Download Experimental Demonstration of Concealed AV

Experimental Demonstration of Concealed
AV Conduction in the Human Heart
By R. LANGENDORF, M.D., A. PICK, M.D., A. EDELIST, M.D., AND L. N. KATZ, M.D.
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THE TERM concealed conduction was introduced in a report' from this department to define in a concise and uniform manner the after effects of partial penetration of
atrioventricular (AV) conduction pathways
by impulses originating in the atria or ventricles.2 This concept proved indispensable
in the proper analysis and understanding of
a great number of arrhythmias encountered
in routine electrocardiography.3-5 Recently,
the validity of our assumptions has been firmly
established by others in the experimental
laboratory"9 with the help of modern technics, particularly the recording of action potentials directly from the different portions of
the conduction system.
With the advent of artificial cardiac pacing
as an accepted method of treatment of StokesAdams disease, it became feasible to produce
in the human heart conditions prone to reveal
the phenomenon of concealed conduction.
Simple experimental procedures can be applied
at the bedside without danger or discomfort
to the patient and various disturbances of
cardiac rhythm can be reproduced at will.
Such observations, some of which shed new
light on the ways concealed conduction inFrom the Cardiovascular Institute, Michael Reese
Hospital and Medical Center, Chicago, Illinois.
Supported by a grant (HE-06375) from the National Heart Institute, U. S. Public Health Service.
Presented at the Seventh Inter-American Congress
of Cardiology, Montreal, Canada, June, 1964.
fluences subsequent impulse formation and
conduction, are the subject of this report.
Material and Methods
All records were obtained on patients in need
of artificial pacing because of frequent StokesAdams attacks. Except in one instance (fig. 7),
the observations were made during the use of
a transvenous catheter pacemaker, prior to
implantation of a permanent one. The experiments
consisted in changing the rate of single stimuli
of constant duration and suprathreshold strength,
or in altering the spacing of paired stimuli
according to a method described elsewhere.10 In
one group of experiments the catheter tip was
kept in the right atrium, in another group the
right ventricle was stimulated. In the case illustrated in figure 7 a permanent pacemaker had
been implanted in the left ventricle.
Results
Catheter Electrode in the Right Atrium
Figures 1 to 4 are records (lead I) obtained
in a patient with arteriosclerotic heart
disease, permanent left bundle-branch block,
and varying degrees of AV block. The four
were performed when,
during artificial pacing, the AV conduction
disturbance had almost subsided. In figures
1 to 3, the atria were driven at frequencies
exceeding those of the natural (sinus) pacemaker, which was completely suppressed. In
figure 4, at a lower stimulation rate, the two
types of impulses were in competition for
atrial control.
Figure 1 shows a replica of the classical
types of experiments
Figure 1
Concealed AV conduction during 2:1 ventricular response to rapid pacing of the atria. Each
stimulus artifact (S) is followed by a P wave (P) (lead I).
Circ8tation, Volume XXXII, September 1965
386
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CONCEALED AV CONDUCTION
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Figure 2
Atrial stimulation by paired artificial impulses. The duration of the phase of concealed AV
conduction and the effects of induced premature impulses on atrial excitability (lead I).
basic experiment of Lewis and Master2 on concealed AV conduction. Driving the atria at a
fast rate of 168/min. produced a 2:1 ventricular response, which changed to a 1:1
response when the frequency of artificial atrial
stimulation was suddenly reduced to 84/min.,
one half the previous rate. Thus, the ventricular rate remained constant but, as alternate "ineffective" atrial impulses were eliminated, the P-R interval shortened from 0.24
Circulation, Volame XXXII, September 19665
sec. during 2:1 conduction to 0.19 sec. during
1:1 conduction. The most likely explanation of
the longer P-R intervals is penetration into the
AV junction (concealed conduction) of the
apparently blocked impulses. An alternative
interpretation, operation of two AV junctional
pathways with different refractory periods
and conduction speeds" appears less likely in
view of subsequent experiments in this patient.
