Bachmann’s bundle ( interatrial tract )

 Bachmann’s bundle (BB), also known as the interatrial bundle, is well recognized as a muscular bundle comprising of parallel aligned myocardial strands connecting the right and left atrial walls and is considered to be the main pathway of interatrial conduction. Disruption of the bundle’s structure causes interatrial conduction block (IAB), which is associated with development of various atrial tachyarrhythmias and with electromechanical dysfunction of the left atrium. Technological progress providing sophisticated mapping and imaging techniques in the past decade has increased our knowledge of specific anatomic structures and their role in development of both atrial brady- and tachyarrhythmias. 

Bachmann’s bundle

In 1963, Thomas N. James described 3 pathways connecting the sinus node to the atrioventricular node (AVN), namely the anterior, medial, and posterior internodal pathways.

Whether these conduction pathways were because of the presence of specialized conduction tissue or because of the anisotropic orientation of the muscle fibers remains controversial. Nevertheless, James described the anterior pathway as leaving the sinus node in anterior direction and giving off a secondary branch at the level of the superior vena cava to form BB. BB stretches subepicardially across the interatrial groove (septal raphe). It is at the interatrial groove that the BB can be identified as a discrete bundle separated by fatty tissues from the infolded right atrial wall that is the limbus of the oval fossa. Notably, the bundle is not surrounded by a fibrous tissue sheath. Instead, the bundle is comprised of strands of atrial myocardium that are similarly aligned in parallel fashion.

Heart viewed from the front has the tips of the right and left atrial appendages (RAA and LAA) pulled back (left). Blunt dissection reveals Bachmann’s bundle (BB) crossing the interatrial groove (white arrows). The black arrows mark the rightward and leftward extensions. The blue oval marks the anticipated site of the sinus node. Longitudinal cut through the left heart (right) displays BB cut in cross-section (*). Note that it adds considerably to the thickness of the left atrial (LA) wall in this heart. LSPV indicates left superior pulmonary vein; RSPV, right superior pulmonary vein; and SVC, superior vena cava.

Its rightward and leftward extensions bifurcate to pass to either side of the right and left atrial appendages. Although they can be traced to varying extents with blunt dissection, both extensions blend into the musculature of the atrial walls. The superior arm of the rightward extension arises in the region of the cavoatrial junction close to the site of the sinus node and in the vicinity of the sagittal bundle. The inferior arm arises in the subepicardium of the right atrial vestibule. 

More details:  📖 Hurst’s the Heart, 14th Edition: Two Volume Set

Leftward, BB buttressing part of the anterior atrial wall with its thickness  is still traceable to where it encircles the neck of the left atrial appendage and blends in with the lateral atrial wall. The superior part traverses in the infolding of the atrial wall, known to arrhythmologists as the left lateral ridge, to pass in front of the orifices of the left pulmonary veins.8 The inferior part descends toward the atrial vestibule to combine with the circumferentially aligned myocardial strands in the subepicardium of the inferior wall.

BB myocytes are organized into relatively well-aligned myocardial strands or myofibers in the subepicardium and differ in orientation and direction from those in the underlying atrial walls. A perpendicular myofiber orientation has been observed at the junction with the superior caval vein, whereas myofibers are orientated more randomly at the level of the interatrial septum BB myocytes are surrounded by thin septa formed of tightly packed collagen fibrils running uninterrupted over distances of 392 µm (ie, ≈4 times the average myocyte length). Over larger distances, those thin septa form several interconnections. Thick septa are present near the surface of BB, where they often encircle groups of myocytes.

Sherf et al suggested that BB contained myocytes specialized for rapid conduction. These myocytes differed from ordinary atrial myocytes by a relative paucity of organized myofilaments and by their lack of a transverse-axial tubular system. Based on electron microscopy,  different types of myocardial cells in BB were distinguished. Dependent on their cytological features, these myocardial cells were labeled as myofibril-rich, myofibril-poor, broad transitional cells, slend transitional cells, and P-cells. Myofibril-rich cells in BB did not differ from myocytes in other parts of the atrial myocardium. Myofibril-poor cells are Purkinje-like cells that are not only numerous in BB but also in other interatrial conduction pathways. P-cells in BB are very similar to P-cells present in the sinoatrial node (SAN) and AVN. Slender transitional cells are shorter and narrower than broad transitional cells, but both broad and slender transitional cells show similarities with myofibril-rich and myofibril-poor cells. They possess many myofibril-dense zones and also intracellular spaces filled with cytoplasm. However, other investigators were not able to find specialized myocytes.

Jean George Bachmann (1877 - 1959), German-American physiologist.

