Atrial fibrillation (AF) is a common rhythm disorder with a prevalence of 1–2% in the total population, with an increasing prevalence with age.1) Although rhythm control of AF may reduce heart failure mortality,2) cardiovascular mortality and hospitalization,3) and incidence of stroke,4) and improve cognitive5) and renal functions,6) most studies have been evaluated after aggressive rhythm control of AF by catheter ablation. As a traditional intervention for rhythm control, elective external cardioversion (ECV) is commonly used to treat persistent AF (PeAF). Although ECV has a high acute success rate for restoring sinus rhythm, its rate of sinus rhythm maintenance is low when using antiarrhythmic drugs (AADs).7) The outcome of ECV can vary greatly with the duration of AF,8) the type of AAD used,7) the ECV shock energy form and energy level,9) the location of the paddles,9) and the rhythm follow-up protocol after ECV.10) In previous studies on ECV, acute and short-term success rates were commonly evaluated within 1 month by combining the 2.8) Only a few previous studies have evaluated the effects of ECV in highly selective patient groups, such as long-standing PeAF or post-AF catheter ablation (AFCA) PeAF.11) Moreover, only a few studies have been conducted on elective ECV performed in an outpatient clinic rather than ECV performed in an emergency room or intensive care unit.12)13)

The purpose of this study was to evaluate the efficacy and safety of outpatient clinic-based ECV in a protocol-based single center prospective registry. The aims of this study were to evaluate the acute success rate after outpatient clinic-based ECV, short-term rhythm outcomes within 3 months, and long-term outcomes after 3 months and to identify factors associated with these rhythm outcomes. We also compared ECV outcomes between patients with long-standing PeAF and those in whom AFCA.

In this single center prospective registry study, we investigated factors associated with acute success, early AF recurrence, and long-term recurrence after outpatient clinic-based ECV. This study has several characteristics. First, we included a relatively homogeneous group of patients who underwent ECV on an elective schedule in an outpatient clinic and excluded ECV cases in the emergency room or intensive care unit. Second, we performed protocol-based ECV with serial increases in ECV energy. Third, rhythm follow-up after ECV was monitored by Holter monitoring in accordance with the fixed period, and recurrence of asymptomatic paroxysmal AF could be found. After protocol-based ECV under AAD, the acute success rate was 88.6%, and the complication rate was 0.47%. Among the patients with a successful ECV, AF recurred in 55.5% within 3 months, and another 52.4% of the patients who maintained sinus rhythm for 3 months, experienced a recurrence of AF after a median follow-up period of 32 (9–66) months. Amiodarone was protective against short-term AF recurrence, and baseline heart failure was protective against long-term AF recurrence with significant improvement of LV function after ECV. We also found that successful ECV energy and early recurrence rate within 3 months were significantly lower in post-ablation patients than in those with long-standing PeAF.

ECV has been used as an effective rhythm control method for PeAF.⁹⁾¹¹⁾ However, it is known that the recurrence rate of AF is relatively high after the procedure.18) AF duration,11) LA size,18) comorbidity,18) method of cardioversion,9) and drug selection7) have been shown to affect the outcomes of ECV. An acute outcome reflects an assessment of the electrical intervention of ECV itself; however, the early recurrence of AF after restoration of sinus rhythm determines AAD responsiveness. In this study, the mean successful ECV energy was 144 J, utilizing biphasic shock and defibrillation patches. Therefore, it would be reasonable to start the ECV at 150 J in similar patients with AF. Although the ECV acute success rate was similar to previous studies,¹⁰⁾¹²⁾ we could not find any independent risk factor for ECV failure.

A major concern of outpatient clinic-based ECV might be complication risk. However, ECV has been deemed as a low risk procedure even in the presence of significant heart disease in previous studies12)13) and in this study. In the present study, there was a complication rate of 0.47%: sinus node dysfunction requiring hospitalization was 0.29% and minor stroke/TIA comprised 0.18%. There were no complications associated with anesthesia. All patients underwent anticoagulation for more than 3 weeks before ECV. One of the patients who experienced peri-procedural stroke/TIA had a CHA₂DS₂-VASc score of 6, and 2 of them had score of 3 and a prior history of ischemic stroke. All 3 patients recovered neurologically.

