SCN5A and long QT syndrome 3: Late-INa and slowed inactivation of the sodium current are also induced in the presence of certain Nav1.5 mutations that underlie LQT3 in humans.39,40 Genetic induction of late-INa results in action potential prolongation, an increase in the ECG QT interval, altered calcium handling, and an increased risk of developing life-threatening torsades de pointes ventricular fibrillation.