In figure 2 are assembled six selected seg-
LANGENDORF ET AL.
388
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S
S
Sx S x2S
S
S
S
s
S XiE X2 S
S
S
Figure 3
Repetitive concealed AV conduction. Note that in
lower panel the ventricles fail to respond to three
consecutive atrial stimuli (X1-S-X2) (lead I).
long records illustrating the effects
conduction of intermittent paired stimulation of the atria in this patient. While the
basic driving cycle (S-S) was kept constant
(at a rate of 94/mmn.) the position of the interpolated extra stimulus (X) was progressively shifted from Si toward S2. In all instances (panels A-F), X elicits an atrial
response but only in panel F, after the longest
S1-X interval, is it propagated through the
entire AV junction to reach and activate the
ments of
on AV
ventricles. In panels A to D, the extra stimulus,
while stopped within the AV junction, progressively slows (panels A and B), and finally
prevents (panels C and D) AV conduction of
S2. In panel E, failure of the atria to respond
to S2 (see below) seems to preclude demonstration of concealed conduction of X, but
the latter can be postulated in view of the
findings in panels D and F. Thus, there is
concealed AV conduction of impulse X over
a period of at least 0.11 and probably over
0.14 sec.
The response of the atria to basic and premature artificial stimulation is prompt throughout panel A to C. In panel D, there is a
latency of 0.08 sec. of the P wave after S2;
in panels E and F, this P wave is absent.
Retardation and failure of atrial excitation by
S2 can be attributed to the progressively
shorter X-S2 interval: in panel D, although
the atrial myocardium is partially refractory,
it still permits interpolation of X (as in panels
A to C); in panels E and F complete refractoriness of the atria at the time of S2 results
in a compensatory pause after X. An obvious
analogy is thus apparent to the effects of ventricular premature systoles on AV junctional
tissues with regard to interpolation or production of a compensatory pause.12
Figure 3 illustrates the artificial production
of repetitive concealed AV conduction. During
a faster basic driving rate of 120/min., which
lengthened the AV conduction time to 0.28
sec., extra stimuli X, and X2 were interpolated
into two consecutive basic cycles, so that the
PI
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Figure 4
Discharge of a subsidiary AV junctional pacemaker by concealed antegrade AV conduction in
artificial atrial parasystole. Pl represents atrial responses to artificial pacemaker stimuli indicated by dots in the diagram and broken lines at A (atrial) level. S-A indicates impulse propagation from and to the S-A node; A-V indicates complete or partial impulse propagation in the
A-V junction (which is divided by a horizontal line into an upper and lower portion) (lead 1).
Circulation, Volume XXXII, September 1965
CONCEALED AV CONDUCTION
389
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in the AV junction and ventricular pauses of
different length ensue. The first (shorter) one
(1.16 sec.) is terminated by a junctional
escape interfering with a simultaneously occurring sinus impulse, the sinus P wave being
superimposed on the QRS of the escaped
beat. In the second (longer) pause (1.28
sec.), such an escape failed to occur at the
expected time, indicating premature extraneous discharge of the subsidiary center by concealed conduction of the third pl impulse.
This impulse penetrated deeper than the
second because its later occurrence after the
preceding conduction of the sinus impulse permitted partial recovery of the AV junction.
This interpretation was fortified by the facts
that (1) the duration of the escape interval
was quite constant (1.16 sec.) and (2) whenever the ventricular cycle exceeded this value
it contained a nonconducted pl, with a R-Pl
interval of at least 0.24 sec.
atria received five stimuli in rapid succession:
S-X1-S-X2-S. With S-X intervals of 0.21 sec.
