Download The Anatomy of the Atrioventricular Conduction System

The Anatomy of the Atrioventricular Conduction
System in Ventricular Septal Defect and
Tetralogy of Fallot: Correlations with the
Electrocardiogram and Vectorcardiogram
By ROBERT H. FELDT, M.D., JAMES W. DUSHANE, M.D.,
AND
JACK L. Tirus, M.D.
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patterns similar to the A-V canal pattern, that is,
an initially counterclockwise QRS loop in the
frontal plane with the mean axis directed superiorly and to the left (fig. 1A). Two of these
cases were studied; these were designated cases
Al and A2. Two other examples of tetralogy
(cases A3 and A4), which had VCG patterns
more representative of tetralogy, also were studied (fig. 1C).
None of the four examples selected had ventricular septal defects of the A-V canal type.8 The
defects in all four cases were located in similar
positions in the ventricular septum.
Ventricular Septal Defect
Seventy-nine examples of isolated VSD of the
usual type came to autopsy from 1958 through
1962. Seven of the 79 (8.9%) had preoperative
VCG patterns similar to the A-V canal pattern
and two of these (cases Bi and B2) were
studied (fig. 1B). Two other examples (cases
B3 and B4) in which the VCG patterns were
more typical of VSD also were studied (fig. 1D).
None of these four examples had the morphological features of VSD of the A-V canal type.8
IN RECENT YEARS the morphological features of the major portions of the atrioventricular (A-V) conduction system of human
hearts that were afflicted with various congenital anomalies have been described.1-6
These reports have contrasted the anatomy
of the A-V conduction system in examples
of ventricular septal defect (VSD) and tetralogy of Fallot with that configuration found
in defects of the persistent atrioventricular
(A-V) canal type.7 8
The vectorcardiographic (VCG) pattern
found in the A-V canal has been considered
to be characteristic.9-12 It is now known, however, that various congenital heart defects have
a similar vectorcardiographic pattern, including occasional examples of VSD and tetralogy
of Fallot.13-l5 Therefore, a study was made
of the morphology of the A-V conduction
system in instances of VSD and tetralogy of
Fallot that had frontal plane VCG patterns
similar to those usually found in the A-V
canal. An effort was made to find a characteristic or characteristics of the anatomy of
the conduction system that might be associated with the unusual VCG pattern observed
in these examples.
Methods
The morphology of the conduction system in
each of the eight cases studied was reconstructed
with the use of a previously established technique
of multiple histological sections.5 The blocks of
tissue for sectioning included the atrial and ventricular septa on each side of the atrioventricular
rings and included the atrioventricular node, the
common bundle of His, and the proximal portions
of the right and left bundle branches. The more
peripheral bundle branches were not studied.
The tissue blocks were sectioned at a thickness
of 8tt, and every twentieth or fortieth section
was stained with hematoxylin and eosin and
examined. The next succeeding section was
stained with the Mallory-Heidenhain stain and
examined. In each example, additional sections
were stained and studied as needed to elucidate
certain details. Approximately 300 sections were
examined in each case.
Material
Tetralogy of Fallot
Of 78 examples of tetralogy in which autopsy
was performed from 1958 through 1962, five
(6.4%) were associated with preoperative VCG
From the Mayo Clinic and Mayo Foundation,
Rochester, Minnesota.
This investigation was supported in part by Training Grant HTS-5515 from the National Institutes of
Health, U. S. Public Health Service.
774
C
irculation, Volume XXXIV, November 1966
ATRIOVENTRICULAR CONDUCTION SYSTEM
775
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Figure 1
The frontal plane projection of the QRS loop schematically adapted from the scalar electrocardiogram of some of the cases selected for study. (A) Pattern noted in an example of tetralogy
of Fallot considered similar to the A-V canal pattern. (B) Pattern noted in a VSD example considered similar to the A-V canal pattern. (C) Pattern considered representative of those usually
seen in tetralogy. (D) Pattern considered representative of VSD.
The lengths of various portions of the conduction system were measured (schematically
represented in fig. 2). The length of the common
bundle measured from its origin at the A-V node
to the point at which left bundle branching began
was termed "A." The length of the common
bundle from which left bundle branches were
Circulation, Volume XXXIV, November 1966
given off was termed "B." The length of conduction fibers destined to become the right bundle
branch but before they came to lie on the right
side of the ventricular septum and to take an
inferiorly (apexward) directed course was termed
"C." ABC was the total length of the A-V
conduction system studied, but in fact represented
776
FELDT ET AL.
