Meir Friedman MD, Paul Schweitzer MD.
Beth Israel Medical Center, New York, NY
Address for
correspondence: Meir Friedman MD, Affiliation Beth Israel
medical Center, Department of Cardiology, New York, NY. E-mail:
mf105@columbia.edu
This ECG was recorded from a 93
year old patient with a previously documented third degree AV block and
an underlying LBBB. The twelve lead ECG demonstrates sinus bradycardia
at a rate of 52 beats per minute, a PR interval of 230
milliseconds, and a left bundle branch block (
Figure 1). In
figure 2, a rhythm strip of leads
V1, II
and V5 shows sinus rhythm with AV block. Note that only P waves in the
T wave of the paced beat are conducted. The non-conducted P waves
are followed by ventricular paced beats at an escape interval of 1200
milliseconds.
Figure
1: Shows sinus bradycardia at a rate of 52 beats per minute, PR
interval of 230 milliseconds and a left bundle branch block.
Figure 2: This is a rhythm
strip where lead V1, II and V5 demonstrates 2:1 AV block. The P waves
in the T wave of the paced beats are conducted while the others are
not. The non-conducted P waves are followed by ventricular paced beats.
What is the
mechanism of AV conduction in this patient? The following
possibilities should be considered:
1. Supernormal conduction
2. Wedensky phenomenon
3. Gap phenomenon
4. Peel back effect
In the sixties
and seventies Pick et al
1,
Schamroth
2 and others
3 suggested supernormal conduction as the
most likely mechanism of unexpected transient improvement of
atrio-ventricular conduction defects. This electrophysiological
phenomenon was defined as better than expected conduction in patients
with depressed conduction during a short interval in the ventricular
cycle
1. However, there is no
definitive proof of supernormal conduction in humans.
The next
possible explanation is the Wedensky effect. This mechanism was
first documented on the nerve-muscle preparation and was defined as a
"stronger" stimulus, where in the case of AV block, a ventricular
premature or paced beat, is followed by transient antegrade conduction
by decreasing the refractoriness of the AV conduction
2. Similarly to supernormal conduction,
its existence in humans remains controversial.
According to
Moe et al
4 and others
5, there are alternative explanations for
electrocardiographic abnormalities suggestive of supernormal conduction
or Wedensky phenomenon. The first of such explanations is the Gap
phenomenon. Experimental and clinical studies
4,5 showed that atrial premature beats with
longer coupling intervals were blocked and earlier ones conducted. The
explanation of Gap Phenomenon is conduction delay in the proximal part
of the AV conduction system, causing recovery of its more distal
portion. The gap phenomenon is an unlikely mechanism in our patient
because the conducted P wave occurred after the paced beat rather than
following a premature beat.
A more likely
mechanism is the "peeling back" effect. It is assumed that
pre-excitation of the AV node by a ventricular or junctional beat
shortens the absolute refractory period of the AV or the His-Purkinje
system and allows conduction of a supraventricular impulse
4.
The final
question is the site of AV block. Because the patient has a left
bundle branch block the possibility of infranodal block has to be
considered. However, without His bundle recording, the site of AV
block remains uncertain.
References
1. Pick A, Langerdorf R, Katz LN. The
supernormal phase of atrioventricular conduction. I Fundamental
mechanisms. Circulation 1962; 26: 388-404.
2. Schamroth L. The disorder of
cardiac rhythm. Blackwell Scientific publications, Oxford and
Edinburgh. 1971; 271-279.
3. Rashid A, Massumi RA, Amsterdan EA,
Mason DT. Phenomenon of supernormality in the human heart. Circulation
1972; 46:264-275.
4. Moe GK, Childers RW, Merideth J.
An appraisal of supernormal AV conduction. Circulation 1968;
38:5-28.
5. Gallagher JJ, Damato AN, Varghese PJ,
Caracta AR, Josephson ME, Lau SH. Alternative mechanisms of
apparent supernormal atrioventricular conduction. Am J Cardiol.
1973;31: 362-371.