(upper panel), the proximal AV junction was
completely refractory to the premature impulse
and hence regular ventricular responses to S
stimuli, as well as the corresponding P-R intervals, were undisturbed. However, when
S-X intervals were increased to 0.24 sec. (lower
panel), a long ventricular pause occurred after
the response to the first (S) of the group of
five rapid stimuli. Evidently, X1 penetrated into
the AV junction to prevent conduction of the
subsequent S. Yet this stimulus must likewise
have entered the AV junction over the same
pathway, for in its wake X2 was stopped before regular ventricular responses to driving
stimuli were resumed (with shortening of the
P-R interval of the first conducted beat). Conceivably, during the preceding repetition of
concealed conduction not only depth but also
speed of partial penetration of the AV junction
by atrial impulses may have declined progressively as is indicated in the diagram (a
"Wenckebach phenomenon of concealed conduction"3 ).
Figure 4 shows the effect of concealed AV
conduction on impulse formation in the AV
junction. By driving the right atrium at a rate
slower than that of the natural (sinus) pacemaker an atrial parasystole has been produced
artificially. Of the five pacemaker stimuli that
fall outside the refractory phase of the atria
and elicit an atrial response (Pl) only the
first, fourth, and fifth produce ventricular
responses; the second and third are stopped
....
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Catheter Electrode in the Right Ventricle
Figure 5 shows a case of complete AV dissociation caused by advanced AV block in
another patient with arteriosclerotic heart
disease. There is competition for ventricular
control between responses to an artificial
ventricular pacemaker rate (54/min.) and
an AV junctional pacemaker discharging
spontaneously at a faster rate (60/min.); the
QRS complexes of the latter are prolonged
due to right bundle-branch block. Whenever
a pacer impulse falls outside the refractory
period following spontaneous ventricular activation, it produces a premature ventricular
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Figure 5
Discharge of an AV junctional pacemaker in complete AV dissociation by concealed retrograde
conduction from an artificial ventricular parasystole (with fixed coupling intervals) (lead II).
Impulses of the ventricular pacemaker are indicated by dots in the diagram, the retrograde
ventricular impulse by broken lines at V (ventricular) level.
Circulation, Volume XXXII, September 1965
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LANGENDORF ET AL.
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Figure 6
Concealed antegrade conduction in unidirectional AV block; a before, b and c during transventricular pacing (all lead II). The shaded area in the diagram indicates a region of
unidirectional conduction in the AV junction.
venous
beat. Concealed backward propagation into
the AV junction of these effective artificial
stimuli is disclosed by a shift in the appearance time of the subsequent spontaneous
junctional beat.
The parasystolic complexes in this case recur
at a precisely fixed "coupling" interval after
every fourth spontaneous beat. This is the case
because (a) each effective artificial impulse
discharges the natural pacemaker and shifts
its timing by a constant interval, (b) the
rates of the two pacemakers are regular, and
(c) the speed of propagation of the two types
of impulses is constant within the AV junction
in both forward and retrograde directions. A
coincidence iof these three factors must result
in a constant and repetitive coupling of artificial to spontaneous beats.
The records of figure 6 (all lead II) were
obtained in a patient with advanced and unstable AV block. A record before artificial
pacing, with AV conduction varying between
4:1 and 3:1 during sinus rhythm, is shown in
panel a. When the ventricles are paced at a
rate of 88/min. (panel b) each ventricular
complex is followed, at a constant interval
of 0.16 sec., by an inverted P wave (1:1 retro-
grade conduction). Sinus activity is completely
suppressed and the entire heart is controlled
by the artificial pacemaker. After reduction of
the driving rate to 55/min. (panel c), sinus
activity is restored and incomplete "reversed"
AV dissociation'3 has developed. All antegrade
(sinus) impulses are blocked but artificial ventricular impulses capture the atria completely
(-P) or partially (PF) when they occur within
a certain time interval after a sinus P wave.
On the basis of these facts, the following
conclusions can be drawn in this case:
1. The block in the AV junction is unidirectional, permitting complete antegrade
conduction only after long recovery times
(panel a); this effect on antegrade conduction contrasts with the ease of retrograde conduction even at a relatively fast driving rate
(panel b).
2. Proximal to this unidirectional block,
there is a pathway common to, and used by,
both antegrade and retrograde impulses. The
state of recovery of this region following
penetration (concealed conduction) of a
sinus impulse determines whether a retrograde
impulse will succeed or fail in reaching the
atria.
