Effect of class Ia antiarrhythmic agents on fasciculoventricular fibers

In four patients with fasciculoventricular (FV) fibers, the electrocardiographic and electrophysiologic characteristics were studied based on their response to class Ia antiarrhythmic agents. Three patients had paroxysmal atrial fibrillation, and one showed atrioventricular nodal Wenckebach-type block with dual pathways. Three of the four patients showed complete block of FV fibers after administration of disopyramide 0.28-0.60 mg/kg, procainamide 1.7-2.6 mg/kg, and ajmaline 0.18-0.26 mg/kg, respectively. Normalization in the remaining patient was not seen after disopyramide 2 mg/kg or procainamide 10 mg/kg, but was achieved by ajmaline 0.86 mg/kg. Ventricular preexcitation beats showed initial q waves (3 patients) or a decrease in the height of the r wave (1 patient) in V1, no initial q wave in V6 (4 patients), and ST-T changes (1 patient). Since these findings resemble ischemic change or intraventricular conduction disturbance, drug testing is necessary for precise diagnosis and proper clinical management.