Mechanisms of Posterior Fascicular Tachycardia: The Relationship Between High Frequency Potentials and the Ventricular Myocardium.

### Case 1A 17-year-old man presented with a history of palpitations and surface ECG demonstrating wide QRS tachycardia (QRS duration=140 ms) with a right bundle branch block pattern and left-axis deviation (Figure 1A). The 12-lead ECG was normal during sinus rhythm.Figure 1. , Surface ECG of tachycardia showed wide QRS complex (QRS duration=140 ms) with a right bundle branch block pattern and left-axis deviation. B , sinus rhythm (left), A–H and H–V intervals were 57 and 55 ms, respectively. P2 was recorded with an antegrade conduction sequence from proximal to distal multielectrode catheter (MEC). The ventricular potentials at the mid MEC were earlier than proximal and distal ventricular potentials. tachycardia (right), the H–V interval was −30 ms. P1 and P2 were recorded with opposite conduction sequences with P1 activated antegradely, and P2 activated retrogradely. P1 transition to P2 activation occurred toward the distal MEC. C , Right and left anterior oblique fluoroscopic images of catheter positions. The 20 pole MEC was positioned at the left septal ventricle via a retrograde aortic approach. CS indicates coronary sinus electrogram; HIS, His bundle electrogram; His, His bundle; LAF, left anterior fascicle; LAO, left anterior oblique; LPF, left posterior fascicle; LV, left ventricular electrogram; RAO, right anterior oblique; RB, right bundle; and RV, right ventricular electrogram.See Editor’s Perspective by Asirvatham and Stevenson During electrophysiology study, mapping was performed using a 20 pole multielectrode catheter (MEC) via a retrograde aortic approach (Figure 1C). The baseline A–H and H–V intervals were 57 and 55 ms, respectively (Figure 1B, left). A wide QRS tachycardia identical to the clinical tachycardia was observed with cycle length of 320 ms, H–V interval of −30 ms (Figure 1B, right). This was consistent with a diagnosis of …