Circulation, Volume XXXII, September 1965
CONCEALED AV CONDUCTION
391
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7
Concealed retrograde conduction of left ven tricular pacemaker stimuli causing a supernormal
phase for antegrade conduction of SA impulsses in a unidirectional AV block (lead II). The first
two pacemaker signals are retouched. (Courttesy of Dr. Samuel Goldfein, Chicago.)
Electrode Implanted in the Left Ventricle
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In figure 7 are reproduced parts of a long
record in a patient after implantation of a
permanent pacemaker. The driving rate of
the ventricles is slower (68/min.) than the
sinus rate (120/min.) and incomplete AV dissociation has developed, with partial captures
of the atria by some of the artificial impulses
(Pp, i.e., atrial fusion beats), and total capture
of the ventricles by some of the sinus impulses;
the latter occurs exclusively and predictably
whenever a sinus impulse falls, within a certain limited time, shortly after an artificial
ventricular beat.
The analysis of this type of complex mechanism is indicated in the diagram and has been
described in detail previously.5 As in the preceding case, it requires the assumption of a
region of unidirectional block (shaded area);
in addition, however, a supernormal phase of
conduction has to be postulated to develop
in the wake of retrograde conduction across
the region of unidirectional block. Such super-
normality following concealed retrograde
conduction then permits passage of an appropriately timed sinus impulse. Here, in
contrast to the depressant effect illustrated in
all preceding figures, concealed conduction
acts to enhance subsequent conduction.
Discussion
While Lewis and Master2 in their classical
experiments demonstrated that an apparently
blocked impulse may delay or prevent conduction of a subsequent impulse, analysis of
spontaneously occurring clinical arrhythmias
has revealed a number of other manifestations
Circulation, Volume XXXII, September 1965
of concealed AV conduction. Thus concealed
conduction may (1) cause a repetition of concealed conduction and thus impair conduction
of several subsequent impulses ("repetitive
concealed conduction"3); (2) disturb the regular action of a subsidiary pacemaker ("by concealed discharge"1); (3) enhance, rather than
inhibit, subsequent conduction by creation of
a supernormal phase.5 These after effects of
partially penetrating impulses-with the exception of the last one-have been confirmed, and
their mechanism has been elucidated in detail, in recent animal experiments.6-9 In the
course of our observations during artificial
pacing, all known manifestations of concealed
conduction could be reproduced in man by
proper adjustment of the driving rate of the
artificial pacemaker controlling atria or ventricles.
Antegrade concealed conduction in its
simplest form was produced in a case of intermittent AV block, during a transient period
of restored AV conductivity, by rapid stimulation of the atria (fig. 1) or by reduction of
the atrial driving rate to less than the natural
(sinus) rate (fig. 4). A scan of the junctional
cycle by paired artificial atrial impulses (fig.
2) yielded values for the duration of the
phase of concealed (antegrade) conduction
corresponding to data found experimentally
in the dog heart.7 Retrograde concealed conduction was clearly demonstrated in complete
AV block during artificial stimulation of the
ventricles "at a rate slower than that of a
spontaneous junctional pacemaker (fig. 5).
Stimulation with faster rates has been shown
392
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to be followed by depression of impulse
formation in, and conduction from, the
junctional pacemaker."4 Furthermore, a wellknown manifestation of concealed retrograde
conduction was observed during artificial ventricular pacing in patients who have preserved
or resumed AV conduction of sinus impulses.
