BACKGROUNDElectrocardiographic signature of escape capture bigeminy that spans generations and clusters in a family has not been linked to a sodium channel voltage sensor mutation.OBJECTIVETo characterize the clinical and biophysical consequences of the R222Q mutation in the voltage sensor of cardiac sodium channels.METHODSComprehensive clinical assessment, invasive electrophysiologic study, genetic analysis, and patch-clamp studies were undertaken.RESULTSUniquely, 5 members had the same electrocardiographic pattern of a junctional escape ventricular capture bigeminy. Genetic analysis of 3 family members revealed the same mutation (R222Q) in the cardiac sodium channel gene, SCN5A (nucleotide change was 665 G→A that led to missense amino acid substitution Arg 222 Gln, located in the S4 voltage sensor in domain I). Catheterization and mapping revealed that there was no consistent evidence of bundle branch reentry or fascicular potentials preceding ectopic beats. The bigeminy was suppressed by the intravenous administration of the sodium channel blocker, lidocaine. Patch-clamp studies revealed unique differential leftward voltage-dependent shifts in activation and inactivation properties of human voltage-gated Na(+) channels with the R222Q mutation, consistent with increasing channel excitability at precisely the voltages corresponding to the resting membrane potential of cardiomyocytes.CONCLUSIONSThe R222Q mutation enhances cardiac sodium channel excitability, resulting in an unusual, highly penetrant phenotype of escape capture bigeminy and cardiomyopathy. These findings support the conclusion that a mutation in the voltage sensor of cardiac sodium channels can cause bigeminal arrhythmia associated with cardiomyopathy.