Abstract Body (Do not enter title and authors here): Background:
In patients with atrial fibrillation (AF), biphasic energy is superior to monophasic energy for cardioversion (CV). However, the optimal initial biphasic energy for CV is unknown. There are no studies in the US comparing the different biphasic energy levels.

Objective:
To compare the efficacy of initial high energy (IHE, 300-360J) versus initial low energy (ILE, ≤200J) biphasic shocks for CV of AF.

Methods:
Patients who received CVs at our institution for AF over a year were identified using the ICD-10 code. Patients who received internal CV, CV during other procedures, or CV for hemodynamic instability were excluded. Shocks were delivered using Lifepak® 20 through self-adhesive electrodes placed in an anteroposterior direction. Statistical analysis was performed using IBM® SPSS® Statistics software. The primary endpoint was conversion to sinus rhythm after the first shock (CSRF) and the secondary endpoint was conversion to sinus rhythm (SR) using multiple shocks.

Results:
A total of 594 patients were screened and 409 patients were included for final analysis. Results are shown in the table. Patients in the ILE group had lower BMI and lower rates of prior ablation. Patients in the IHE group had higher rates of CSRF (95.5% vs 87.6%, p=0.018). On subgroup analysis, higher rates of CSRF were observed with IHE even in patients with BMI>30 (97.4% vs 84.2%, p=0.019). More patients in the ILE group required multiple shocks. Even when multiple shocks were used, there was a higher rate of conversion to SR in IHE vs ILE (95.5% vs 92%, p=0.24). On multivariate analysis, the odds of CSRF were 6-fold higher with IHE after adjusting for sex, ejection fraction, prior ablation, obesity, and history of heart failure.

Conclusion:
Our study shows that IHE is better than ILE at converting AF into SR with the first shock. Even though the latest AF guidelines recommend 200J for initial cardioversion, select patients may benefit from >300J to avoid multiple shock attempts.