Objective: Loss of Na⁺ channel function has been implicated in idiopathic ventricular fibrillation (IVF) and Brugada syndrome. We have studied the biophysical properties of an IVF mutation (S1710L) that exhibited an unusual clinical phenotype: rate-dependent bundle branch block without manifestation of Brugada-type ECG pattern. Methods: The mutant S1710L channels were expressed in mammalian cells and their gating properties, studied using whole-cell patch clamp techniques, were compared with wild-type (WT) and a Brugada syndrome mutant channel T1620M. Results: The S1710L channel exhibited significantly faster macroscopic current decay than WT or T1620M. In addition, S1710L showed a negative shift in the voltage-dependence of fast inactivation and slower recovery from fast inactivation than in WT or T1620M. In addition to the alterations in fast inactivation most commonly observed in Brugada syndrome mutations, S1710L exhibited marked enhancement in slow inactivation and a large positive shift of activation that potentially decreases conduction velocity. Conclusions: These functional abnormalities may be responsible for the overlapping clinical phenotypes associated with Brugada syndrome and the cardiac conduction defect, a novel cardiac Na⁺ channelopathy.