Amiodarone is a mixed ion channel blocker with additional anti-adrenergic effects and significantly reduce the rate of recurrence of paroxysmal and PeAF.19)20) Because of a high rate of long-term adverse effects, amiodarone is generally reserved for patients with congestive heart failure or as a second-line AAD.14) However, rhythm control effects after restoring sinus rhythm were significantly superior with amiodarone than with sotalol or propafenone in patients with PeAF.19)20) In the SAFE-T trial,20) 1-year sinus rhythm maintenance rate was significantly higher in the amiodarone group than in the sotalol group (52% vs. 32%), although spontaneous sinus conversion rate and ECV success rate did not differ. In this study, amiodarone was superior to other AADs in regards to short-term rhythm outcomes within 3 months after ECV; however, there was no difference in ECV success rate or long-term rhythm outcomes.

The energy requirement for successful ECV was significantly lower in post-AFCA patients than in the long-standing PeAF patients. AFCA for AF has beneficial effects via multiple mechanisms,21) including isolation or abolition of trigger foci, in both PV and extra-PV sites,22) as well as modulation of autonomic innervation.23) Hwang et al.21) reported that wide circumferential PV isolation and linear ablations also reduce cardioversion threshold by reducing atrial critical mass. Although the short-term rhythm outcomes of ECV were better in patients with post-AFCA recurrence than in those with long-standing PeAF, acute success and long-term recurrence rates did not differ between the 2 groups, consistent with a previous report.

The presence of symptomatic systolic and diastolic LV dysfunction has been shown to have a negative impact upon ECV success and 30-day outcomes of cardioversion.7) LV dysfunction can lead to increases in atrial pressure and LV hypertrophy, facilitating AF maintenance or AF recurrence after ECV. However, better long-term success of ECV has been shown in patients with associated heart failure in this study with significant improvement of LV function in follow-up echocardiogram. For this reason, the majority of heart failure patients included in this study were estimated to have tachycardiomyopathy24) or AF induced cardiomyopathy.25) AF itself is a major risk factor of new-onset heart failure,26) and irregular ventricular activation alters myocardial gene expression, modulating calcium handling, cell function, and sympathetic activation.27) Therefore, long-term maintenance of sinus rhythm after successful ECV improved heart failure and reduced LA size, vice versa. It was reported that the absence of coronary artery disease, significant structural heart disease, or late gadolinium enhancement in cardiac magnetic resonance imaging predict improvement in heart failure after rhythm control of AF in patients with associated LV dysfunction.28)29)

This study was an observational cohort study from a single center that included highly selected patients referred for AF rhythm control. This study might have a selection bias for the baseline characteristics of the patients, such that our outcomes of outpatient clinic-based ECV cannot be generalized to all AF patients. The follow-up duration had a non-standard distribution in this study (32 [9–66] months). It was due to the very high chance of an early AF recurrence after the ECV, but a minority of the patients maintained sinus rhythm over a long follow-up period. We determined AF duration based on the first ECG documentation point because asymptomatic AF is common. The AAD dose was dependent on the physician's judgment rather than the strict protocol. Because we included the post-AFCA patient group, the study population was heterogeneous and could not represent the pure anti-arrhythmic drug effects. The ECG parameters are utilized in the prediction of the rhythm outcome after the ECV, but anti-arrhythmic drugs also remarkably affect the ECG parameters depending on their kind and dosage. Although we used the anterior-posterior placement for the electrode pads, the results of a pooled study analysis failed to identify any difference between the anterior-posterior and anterio-lateral pad placement in restoring sinus rhythm by the ECV. We performed protocol-based regular rhythm follow-up by Holter, the patients still have a high chance of subclinical paroxysmal AF between Holter evaluations.

In conclusions, the success rate of outpatient clinic-based ECV was 88.6% in this study, with a low complication rate, although 55.5% experienced recurrence of AF within 3 months. Amiodarone was protective against short-term AF recurrences, and long-term AF recurrences were less in patients with baseline heart failure and potential recovery of their ventricular function.