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.