within 1 min of 20% of episodes (81).
Atrial shock therapies may be programmed in three modes: automatic (as in ventricular ICDs), timed (to delay a shock for AF until a programmed time, typically during sleep), or patient-activated using a separate activator device. The nonautomatic modes have dominated, both because of patient preference and to allow episodes an opportunity to convert simultaneously. Lastly, the device has dual-chamber pacemaker capability, and importantly, can utilize pacing algorithms that may be useful for prevention of AF, again potentially limiting the frequency of painful shocks. Because the atrial/ ventricular defibrillator has ventricular defibrillation capabilities, it may allow higher dosing of antiarrhythmic drugs and use of the atrial defibrillator in high-risk patients, such as patients with heart failure, cardiac disease, and ventricular arrhythmias, all of whom were excluded in trials with the Metrix IAD. Among 146 patients implanted for AF only with the Jewel AF device, the mean atrial defibrillation threshold was 6.8 J. Symptom scores and quality of life improved over the course of the trial (81).
An interesting inference regarding pathophysiology has been gleaned from the follow-up of patients implanted with either the Metrix or Jewel AF device. There is a subset of patients in whom the frequency and duration of AF episodes, or "AF burden," decreases with time. The explanation of this is a reverse remodeling effect of sinus rhythm: just as AF is accompanied by atrial electrophysiologic remodeling which makes the atria more conducive to maintaining AF, restoration and maintenance of sinus rhythm fosters reversal of these changes. It remains to be seen how complete this remodeling can be, and how to select refractory AF patients for whom aggressive efforts to maintain sinus rhythm will ultimately be successful.
Finally, an unexpected benefit has been noted in a significant proportion of patients who received the Jewel AF device in the "AF only" trial. Although patients had no history of a sustained ventricular tachyarrhythmia prior to implant, 11 of the 144 patients in the trial (7.6%) had a total of 67 spontaneous episodes of VT or VF during follow-up (81). Of these, 10 episodes reverted spontaneously, and 57 (85%) were successfully treated by the device. This relatively high incidence of ventricular tachyarrhythmias during follow-up probably stems from the fact that thus far, the device has been implanted primarily in patients with significant structural heart disease.
Although the IAD appears to be efficacious and safe in the selected patients from initial studies (82,83), the limitations of the IADs include pain from atrial cardioversion, inappropriate shocks from lead damage, dislodgment, or oversensing, early recurrence of AF after a shock resulting in multiple shocks, and the high cost. However, the pain associated with shocks is probably the most important factor limiting the widespread use of IADs. Since patients usually do not tolerate frequent shocks, the device is limited to patients with relatively rare episodes of persistent AF.
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