A
AVN
B
C
AVN
| RB
A
B
0D
LB
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Figure 2
Schematic representation of the two configurations
(1 and 2) of the conduction system found in this
study. A represents the length of common bundle from
its origin at the A-V node (AVN) to the point at which
left bundle branching (LB) begins. B represents that
portion of common bundle involved in left bundle
branching. C represents the length of the fibers destined to become the right bundle branch (RB) but
before they take a definite rightward and inferior
course. D represents the distance from the A-V node
to the point at which the right bundle branch took a
definite rightward and inferior course in those examples in which the right bundle branch originated before left bundle branching was completed.
the length of conduction tissue from the A-V
node to the point at which the right bundle branch
assumed its usual anatomic relationships on the
right side of the ventricular septum.
In instances in which the right bundle branch
originated before left bundle branching was completed (configuration 2, fig. 2), the distance from
the A-V node to the point at which the right
bundle branch turned to the right side was
called "D." In comparisons of various measurements, the ABC distance in configuration 1 of
figure 2 was assigned the same meaning as the
D distance in configuration 2. The actual measurements obtained for the eight specimens studied
are listed in table 1.
Results
Morphological Findings
In all eight examples, the A-V node
appeared to be formed by the fusion of
finger-like fascicles of atrial myocardium in
the region of the artery to the node (fig. 3A
and B). The majority of these fibers entered
the region of the node from the posterior
aspect of the septum, with lesser numbers
from the region of the atrial septum superior
to the node. In two examples, cases A2
and B2, the node was situated more posteriorly relative to the coronary sinus ostium than
in the others.
In all examples, the common bundle
(bundle of His) was identifiable in the floor
of the right atrium for only a short distance
before it penetrated the atrioventricular ring
and central fibrous body (fig. 4A). It was
situated on the left endocardial surface of
the ventricular septum after passing through
the fibrous tissue.
The left bundle branching in each case
consisted of a series of fascicles given off
the common bundle in a continuous fashion
(fig. 4B). In none of the examples could
Table 1
Actual Distances* Measured
Case
Age (yr)
Heart weight
g
Al
A2
A3
A4
18
17
9
5
640
460
207
184
Case
BI
B2
B3
B4
Age (yr)
2
232u
5/12
7
Tetralogy of Fallot
A
B
Distances (mm)
C
AB
ABC
2.0
2.0
3.6
5.6
4.8
3.6
4.4
6.4
10.0
7.0
1.0
1.0
6.8
5.6
8.0
12.0
16.8
12.6
9.0
13.0
Distances (mm)
C
AB
ABC
Ventricular septal defect
Heart weight
g
A
B
109
81
62
186
2.0
2.8
3.6
1.8
2.3
8.4
1.0
5.6
12.5
-
1.0
-
4.3
11.2
9.2
14.3
D
5.3
12.2
3.6
6.0
*See text and figure 2 for definitions.
Circulation, Volume XXXIV, November 1966
ATRIOVENTRICULAR CONDUCTION SYSTEM
777
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Figure 3
Histology of the conduction system. AS designates the atrial septum; VS, the ventricular
septum; MV, the mitral valve; TV, the tricuspid valve; VSD, the ventricular septal defect; CB,
the common bundle; LB, a left bundle branch; RB, the right bundle branch. All figures represent sections stained with hematoxylin-eosin. (A) Region of the artery to the node (arrow) showing fibers destined to enter the A-V node (x 10). (B) Region of the A-V node (arrow) (x 10).
these fascicles be separated into distinct radiating bands in their proximal 1 to 2 mm.
In each case the right bundle branch was a
discrete bundle of fascicles (fig. 4C).
Conduction System in Tetralogy of Fallot
The four examples of tetralogy
were
similar
in that conduction tissue was in close approximation to the postero-inferior rim of the septal
defect (fig. 5A and C). Cases Al and A2,
Circulation, Volume XXXIV, November 1966
the examples with a VCG pattern similar to
the A-V canal pattern, differed from cases
A3 and A4, the examples with the usual VCG
pattern of tetralogy of Fallot, in that the
fascicles destined to become the right bundle
branch took an elongated and circuitous
course. In case A2 this abnormal course appeared to result from the position of the
septal defect, but in case Al this explanation
did not pertain.
FELDT ET AL.
778
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Figure 4
Histology of conduction system (continued from figure 3). Abbreviations same as in figure 3. (A)
Region at which the common bundle (arrow) penetrates the fibrous tissue separating atria and ventricles
(x1O). (B) Region of the common bundle (CB), posterior and inferior to the VSD, showing the left bundle-branch fascicles (LB) (x 10). (C) Longitudinal
course of the right bundle branch (RBB) in the upper
part of the ventricular septum (x 8).
