(Circulation. 2001;103:e63.)
© 2001 American Heart Association, Inc.
Correspondence |
Anatomy of the Atrioventricular Conduction System
Professor of Pathology, Rush Medical College, Rush-Presbyterian-St. Luke’s Medical Center, Chicago, Illinois
To the Editor:
I am writing regarding the article by Racker and Kadish1 that appeared in a previous issue of Circulation. Their statement that "the atrioventricular (AV) node and AV bundle consist of myocardial fascicles, not myofibers"1 is inappropriate.2 Dorland’s Illustrated Medical Dictionary defines fascicle as "a small bundle or cluster, especially of nerve or muscle fibers." The human conduction system has more collagenous connective tissue and elastic tissue than the surrounding myocardium, and this may be considered the hallmark of the conduction system histologically.2 Lev et al2 did not mention anything about nerves in their work cited by Racker and Kadish.
Racker and Kadish’s interpretation of Figures 1 and 4 from the article by Lev et al2 needs further clarification.1 They state that "Lev et al’s Figure 1 partially overlooked the proximal AV bundle, as seen in schematic AV junction region tissue blocks."1 This statement is inaccurate. Figure 1 in the article by Lev et al2 clearly demonstrates the entire AV conduction system, including the approaches to the AV node. Likewise, Racker and Kadish’s statement that "Lev et al’s Figure 4 is, in fact, restricted to the proximal AV bundle/AV node junction"1 is inaccurate. Lev et al’s Figure 42 is a photomicrograph of the AV node and nothing but the AV node. Further, my colleagues and I previously documented that the electrophysiological data from the AV junction more closely approximated pathological findings.3 4 5
The conduction system can be sectioned in any method one chooses and may be labeled in any way one wishes. However, it should be emphasized that the atria, ventricles, and AV conduction system are not straight in size, shape, or form. The AV conduction system is in the form of a curve or an arc. The significance of anatomy is its function. There is no anatomy without function and there is no function without anatomy. A semiquantitative analysis of the conduction system for correlative studies with electrophysiological studies can only be obtained if blocks are taken in such a manner that the entire conduction system can be followed from the beginning to the periphery and every 20th section is examined at levels of 5 to 7 µm.
References
1.
Racker
DK, Kadish AH. Proximal atrioventricular bundle, atrioventricular node,
and distal atrioventricular bundle are distinct anatomic structures
with unique histological characteristics and innervation.
Circulation. 2000;101:1049–1059.
2. Lev M, Widran J, Erickson EE. A method for the histopathologic study of the atrioventricular node, bundle, and branches. Arch Pathol. 1951;52:73–83.
3.
Bharati S,
Bicoff JP, Fridman JL, et al. Sudden death caused by benign tumor of
the atrioventricular node. Arch Intern
Med. 1976;136:224–228.
4. Narula OS, Bharati S, Chan MC, et al. Micro transection of the His bundle with laser radiation via a pervenous catheter correlation of histological and electrophysiological data. Am J Cardiol. 1984;54:186–192.[Medline] [Order article via Infotrieve]
5. Huang SK, Bharati S, Graham AR, et al. Closed chest catheter desiccation of the atrioventricular junction using radiofrequency energy: a new method of catheter ablation. J Am Coll Cardiol. 1987; 9:349–358.
Response
Departments of Medicine and Cardiology and the Feinberg Cardiovascular Research Institute, Northwestern University Medical School, Chicago, Illinois
Unlike what Bharati states, myocardial (and nerve terminal) fascicles with collagen encasement of the atrioventricular (AV) node and distal (and proximal) AV bundle are corroborated by direct observation in 2 different orthogonal planes and exemplify Dorland’s definition. Dr Lev and his colleagues performed a series of meticulous studies and made major contributions to the understanding of cardiac anatomy and pathology. We are aware that Dr Bharati worked closely with Dr Lev for several years and may have insights into Dr Lev’s thinking that are not in press. We were only able to comment on his published work, and we are sure that were Dr Lev alive today he would deny neither the simple, clear-cut anatomic demonstrations nor the technological advances that permitted their delineation.
Bharati is correct in stating that the article by Lev et alR1 that we cited did not mention nerves. However, no mention of collagen was made either. Instead, they reported erroneously that elastic tissue surrounds the myofibers. Lev’s node is composed of a loose network of myofibers, is associated with only mitral valve connective tissue, and is not the interwoven node described by Tawara.R2
We rightly claimed that Lev et alR1 overlooked the proximal AV bundle (Tawara’s atrial bundle), because the proximal AV bundle is a relatively broad tissue, comprised of parallel fascicles, that extends to the mouth of the coronary sinus, outside the right atrium and at the level of the left atrium. Innumerable ganglia are present, but they are only apparent in transverse sections, as are the 4 heart chambers. None of these attributes are depicted in Lev et al’s histological sections, which are based on the transverse plane.
Bharati claims the right "to label the conduction system as one wishes." However, we followed the precedent set by Tawara because Tawara’s atrial bundle, node, and (distal) AV bundle are replicated in our sections. Bharati is correct in stating that blocks must be taken so that the full conduction system can be followed. However, such block(s) must be inclusive of the proximal AV bundle (and the atrionodal bundles), and the plane of the section is critical for specific observations.
Significantly, correlated functional studies corroborate our anatomic designations for the AV junction’s specialized and ordinary atrial tissuesR3 by (1) tissue-specific extracellular and intracellular electrical potentials and conduction properties recorded during direct observation of the AV node and simultaneous recordings from all the tissuesR4 R5 R6 and (2) by restriction of iontophoresed Lucifer yellow to fascicular compartments of the dye-injected myocyte.R7
References
1. Lev M, Widran J, Erickson EE. A method for the histopathologic study of the atrioventricular node, bundle, and branches. Arch Pathol. 1951;52:73–83.
2. Tawara S. Die topographie und histologie der bruckenfasern: ein beitrag zur lehre von der bedeutung der Purkinjeschen faden. Zentralbl Physiol. 1906;19:70–76.
3.
Racker DK, Ursell
PC, Hoffman BF. Anatomy of the tricuspid annulus: the circumferential
myofibers as the structural basis for atrial flutter in a canine model.
Circulation. 1991;84:841–851.
4. Racker DK. Sinoventricular transmission in 10 mM K+ by the canine atrioventricular inputs, the superior atrionodal bundle, and the proximal atrioventricular bundle. Circulation. 1991;83:1745–1758.
5. Racker DK, Whaler GM, Kadish AH. Atrionodal bundles have unique intracellular and extracellular properties confirming the presence of a specialized conduction system in the right atrium. Pacing Clin Electrophysiol. 1998;21:920. Abstract.
6. Racker DK, Kadish AH. The canine atrionodal bundle multilimb input to the AV node possess unique intracellular and extracellular potentials. FASEB J. 1999;13(4):A105. Abstract.
7. Racker DK. The demonstration of intercellular coupling between cells of the canine distal AV bundle using Lucifer yellow-CH and histological sections. J Mol Cell Cardiol. 1989;21:479–493.[Medline] [Order article via Infotrieve]
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