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Electrical cardioversion (ECV), a mainstay in atrial fibrillation (AF) treatment, is unsuccessful in up to 10–20% of patients. An important aspect of the remodeling process caused by AF is the constitutive activition of the atrium-specific acetylcholine-dependent potassium current (I K,ACh  → I K,ACh-c ), which is associated with ECV failure. This study investigated the role of I K,ACh-c  in ECV failure and setting the atrial defibrillation threshold (aDFT) in optically mapped neonatal rat cardiomyocyte monolayers. AF was induced by burst pacing followed by application of biphasic shocks of 25–100 V to determine aDFT. Blocking I K,ACh-c  by tertiapin significantly decreased DFT, which correlated with a significant increase in wavelength during reentry. Genetic knockdown experiments, using lentiviral vectors encoding a  Kcnj5 -specific shRNA to modulate I K,ACh-c , yielded similar results. Mechanistically, failed ECV was attributed to incomplete phase singularity (PS) removal or reemergence of PSs ( i.e.  re-initiation) through unidirectional propagation of shock-induced action potentials. Re-initiation occurred at significantly higher voltages than incomplete PS-removal and was inhibited by I K,ACh-c  blockade. Whole-heart mapping confirmed our findings showing a 60% increase in ECV success rate after I K,ACh-c  blockade. This study provides new mechanistic insight into failing ECV of AF and identifies I K,ACh-c  as possible atrium-specific target to increase ECV effectiveness, while decreasing its harmfulness.

Constitutively Active Acetylcholine-Dependent Potassium Current Increases Atrial Defibrillation Threshold by Favoring Post-Shock Re-Initiation