In addition, in cases Al and A2 the left
bundle branching (the A distance) began a
shorter distance from the A-V node than in
A3 and A4. The actual measurements
(table 1) of this distance were 2.0 mm in
both Al and A2, whereas in cases A3 and
cases
Circulation, Volume XXXIV, November 1966
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779
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Figure 5
Schematic representation of the conduction system. CS indicates the coronary sinus; TV, the
tricuspid valve; VSD, the ventricular septal defect. The AV node is a white oval; the common
bundle before left bundle branching is represented by a bar line with close interruptions; the
common bundle with left bundle branching is the bar line with longer interruptions; the solid
white or black bar lines represent the course of the right bundle branch. (A) The configuration
found in an example of tetralogy with a representative VCG pattern. (B) The configuration in
an example of VSD with a representative VCG pattern. (C) The configuration in an example
of tetralogy with a VCG pattern samilar to the AV canal pattern. (D) The configuration in an
example of VSD with a VCG pattern similar to the AV canal pattern.
A4 the distances were 3.6 and 5.6 mm.
Specimens in cases Al and A2 were larger
than in cases A3 and A4.
In order to compare measurements among
different sized hearts, various ratios were
calculated (table 2). These ratios represented
the length of a portion of the conduction
system to the total length of the system as
defined.
Circulation, Volume XXX1V, November 1966
Comparisons of the ratios obtained from
measurements of length indicated differences
in cases Al and A2 from cases A3 and A4
(table 2): The ratios of B to ABC were
relatively smaller in cases Al and A2 than
in cases A3 and A4. This indicated that the
length of common bundle involved in left
bundle branching was relatively smaller in Al
and A2. The relatively larger ratios of C to
FELDT ET AL.
780
Table 2
Measurement Ratios*
Case
Al
A2
A3
A4
Bi
B2
B3
B4
A/ABC
B/ABC
C/ABC
Tetralogy of Fallot
0.60
0.12
0.29
0.16
0.29
0.56
0.40
0.49
0.11
0.08
0.43
0.49
Ventricular septal defect
0.43
0.19
0.38
0.23
0.69
0.08
A/D
B/D
CID
1.56
1.0
1.0
1.0
2.0
0.3
AB/ABC
0.40
0.44
0.89
0.92
0.81
0.91
AB/D
2.56
2.38
*See text and figure 2 for definitions.
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ABC in cases Al and A2 substantiated the
impression that the right bundle branch was
elongated in these cases in comparison to
cases A3 and A4. The relatively smaller A
to ABC ratios in cases Al and A2 reflected
the differences already noted in actual measurements.
Conduction System in VSD
All four examples of VSD studied were
similar to the examples of tetralogy of Fallot
in that conduction tissue was in close approximation to the edge of the ventricular
septal defect. Two different morphological
configurations were observed (fig. 5 B and D).
One pattern (cases BI and B2) was similar
to that observed in the cases of tetralogy,
whereas the other configuration resulted from
the fact that the right bundle branch originated from the common bundle before left
bundle branching was completed (cases B3
and B4).
Comparisons of the actual measurements of
the A distances (table 1) did not show consistent differences between those cases with
a vectorcardiographic pattern similar to that
of A-V canal (cases Bi and B2) and those
with representative vectorcardiographic patterns of VSD (cases B3 and B4). Although
the A distances in cases Bi and B2 were
less than in case B3, this measurement in
case B4 was less than that of any of the
others.
The ratio of AB to ABC or AB to D was
the only measurement ratio that appeared to
distinguish cases Bi and B2, the examples
with VCG patterns similar to the A-V canal
pattern, from cases B3 and B4. The ratios
indicated that cases Bi and B2 had a relatively longer distance from the A-V node to
the point at which the right bundle actually
turned to the right than cases B3 and B4.
Discussion
The source of the fibers contributing to
the formation of the A-V node was similar
to that previously reported."' The position
of the A-V node was normal in all but one
example of tetralogy of Fallot and one of
VSD. The close relationship of the major
A-V conduction tissue and the posterior rim
of the ventricular septal defect has been reported many times in tetralogy of Fallot and
the usual variety of VSD.'-0
The finding of a distinct, compact right
bundle branch is well known. Its apparent
"early" origin (cases B3 and B4) has been
reported previously in VSD and has been
considered a variant of normal.17
Previous studies2' 5 have indicated that the
left bundle-branch system is a continuous
sheet of fascicles leaving the common bundle
as observed herein. We were unable to recognize the division of these left bundle-branch
fibers in their proximal 1 to 2 mm into specific
anterior and posterior radiations as has been
described previously.18
Although apparent elongation of the major
A-V conduction tissue has been reported
previously in A-V canal defects and was
presumably due to distortion by the defect
itself,7' 8 previous studies have not indicated
that similar elongation may occur in tetralogy
of Fallot.4 0) It seems significant that both
examples of tetralogy with VCG patterns
similar to the A-V canal pattern had elongation of the course of the fibers destined to
become the right bundle branch. If this
elongation were not related necessarily to the
position of the septal defect, which appeared
to be the situation in case Al, it might be
considered to be a specific anomaly per se.
