Introduction For elective electrical cardioversion of atrial fibrillation, the recommended standard is the administration of a biphasic direct current shock. However, there is considerable variability among physicians regarding the choice of the initial shock energy, as current clinical guidelines do not specify it precisely. The guidelines of American cardiology societies recommend a shock intensity of at least 200 J, while the European Society of Cardiology guidelines do not comment on shock energy at all.

Evidence from a previous randomized clinical trial has shown that using a higher initial shock energy (360 J) is more effective, safe, and reduces the cumulative delivered energy, since lower initial shock energy more frequently requires repeated shocks. The safety of the maximal-energy protocol was demonstrated in this study, among other by the absence of myocardial injury as assessed by cardiac troponin I levels measured 4 hours after electrical cardioversion. However, in this study, the maximal initial shock energy protocol was compared with a low-energy escalating protocol (125-150-200 J), which is no longer supported by current clinical guidelines.

Objective The primary objective of the study is to compare the efficacy (short-term maintenance of sinus rhythm and the need for repeated shocks) of two regimens commonly used in clinical practice: 200-360-360 J and 360-360-360 J.

Secondary objectives are to identify factors that would justify the use of either lower or higher initial shock energy. Another secondary objective is to confirm the safety of the maximal initial shock energy protocol by assessing the biomarker of acute stress (copeptin) in a subpopulation of patients (n = 60).

Methods The main inclusion criterion for the study is the presence of atrial fibrillation or atypical atrial flutter and a clinical indication for electrical cardioversion raised an independent cardiologist. The main exclusion criterion is the presence of typical atrial flutter or focal atrial tachycardia, for which lower shock energy is recommended.

This is a prospective, randomized, single-blind (patient) study in which patients will be randomly assigned in a 1:1:1 ratio either to (i) 200-360-360 J protocol, (ii) 360-360-360 J protocol, or (iii) individualized selection of one of the above protocols based on additional parameters. I one of parameters next parameters is met, the 360-360-360 J protocol will be applied, if not, the 200-360-360 J protocol will be applied: Left atrial size > 55 mm, duration of the arrhythmia > 12 months, BMI > 30 kg/m2, chest circumference > 120 cm.

Since chest dimensions and the amount of subcutaneous fat may influence resistance to electrical current and thereby reduce the delivered electrical energy, chest dimensions and configuration will be evaluated prior to electrical cardioversion in addition to height and weight. The procedure itself (electrical cardioversion) will then be performed according to the standard institutional clinical practice.

Inclusion Criteria:

• Atrial fibrillation
• Atypical atrial flutter
• Indication for elective electrical cardiac version made by independent physician

Exclusion Criteria:

• Typical atrial flutter
• Focal atrial tachycardia