Abstract
His bundle pacing comes with the expectation of avoiding morbidity and mortality associated with pacing the ventricular myocardium. The paced complex typically exhibits a delta wave and resembles the relatively normal QRS complex of fasciculoventricular preexcitation. In patients with atrioventricular conduction disease, His bundle pacing ‘corrects’ both right and left bundle branch block. Conduction block is known to have a diffuse substrate and mechanism of correction remains unclear. In 51 patients with normal QRS complex, 35 patients with right bundle branch block and left axis deviation, and 41 patients with left bundle branch block we analyzed the changes in the paced QRS complex. In all patients His bundle pacing caused a decrease in ventricular activation time, an increase in lead 1 voltage, an inferior shift in the frontal QRS axis, and a rightward shift in R wave transition in the precordial leads. In patients with normal QRS, despite these changes, the paced QRS remained in normal range. In left bundle branch block, these changes resulted in decrease of prolonged ventricular activation time, increased lead 1 voltage , and normalized the delayed R wave transition in precordial leads. The correction of right bundle branch block suggested that the right ventricular free wall was being activated earlier. This was consistent with early rightward transition of precordial R waves and the inferior QRS axis and the known base to apex activation of right ventricular wall in fasciculoventricular preexcitation. We conclude that the QRS complex in normal range, seen in fasciculoventricular preexcitation, is caused by rapidly conducting, base-to-apex, and septum to lateral wall activation in both left and right ventricles. This would explain the correction of left axis deviation and reversal of delayed lateral wall activation in right and left bundle branch block, by His bundle pacing.