The main finding of the present study is that internal cardioversion by means of ICD shock results in conversion to sinus rhythm in only one third of patients. Factors associated with successful conversion in our study include smaller left atrium, longer right atrial fibrillatory cycle length, shorter total AF duration and use of dual-coil rather than single-coil ICD shocks. Our data support the notion that internal cardioversion of persistent AF in patients with single-coil ICD should be reserved for patients with a favourable arrhythmia profile.
Factors associated with successful cardioversion with an ICD
Only limited data are available on the cardioversion efficacy in persistent AF using the ICD. Two studies showed high cardioversion efficacy of ICD shocks in patients with paroxysmal or acutely induced AF [
6,
7]. Turco et al. [
4] evaluated cardioversion efficacy of ICD shocks in patients with a CRT-D device and permanent AF for more than 1 year. They found that 82 % of patients could be converted to stable sinus rhythm. The difference with our study concerning conversion efficacy is remarkable. Patients were very similar concerning parameters known to be associated with successful cardioversion including AF duration, and left ventricular and atrial sizes. However, in their study, the conversion procedure was only performed after at least 3 months of biventricular pacing which may have optimised atrial electrophysiology before conversion. In addition, all patients were pretreated with amiodarone which enhances electrical cardioversion [
8,
9]. Also, their cardioversion protocol required as many as 3 shocks before cessation of the cardioversion attempt. Finally, although not reported in the manuscript [
4], all patients had dual-coil defibrillator leads (personal communication with the authors). It is well known that the shock vector, determined by the position of the shock coil(s) and the can of the ICD, has major impact on shock efficacy. For defibrillation of ventricular arrhythmias the defibrillation threshold is, for example, higher when the can is placed in the right rather than the left sub-pectoral position [
10]. With respect to atrial arrhythmias it was previously shown that dual-coil shocks are associated with a lower atrial defibrillation threshold [
7]. Electrical cardioversion of AF with temporary internally placed catheters in the right atrium and coronary sinus is associated with a high cardioversion efficacy [
11‐
13]. The internal defibrillation catheters were encompassing both atria, producing the most ideal shock vector for atrial defibrillation.
In our study population the majority of patients had a single-coil shock lead located in the right ventricle. This resulted in a less ideal shock vector for atrial defibrillation, which may explain the low cardioversion efficacy. Although the number of patients with dual coils was small in the present study, cardioversion efficacy was indeed remarkably higher in patients with a dual coil (75 %, 3 out of 4) compared with single coil (26 %, 6 out of 23). Worldwide, dual-coil ICD lead systems are more popular than single-coil ICD systems [
14]. The main reason that the majority in this study population had a single-coil lead is the difficult extraction procedure of dual-coil leads due to the location in the proximal coil [
15].
Many other factors may influence acute external cardioversion outcome, including AF duration, atrial size, patient age, presence of heart failure or structural heart disease, and pretreatment with drugs [
3,
8,
16‐
18]. One interesting finding in the present study was the association between long atrial fibrillatory cycle length, as measured from an atrial electrogram from the atrial pacing lead, and high internal cardioversion efficacy. Shortening of the atrial refractory period and hence also of the atrial fibrillatory cycle length is associated with increasing electrophysiological complexity and reduced response to anti-arrhythmic treatment [
19‐
22]. A short cycle length may therefore represent an advanced atrial substrate. This supports the notion that a longer atrial fibrillatory cycle length as measured from the atrial lead in the device may help identify AF patients who may respond to internal atrial defibrillation.
In this study the AF recurrence rate after 1-year follow-up is comparable with that seen after external cardioversion [
23]. This is somewhat surprising since most patients in the present study had significant structural heart disease with the majority having reduced systolic LV function a marked atrial dilation. Interestingly, patients who underwent successful internal cardioversion had a recurrence rate of only 44 %. Although the number of patients is too small to draw a firm conclusion, it is tempting to speculate that these patients had a less advanced atrial substrate since they responded to single-coil internal cardioversion.
Limitations
Although we collected data prospectively in our pacemaker and ICD database, this study was retrospective and at best reflects real-life internal cardioversion practice. The majority of patients had a single-coil ICD hampering drawing conclusions for dual-coil configurations. Nevertheless, the findings concerning the effects of (single-coil) ICD shocks are noteworthy, especially since data concerning clinical internal ICD cardioversion of persistent AF are largely lacking. A randomised study comparing external cardioversion and ICD cardioversion with inclusion of more patients would be desirable. However, due to the remarkable low success percentage of internal cardioversion, our protocol has been adapted and the first choice is external cardioversion for persistent AF in patients with an ICD.