Conceivably, the anomaly might exist without
a septal defect at all and thus suggest an
Circulation, Volume XXXIV, November 1966
ATRIOVENTRICULAR CONDUCTION SYSTEM
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explanation for the finding of an abnormal
VCG in normal relatives of children with A-V
canal defects.19
The course of the fibers destined to become
the right bundle branch was not obviously
elongated in any of the examples of VSD
studied; however, comparison of the measurement ratios indicated differences similar to
those found in the cases of tetralogy of
Fallot. Thus, the distance from the A-V node
to the origin of the right bundle branch was
relatively greater in the two examples of VSD
with VCG patterns similar to the A-V canal
pattern than in the examples with the usual
VCG pattern.
Comparisons of the actual lengths of the
major portions of the A-V conduction system
in different hearts revealed that, with one
exception, short A distances were associated
with vectorcardiographic patterns similar to
those seen in A-V canal. The converse situation
would be expected if heart size had been
the determining factor; that is, the A distance
would have been longer in cases Al and A2
than in cases A3 and A4. In recognition
of the relationship between heart size and
such measurements, the measurement ratios
were defined to permit comparisons between
various specimens. The actual linear measurements were readily verified in each instance
since the thickness of each histological section
and the exact number of sections were known.
Relative elongation of the right bundle
branch, in combination with a short distance
from the A-V node to the start of left bundle
branching noted in instances of tetralogy of
Fallot and in one of two examples of VSD
that had VCG patterns similar to A-V canal,
was considered to have created an anatomic
situation in which the left bundle branches
could have received a conducted impulse
relatively earlier than in the normal heart.
This anatomic configuration is compatible
with the findings of Roos and Durrer20 in
which epicardial excitation times were measured in examples of ostium primum defects.
They found that the posterobasal portion of
the septum, presumably supplied by the initial
branches of the left bundle, was excited
Circulation, Volume XXXIV, November 1966
781
earlier than normal. In addition, the anatomic
configuration found in this study would permit
early excitation of the left bundle as suggested
by Grant and associates2' to occur in preexcitation syndrome. In regard to Grant's
hypothesis, however, our findings indicated
that the impulse may not reach the left
bundle branch earlier because of an anomalous route of excitation as has been suggested
to occur in pre-excitation, but rather because
the conduction system was anatomically suited
for relatively early impulse conduction to the
left bundle branches.
Burchell and associates22 had suggested
earlier that the abnormal vectors observed in
A-V canal may result from an imbalance of
the electric forces which might be related to
an anatomic abnormality of the left bundle
system. Since our study did not include the
peripheral portions of the left bundle branching system, we cannot comment on this
possibility as an alternate view to that suggested above.
Summary
The atrioventricular conduction system was
studied by a technique of selected serial
histological sections in examples of tetralogy
of Fallot and ventricular septal defect to
correlate the anatomic findings observed with
the electrophysiological findings as manifested
by the vectorcardiographic pattern. Eight
hearts were studied; two examples of each
anomaly had electrocardiographic and VCG
patterns in the frontal plane similar to A-V
canal defects and two examples of each had
ECG and VCG patterns usually associated
with these lesions.
Of the four examples of tetralogy, the two
examples with VCG patterns similar to the
A-V canal pattern differed from the other
examples in that the distance between the
A-V node and the origin of left bundle
branching was shorter and the fibers destined
to become the right bundle branch took an
elongated course. The VSD examples offered
less striking differences. A short distance
from the A-V node to the onset of left bundle
branching was seen in both examples with
FELDT ET AL.
782
VCG patterns similar to the A-V canal pattern, but one example of VSD with a representative VCG pattern had a similar anatomic
configuration. When measurement ratios were
compared, the right bundle took its origin at
a relatively greater distance from the A-V
node in the examples with VCG patterns
similar to the A-V canal pattern.
The anatomic configuration of those examples of tetralogy and VSD with VCG patterns
similar to the A-V canal pattern was such
that relatively early conduction to the left
bundle branch system could occur.
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1.
2.
3.
4.
5.
6.
7.
8.
9.
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Circulation, Volume XXXIV, November 1966
The Anatomy of the Atrioventricular Conduction System in Ventricular Septal
Defect and Tetralogy of Fallot: Correlations with the Electrocardiogram and
Vectorcardiogram
ROBERT H. FELDT, JAMES W. DUSHANE and JACK L. TITUS
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Circulation. 1966;34:774-782
doi: 10.1161/01.CIR.34.5.774
Circulation is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
Copyright © 1966 American Heart Association, Inc. All rights reserved.
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