Discussion

A differential diagnosis of the LBBB morphology tachycardia with AV dissociation includes: (a) myocardial ventricular tachycardia; (b) bundle-branch reentrant ventricular tachycardia (BBR-VT); (c) verapamil-sensitive upper septal idiopathic left ventricular tachycardia (US-ILVT); (d) ventricular tachycardia associated with longitudinally dissociated His bundle1.
We initially considered the mechanism of this tachycardia was bundle-branch reentry, because the patient underwent the valve replacement2 and the QRS duration was increased with a slight deviation of the QRS axis following the operation. Moreover, the baseline HV interval was exceeded the normal range3. This indicated the His-Purkinje system (HPS) was disordered. However, the H-H (i.e. Spike-Spike) cycle length during the tachycardia did not determine the V-V interval, but rather the contrary. Besides, the long post pacing interval (PPI) (624 ms; a PPI – TCL > 30 ms) following RVA pacing without manifest fusion (Figure 2A, 2B) made BBR-VT unlikely 4,5.
US‐ILVT is a unique type of VT, half of which exhibited an identical QRS configuration as sinus rhythm and occurred after ablation of the common form of verapamil-sensitive fascicular VT6. US-ILVT is considered to be caused by a reentrant circuit incorporating the abnormal Purkinje fiber with slow conduction. However, the His bundle electrogram usually preceded the QRS complex during US-ILVT with a shorter HV interval than that during sinus rhythm.6
Arai H, et al reported myocardial ventricular tachycardia from the vicinity of the His bundle7. The entrainment from the RVA showed constant and progressive fusion, which was in contrast with our case (Figure 2A and 2B). In addition, pacing from the His bundle area during the tachycardia exhibited two different responses. In most instances, the Spike potential was captured as shown in the left panel of Figure 2C. The PPI was 578 ms and an interval between the last stimulation to fragmented potential (380 ms) was shorter than the TCL. Occasionally, the fragmented potential was captured selectively and concealed entrainment was observed with the stimulus to QRS interval of 162 ms, which was close to the fragmented potential to QRS interval (195 ms) during the tachycardia (the middle panel in Figure 2C). Shortening of the stimulation interval to 340 ms gradually prolonged the stimulus to QRS interval and terminated the tachycardia at the time between the fragmented and Spike potentials (the right panel in Figure 2C). The fourth stimulation (in the figure) directly captured ventricular myocardium with a different QRS morphology from that during the tachycardia, indicating some tissue insulated from the ventricular myocardium participated in the reentrant circuit. We thus considered that the longitudinally dissociated right-sided His bundle caused the fragmented potential during the tachycardia, whereas the Spike potential was attributable to retrograde activation of the main body of the His bundle (Figure 3A). As a turnaround area of the fragmented to Spike potentials, transverse spread of the propagating impulses had been demonstrated in the His-Purkinje system with longitudinal dissociation8. The tachycardia resembled ventricular tachycardia with an area of slow conduction in the longitudinally dissociated left-side His bundle1. In that case, the left-sided His bundle potential was converted into fragmented potentials at the initiation of the ventricular tachycardia (Figure 2A1).
A 3.5 mm tip ablation catheter (ThermoCool SmartTouch SF; Biosense Webster, Diamond Bar, CA, USA) was placed at the slightly more ventricular site (as compared to the His bundle area) where the fragmented potential was recorded 49ms earlier from the onset of QRS complex (Figure 3B). Pacing from this site demonstrated concealed entrainment (Figure 3A). A delivery of radiofrequency energy (30W) terminated the tachycardia in 20 seconds with no change in the QRS configuration during the baseline rhythm. The tachycardia became no inducible after this radiofrequency application.