Neonatal long QT syndrome type 3 predicted by positive lidocaine challenge.

A female infant presented with bradycardia and an electrocardiogram demonstrating 2:1 atrioventricular depolarization, a prolonged QT interval, and T wave alternans. After propranolol therapy was initiated, a lidocaine challenge was performed with progressive shortening of the QT interval. This positive lidocaine challenge prompted clinical suspicion of long QT syndrome type 3 (LQT3) and early initiation of mexiletine therapy. Subsequent genetic testing confirmed the infant's diagnosis of LQT3.