Cardiac Electrophysiology Review 2003;7:292–296 C 2003 Kluwer Academic Publishers. Manufactured in The Netherlands.
Effect of Electrode Position on the Outcome of Cardioversion Paulus Kirchhof,1 Martin Borggrefe 2 and Gunter Breithardt1 ¨ 1 Department of Cardiology and Angiology, Hospital of the University of Munster, Germany; 2 Department of Cardiology ¨ and Angiology, Hospital of the University of Mannheim, Medical Faculty of Heidelberg, Germany
Abstract. Acute termination of atrial fibrillation is the starting point of any therapy aimed at preventing atrial fibrillation and improves cardiac output in patients with hemodynamic compromise due to the arrhythmia. External electrical cardioversion is a simple procedure to terminate persistent atrial fibrillation in the majority of cases. Initially, Lown et al. used an anterior-lateral position of electrodes to apply a cardioversion shock. Recent pathophysiological studies have shown that atrial fibrillation is maintained by functional re-entry circuits anchored in the left atrium. As the left atrium is located posteriorly in the thorax, an anterior-posterior electrode position may be more efficient for external cardioversion of atrial fibrillation than the commonly used anterior-lateral electrode position. Several recent studies have confirmed that an anterior-posterior electrode position is superior to an anterior-lateral electrode position for external cardioversion of atrial fibrillation. There are no indications that an anterior-posterior electrode position is less safe than an anterior-lateral electrode position. We therefore suggest that an anterior-posterior electrode position should initially be used for external cardioversion of atrial fibrillation. Key Words. electric countershock, clinical trial, randomized clinical trial
Impact of atrial fibrillation on mortality and morbidity and limitations of current therapies aimed at maintaining sinus rhythm Atrial fibrillation, the most common sustained cardiac arrhythmia, is increasing in prevalence in an aging population [1,2]. The hemodynamic consequences of atrial fibrillation and its thromb-embolic complications cause relevant morbidity and mortality [3,4], especially in patients with heart failure [5]. Therefore, prevention of atrial fibrillation appears a reasonable goal of medical treatment. So far, however, maintaining sinus rhythm has been difficult, mainly due to re-occurring atrial fibrillation after successful cardioversion, but also due to failure to acutely terminate atrial fibrillation [6–9]. In consequence, a number of recent trials have not demonstrated a benefit for a treatment strategy aimed at preventing recurrent atrial fibrillation [6–9]. These clinical findings
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are in contrast to our knowledge that the persistence of atrial fibrillation predisposes the heart to atrial fibrillation through electrical and structural adaptation processes (“atrial fibrillation begets atrial fibrillation”, [10–12]), and to the adverse effects of atrial fibrillation on cardiac function and thrombus formation. Considering that atrial fibrillation has multiple etiologies that may respond differently to different treatments, a better understanding of the pathophysiologic factors causing recurrent atrial fibrillation may allow to improve rhythm control therapies in patients with atrial fibrillation. Whatever types of treatment for the prevention of recurrent atrial fibrillation may evolve in the future, every treatment aimed at preventing recurrent atrial fibrillation requires initial restoration of sinus rhythm. In addition to initiating a therapy aimed at maintaining sinus rhythm, restoration of sinus rhythm can directly improve cardiac function [13] and may therefore be helpful in the acute treatment of patients with acute cardiac failure due to the arrhythmia [14].
Clinical use and limitations of external electrical cardioversion External electrical cardioversion is a readily available, often effective treatment for the acute termination of atrial fibrillation [15–17]. Its basic technique, application of a strong electrical shock through the thorax via two electrically conducting electrodes that oppose each other [1,18,19], has not changed since its first description by Lown et al. [15,20]. Sinus rhythm is often, but not always restored by external cardioversion [15–17]. Therefore, several groups have tried to improve the technique of electrical cardioversion, e.g., by using internal, catheter-based cardioversion electrodes [21,22], by using biphasic shock wave forms [16], or by using multiple external cardioversion electrodes [23].