If, under such circumstances, the artificial beat
becomes interpolated, the postectopic sinus
beat may show prolongation of its P-R interval.15
By varying the rate of ventricular pacing
in an instance of AV block of high degree,
a simple mechanism was readily changed to
a complex one attributable to unidirectional
block with an interplay of both antegrade and
retrograde concealed conduction (fig. 6, b and
c). Observations of this kind not only justify
the assumption of concealed conduction of
antegrade impulses in complete AV block13' 16
but also provide evidence that unidirectional
block is indeed the explanation of retrograde
P waves under such circumstances, a concept
still questioned by some investigators.17 18
Facilitation of antegrade conduction in advanced AV block by a supernormal phase
induced by concealed retrograde conduction
was postulated5 19 to account for the exclusive
occurrence of ventricular captures after an
idioventricular beat. Actual occurrence of
retrograde or fusion P waves, as illustrated
in figure 7, supports the same assumption
in cases in which such direct evidence of
traversion of a region of unidirectional block
by retrograde impulses is missing.
The distinction as to whether concealed conduction causes a total block of a subsequent
impulse or its partial conduction (repetitive concealed conduction) cannot always
be made. In the case of figure 3, penetration of impulse S following X1 could not
have become manifest without the presence
of impulse X2. Repetitive concealed conduction seems to play an important role in the
ventricular response to rapid atrial rates, particularly atrial fibrillation and atrial flutter4' 20
and can be the mechanism of prolonged ventricular asystole in cases of second-degree
AV block.3' 21
LANGENDORF ET AL.
Finally, two ancillary aspects of our experience with pacing of atria and ventricles
at varying rates merit comment, because of
their bearing on our understanding of mechanisms and consequences of single premature beats. In figure 2 is illustrated an experimental model that permits an estimation
of the atrial refractory period gauged by the
length of the returning cycle after an atria]
premature systole, comparable to conditions
created by ventricular premature beats during
idioventricular rhythms.22 Figure 5 demonstrates the fallacy of too strict separation of
parasystolic rhythms from coupled "extrasystoles."23
Summary
Artificial pacing of patients with StokesAdams disease provided an opportunity to
study experimentally the ways of operation
of concealed antegrade or retrograde conduction, or both, in the AV junction.
With a catheter electrode in the right atrium
the classical experiment of Lewis and Master
was repeated, revealing, during a 2:1 ventricular response to an atrial tachyeardia, the
delaying effect of seemingly blocked atrial
impulses on subsequent AV conduction.
Shifting the position of a single premature
atrial impulse within a constant driving cycle
of the atria produced graded effects of
"blocked" atrial impulses on AV junctional
refractoriness, permitting an estimation of the
duration of the "phase of concealed AV conduction."
Interpolation of such premature atrial impulses into successive driving cycles resulted
in "repetitive concealed conduction."
In an artificially produced atrial parasystole
there was observed "concealed discharge" of
a subsidiary (escaping) AV junctional pacemaker by an apparently nonconducted atrial
impulse.
With a catheter electrode in the right ventricle in a case of advanced AV block, concealed retrograde conduction of pacer stimuli
disturbed the rhythmicity of a spontaneous
AV junctional pacemaker.
In a case of advanced AV block with preCirctdation, Volume XXXII, September 1965
CONCEALED AV CONDUCTION
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served retrograde conduction (unidirectional
block), evidence of penetration of the upper
AV junction by the "blocked" antegrade impulse was found.
With electrodes implanted in the left ventricle in a case of advanced AV block, concealed retrograde conduction of the artificial
pacemaker stimuli enhanced antegrade conduction by transiently changing an area of
unidirectional block to one of supernormal
conduction.
Thus, all known manifestations of concealed
atrioventricular and ventriculo-atrial condluction, occurring spontaneously in clinical records or induced in animal experiments, were
artificially reproduced in the human heart.
References
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The effect of blocked impulses on the formation and conduction of subsequent impulses.
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3. LANGENDORF, R., AND PICK, A.: Concealed conduction. Further evaluation of a fundamental
aspect of propagation of the cardiac impulse.
Circulation 13: 381, 1956.
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Experimental Demonstration of Concealed AV Conduction in the Human Heart
R. LANGENDORF, A. PICK, A. EDELIST and L. N. KATZ
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Circulation. 1965;32:386-393
doi: 10.1161/01.CIR.32.3.386
Circulation is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
Copyright © 1965 American Heart Association, Inc. All rights reserved.
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