Commentary
SVT is a narrow QRS tachycardia with intermittent AV dissociation. The
differential diagnosis of SVT in this patient included AV-nodal
reentrant tachycardia (AVNRT) with upper common pathway block,
junctional tachycardia (JT) with junctional-atrial block, intra-Hisian
reentrant tachycardia, or orthodromic reentrant tachycardia (ORT) using
a concealed nodofascicular or nodoventricular accessory pathway with
nodal atrial block.
The absence of split His bundle potentials during the procedure excludes
intra-Hisian reentrant tachycardia. SVT was necessarily initiated
depending on a critical AH interval prolongation (Figure 1). Moreover,
atrial overdrive pacing during SVT accelerated the His bundle potentials
to the paced CL, and produces an atrial-His-atrial (AHA) response upon
pacing cessation. These findings rule out the diagnosis of
JT.1
When the H-H interval is relatively stable, no premature ventricular
extrastimuli delivered during His bundle refractoriness preexcited the
next His bundle potential. After infusion of isoproterenol, SVT with the
same earliest atrial activation at the CS ostium with a fixed 1:1 AV
relationship was induced by atrial extra stimulation with a finding of
an AH jump up. A ventricular extrastimuli delivered during SVT when the
His bundle was refractory did not reset the atrial cycle. A
ventricular-atrial-ventricular (VAV) response was observed upon
resumption of the tachycardia, on cessation of ventricular entrainment
pacing, and the difference between the postpacing interval and
tachycardia CL was 139 ms, which is longer than 125
ms.2 Figure 3 shows RV overdrive pacing during SVT at
a faster pacing rate. During RV pacing, 2 consecutive His bundle
potentials occur at the pacing CL shortly after the fourth and fifth
pacing stimuli, but there were no change of the A-A intervals after
those pacing stimuli, and VA block was observed after the sixth pacing
stimulus. That is, RV overdrive pacing during SVT retrogradely captured
the His bundle but failed to accelerate the atrium.3These findings consistent with AVNRT, and, thus, ruled out the
possibility of ORT.
We performed electroanatomical mapping to identify the earliest site of
atrial activation during SVT, and observed that the arrhythmia was not
inducible with disappearance of VA conduction after radiofrequency
ablation at the CS ostium, consistent with successful ablation of the
slow pathway. Thus, we diagnosed SVT as atypical AVNRT using the slow
pathway as an anterograde limb and the slow pathway as a retrograde limb
(slow-slow AVNRT) with upper common pathway block.
In this case, the fluctuation of the H-H intervals during slow-slow
AVNRT with AV dissociation can be explained by the timing of the
dissociated sinus rhythm; late (Figure 2A) and early (Figure 2B) coupled
sinus complexes (see the asterisk in the figure). In Figure 2A, the
second and third sinus complexes advanced the next His bundle
potentials. That is, the sinus complexes delivered during the
refractoriness of the His bundle anterogradely conducted down the slow
pathway, reached the His bundle, conduced to the ventricle, and returned
through the slow pathway. Figure 2B shows that the early atrial
contraction from the sinus node advanced the immediate His bundle
potential by 47 ms with continuation of the tachycardia. The first to
third H-H intervals were 625 ms, and the fourth was shortened to 578 ms
after an atrial contraction from the sinus node. The fifth H-H interval
was prolonged to 672 ms, while the sixth and seventh H-H intervals
returned to 625 ms. The interval from the fourth to the sixth His bundle
potentials was exactly identical to twice the CL of tachycardia. Atrial
excitement from the sinus node descends the fast pathway, reaches the
His bundle, and conducts to the ventricle, though it does not affect the
tachycardia circuit. That is, because the fast pathway was not included
in the tachycardia circuit, slow-slow AVNRT continued even when the
immediate His potential was advanced. We think that these might be the
reason why the H-H intervals fluctuated during slow-slow AVNRT.