Address correspondence to: Paulus Kirchhof, M.D., Medizinische Klinik und Poliklinik, Innere Medizin C-Kardiologie und ¨ ¨ Angiologie, Universitatskliniken Munster, Albert-Schweitzer¨ Straße 33, D-48129 Munster, Germany. E-mail:
[email protected]
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Atrial fibrillation is maintained by different mechanisms anchored to the left atrium Atrial fibrillation is a heterogeneous disease that can be initiated by focal ectopy [24] or alterations in atrial repolarization [25–27]. In addition, atrial hypertrophy, atrial dilatation and atrial ischemia will contribute to the formation of a substrate for fibrillatory activity in the atria. These electrophysiological and structural changes interact and form a substrate for functional re-entry, most likely either spiral wave reentry or left atrial micro re-entry [28,29]. Once initiated, atrial fibrillation will perpetuate itself unless terminated in time [10–12]. Of importance for electrical cardioversion is the fact that both the initiating electrophysiological events [24] and the re-entrant circuits maintaining atrial fibrillation [30,31] often occur in the left atrium, and more specifically at the posterior wall of the left atrium close to insertion of the pulmonary veins. Creation of a strong electrical field in the left atrium Despite persistent gaps in our understanding of the exact electrophysiological mechanisms by which a strong electrical shock terminates fibrillatory activity in the myocardium [32–37], it is known that a shock field gradient of 5 V/cm is needed to successfully defibrillate the myocardium [38,39]. The left atrium is positioned posteriorly in the thorax (Figure 1). Therefore, an anterior-lateral shock field will not always include the entire left atrium in its shock field, and in most cases not center on its posterior wall. An anterior-posterior electrode position, in contrast, will be more likely to include the entire left atrium and the ostia of the pulmonary veins in the region with the highest shock field. These considerations led us and others to the hypothesis that an anterior-posterior electrode position may be more effective for external cardioversion than an anteriorlateral electrode position. Initial evaluation of anterior-posterior electrodes for cardioversion Already in the late 1970s, some researchers suspected that an anterior-posterior electrode position may be more effective for external cardioversion of atrial fibrillation than the anterior-lateral electrode position that had initially been used by Lown et al. [15] and that to date is most often used [1,40]. At that time, this hypothesis was supported by measurements of transthoracic impedance which may be lower in the anterior-posterior electrode position [41]. A large, sequential trial of the two electrode configurations, published in 1981, however, did not detect a difference between the two techniques [42]. Retrospectively, this study probably underestimated the beneficial effect of an anterior-posterior electrode position due to a large number of patients with atrial
Fig. 1. Posterior position of the left atrium in the thorax. (A and B): Radiograph taken during a catheter ablation procedure. Two multipolar electrophysiology catheters, positioned in the coronary sinus (inserted from the jugular vein) and along the lateral right atrial wall (inserted through the femoral vein), delineate the mitral anulus (catheter in the coronary sinus) and the anterior wall of the right atrium. The mitral annulus indicates the basal left atrium that is located posteriorly in the thorax. (A) Right-anterior oblique view (30 degree angulation). (B) Left anterior oblique view (−60 degree angulation). (C) Sagittal magnetic resonance thoracic cross section through the heart. LA = left atrium; RA = right atrium; LV = left ventricle; RV = right ventricle. Note that the left atrium is positioned posteriorly in the thorax. MR image provided by K.-U. Jurgens, ¨ MD, Department of Clinical Radiology, Hospital of the University of Munster. ¨
flutter that were included in the study. As atrial flutter has a macro re-entrant circuit confined to the right atrium, right atrial cardioversion will be sufficient to restore sinus rhythm in patients with atrial flutter. To the credit of the authors, the electrophysiological mechanism underlying atrial flutter was not known at that time. At the time of publication, this trial brought a premature end to the search for better electrode positions for external cardioversion [18,43].
Current evidence supporting the use of anterior-posterior cardioversion electrodes Based on the clinical observation that some patients can only be cardioverted in an anterior-posterior
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cardioversion electrodes [17] (Figure 2), consistent with the pathophysiological and anatomical considerations mentioned above. Furthermore, in 66% of the patients that could not be cardioverted using an anterior-lateral electrode position, a single 360 J shock through anterior-posteriorly positioned electrodes successfully restored sinus rhythm in that study (Figure 2). Within the methodological limitations of such an analysis, a meta-analysis of all randomized trials confirmed that anterior-posterior electrodes are more effective for external cardioversion of atrial fibrillation [17]. Complications related to an anterior-posterior electrode position have so far not been reported. Conversely, an anterior-posterior electrode position appears safer in patients with an implanted pacemaker or defibrillator [1]. Therefore, it appears reasonable to choose an anterior-posterior electrode position for external cardioversion of atrial fibrillation.
Conclusion External cardioversion of atrial fibrillation is an often successful intervention to acutely restore sinus rhythm in patients with persistent atrial fibrillation. Based on recent publications, it appears reasonable to place the cardioversion electrodes in an anteriorposterior electrode position to achieve a higher cardioversion success rate.
References
Fig. 2. Flow chart of the most recent large trial comparing an anterior-posterior and an anterior-lateral electrode position. The anterior-posterior electrode position was more effective in cardioversion, and cross-over from an ineffective anterior-lateral electrode position to the anterior-posterior electrode position restored sinus rhythm in 66% of the cases. Published with permission from [17].
electrode position and on the pathophysiologicalanatomical considerations described above, several investigators started anew to compare the two electrode positions in the late 1990s [44–46]. These initial studies yielded conflicting results, partially due to insufficient study power: One study by Botto et al. found a higher cardioversion success rate when an anterior-posterior electrode position was used [44]. Another study found no difference between the two electrode positions [45], while a third, small study even appeared to demonstrate a better success rate when an anterior-lateral electrode position was used [46]. These studies did not enroll sufficient patients to exclude a relevant difference between the two electrode positions. A large randomized trial recently confirmed that the anterior-posterior electrode position was superior to anterior-laterally positioned
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