Jean George Bachmann

Jean George Bachmann discovered the function of the intraauricular Bundle. Born in Germany, he studied at the Jefferson Medical College, Philadelphia, where he was conferred doctor of medicine in 1907. He worked as a resident in physiology at the Jefferson Hospital, and was demonstrator of physiology at the college. He was professor of physiology at the Atlanta College of Physicians and Surgeons 1910-1915, and in 1915 followed a call to the Emory University, School of Medicine in Georgia.

References

1: Israel CW, Burmistrava T, Berger C. Bachmann-Bündel-Pacing [Bachmann bundle

pacing]. Herz. 2018 Nov;43(7):584-595. German. doi: 10.1007/s00059-018-4755-6.

PMID: 30255305.

2: Murariu E, Frigy A. Bayés' Syndrome-A Comprehensive Short Review. Medicina

(Kaunas). 2020 Aug 13;56(8):410. doi: 10.3390/medicina56080410. PMID: 32823777;

PMCID: PMC7466178.

3: Russo V, Rago A, Papa AA, Arena G, Politano L, Nigro G. Bachmann bundle

pacing reduces atrial electromechanical delay in type 1 myotonic dystrophy

patients. J Interv Card Electrophysiol. 2018 Apr;51(3):229-236. doi:

10.1007/s10840-018-0331-5. Epub 2018 Feb 27. PMID: 29488106.

4: Das A. Electrocardiographic features: Various atrial site pacing. Indian

Heart J. 2017 Sep-Oct;69(5):675-680. doi: 10.1016/j.ihj.2017.08.030. Epub 2017

Sep 1. PMID: 29054201; PMCID: PMC5650557.

5: Teuwen CP, Does LJMEV, Kik C, Mouws EMJP, Lanters EAH, Knops P, Taverne YJHJ,

Bogers AJJC, de Groot NMS. Sinus Rhythm Conduction Properties across Bachmann's

Bundle: Impact of Underlying Heart Disease and Atrial Fibrillation. J Clin Med.

2020 Jun 16;9(6):1875. doi: 10.3390/jcm9061875. PMID: 32560096; PMCID:

PMC7355925.

6: Bailin SJ. Is Bachmann's Bundle the only right site for single-site pacing to

prevent atrial fibrillation? Results of a multicenter randomized trial. Card

Electrophysiol Rev. 2003 Dec;7(4):325-8. doi:

10.1023/B:CEPR.0000023131.13609.ab. PMID: 15071246.

7: Kliś M, Sławuta A, Gajek J. Antiarrhythmic properties of atrial pacing. Adv

Clin Exp Med. 2017 Mar-Apr;26(2):351-357. doi: 10.17219/acem/61429. PMID:

28791857.

8: Khaja A, Flaker G. Bachmann's bundle: does it play a role in atrial

fibrillation? Pacing Clin Electrophysiol. 2005 Aug;28(8):855-63. doi:

10.1111/j.1540-8159.2005.00168.x. Erratum in: Pacing Clin Electrophysiol. 2005

Sep;28(9):1012. PMID: 16105015.

9: Ishiguchi H, Yoshida M, Ishikura M, Kawabata T, Oda T. Successful ablation of

biatrial tachycardia with preserved electrical activation of left atrial

appendage by unidirectional connection via Bachmann's bundle: A case report.

Indian Pacing Electrophysiol J. 2020 Jul-Aug;20(4):166-169. doi:

10.1016/j.ipej.2020.04.006. Epub 2020 Apr 17. PMID: 32305464; PMCID: PMC7384361.

10: Seoane L, Cortés M, Conde D. Update on Bayés' syndrome: the association

between an interatrial block and supraventricular arrhythmias. Expert Rev

Cardiovasc Ther. 2019 Mar;17(3):225-235. doi: 10.1080/14779072.2019.1577137.

Epub 2019 Feb 6. PMID: 30715961.

bachmann's bundle function, bachmann bundle pronunciation, bachmann's bundle location, bachmann bundle block ecg, bachmann bundle pacing, bachmann's bundle quizlet, bachmann's bundle atrial fibrillation, the bachmann bundle and interatrial conduction, bachmann's bundle, bachmann's bundle rate, bachmann's bundle anatomy, bachmann bundle ablation, what is bachmann's bundle, bachmann bundle block, bachmann's bundle vs bundle of his, bachmann bundle definition, bachmann bundle ecg, what is the function of vascular bundle, bachmann's bundle heart, what does a heart bundle mean, what is a heart bundle, bachmann's bundle is located in the left atrium, bachmann's bundle is located in the, what is left atrium in heart, what supplies the left atrium, where is left atrium located, how does the left atrium work, bundle of bachmann, why is av bundle called bundle of his, av bundle vs bundle of his, bachmann's bundle purpose