Corresponding author:
Ulhas M Pandurangi, Chief of Department of Cardiac Electrophysiology and
the Arrhythmia Heart Failure Academy, The Madras Medical Mission,
Chennai, Tamil Nadu, India
Email ID- epulhas@gmail.com
Total Word Count- 1131
CASE
A 68-year-old lady with a history of diabetes mellitus and hypertension
was diagnosed with nonischemic dilated cardiomyopathy (left ventricular
ejection fraction of 30%). She presented with one episode of syncope.
The 12-lead electrocardiogram (ECG) revealed what seems to be sinus
rhythm with 4:3 AV Wenckebach pattern, atypical right bundle branch
block (RBBB) with left anterior hemiblock, QRS duration of 150 ms and a
frontal QRS axis of -30 degrees. Figure 1 shows 12-lead ECG along with
the distal and proximal bipolar His catheter signals obtained during the
electrophysiology study. What is the rhythm that the intracardiac
signals reveal?
DISCUSSION
A closer look at the 12-lead ECG (Figure 1) reveals findings suggestive
but not typical of Wenckebach pattern because the 1stPR interval after the pause is longer than that of the first beat of the
tracing and hence raises a question about the nature of the rhythm.
The P waves are of sinus morphology and appear to occur at constant
intervals. Further analysis of the intracardiac electrograms (EGMs)
points to three distinct signals. Apart from the obvious ventricular (V)
signals on the distal His electrode, the signals with largest maintain a
constant relationship with the P waves and likely represent atrial
activity. The lowest amplitude signals which precede the apparent atrial
signals (best seen in the first beat of the trace), occur at a regular
timing and are likely to be the His potentials (as annotated in Figure
2). They are related with Ventricular signals in a 4:3 Wenckebach
pattern. Of note, the varying HA intervals suggest dissociation. As
inferred from the explanation in figure 2, it is clear that A-A and H-H
intervals are slightly dissimilar, the HA intervals are not fixed and
hence, are dissociated isorhythmically suggesting Hisian automaticity
competing with sinus rhythm. The H-V intervals, however, are
progressively increasing until the fourth H blocks and fails to activate
the ventricle, thus suggesting a 4:3 infra-Hisian Wenckebach. The
characteristic V-V interval shortening before the pause during a typical
Wenckebach phenomenon is also seen (depicted with a ladder diagram in
figure 2). To note, in this tracing, there is no opportunity for the
Hisian impulse to capture the atrium retrogradely.
One other possibility could be that each atrial impulse conducts
antegradely to the subsequent His with a very long AH interval and then
to the ventricle in a Wenckebach pattern. However, the constant HH
intervals with varying AA intervals is against this possibility.
At times (Figure 3A), when the Hisian automaticity usurped the sinus
rate (represented by asterisk), the atrium was captured retrogradely as
can be inferred by the inverted P waves in inferior leads (Figure 3B).
Thus, there was intermittently an H-A association with regular HA
intervals. However, the infra-Hisian Wenckebach pattern persisted in
varying ratio, throughout the study. Here again, the possibility of an
ectopic atrial rhythm conducting down to the next H cannot be ruled out
despite the constant HH intervals.
During the EP study, it was noted that whenever the ventricle was
captured by RV apical pacing, the infra-Hisian conduction system
remained perpetually refractory and there was no antegrade ventricular
capture by Hisian automaticity even at a ventricular pacing rate of 50
bpm.
No ventricular tachycardia was inducible during the EP study.
The patient eventually had a dual-chamber pacemaker without any drugs to
suppress the automatic focus. She had nonischemic cardiomyopathy, and
cardiac MRI had ruled out inflammatory or infiltrative pathologies.
Hence the rhythm in figure 1 can be described as ‘Hisian automaticity in
isorhythmic dissociation with the atrium and infra-Hisian typical
Wenckebach conduction’.
This is in accordance with Occam’s Razor where a single explanation (of
Hisian automaticity) gains precedence over two separate anomalies
(abnormally long AV nodal conduction and an ectopic atrial rhythm) to
explain the same phenomenon.
Gradual prolongation of PR interval followed by a pause usually
represents Atrioventricular (AV) Wenckebach and has classically been
considered an AV nodal phenomenon. However, an open mind should be kept
regarding less common mechanisms as presented in this unusual case.
Infra-Hisian Wenckebach has been scarcely reported in the literature.
(1) The distinction, sometimes, becomes critical as infra-Hisian
Wenckebach is a harbinger of complete AV block and demands permanent
pacing contrary to AV nodal Wenckebach. (2)
In summary, our case documents a rare and hitherto unreported phenomenon
of persistent Hisian automaticity in isorhythmic dissociation with the
atrium and varying patterns of infra-Hisian Wenckebach conduction.