The Cardiac Conduction System





The Sinoatrial Node


The sinoatrial node (SAN) is the pacemaker of the human heart ( Fig. 28.1 ). In visceroatrial situs solitus, the normal pattern of anatomic organization of the viscera and atria, the SAN normally is located to the right of the superior vena cava (SVC; see Fig. 28.1 , left upper diagram ). The SAN is located in the sulcus terminalis (the terminal sulcus), a shallow sulcus or depression that marks the termination of the right horn of the sinus venosus—the smooth venous tissue that includes the terminations of the SVC and the inferior vena cava (IVC). The sulcus terminalis also marks the beginning of the free wall of the right atrium (RA) with its characteristic musculi pectinati (pectinate muscles). Pecten is a Latin noun meaning “comb.” The pectinate muscles of the right atrial appendage (RAA) are remarkably parallel, like the teeth of a comb. The SAN is also known as the sinus node because it is located in the right horn of the sinus venosus (venous sinus). However, the pacemaker is immediately adjacent to the RA; hence sinoatrial node.




Fig. 28.1


Where is the sinoatrial node (SAN) ? This is a reasonable question because to the unaided and uninformed eye, the SAN is invisible. Normally, in visceroatrial situs solitus, the SAN lies to the right of the superior vena cava (SVC) where it enters the right atrium (upper left diagram). The SAN is also called the sinus node because it is located in the smooth venous tissue that is confluent with the SVC inferior venae cava (IVC). This smooth (nontrabeculated) venous tissue is the right horn of the sinus venosus (venous sinus). Hence, the pacemaker is often called the sinus node, because that is where it is located—in the right horn of the sinus venosus. The pacemaker is in a shallow depression, the sulcus terminalis (the terminal sulcus). This is where the sinus venosus ends and where the trabeculated right atrial appendage begins. So the SAN is immediately adjacent to the right atrial appendage. Consequently, the pacemaker’s most widely used name is the sinoatrial node. The “head” of the SAN is laterally and to the right of the SVC, in the sulcus terminalis. The “body” of the SAN may extend downward in the sulcus terminalis toward the IVC. In viceroatrial situs inversus (right upper diagram), the SAN is inverted (right-left switched). The SAN is to the left of the left-sided SVC. In the heterotaxy syndrome with congenital asplenia (left lower diagram), there can be bilateral SVCs with bilateral SANs. In the heterotaxy syndrome with polysplenia (right lower diagram), the SAN can be absent, or difficult to find.

Reproduced with permission from Mullin MP, Van Praagh R, Walsh EP. Development and anatomy of the cardiac conducting system. In: Walsh EP, Saul JP, Triedman JK, eds. Cardiac Arrhythmias in Children and Young Adults with Congenital Heart Disease. Philadelphia, PA: Lippincott Williams & Wilkins; 2001:3.


The sulcus terminalis externally, where the SAN is located, and the crista terminalis (terminal crest) internally both mark the junction of the right horn of the sinus venosus with the RA. The SAN is an invisible subepicardial structure. That is why you have to know where the SAN is, because you cannot see the SAN with the naked eye. This is also why the SAN was not discovered until 1907 by Sir Arthur Keith (1866–1955), while he was working with Martin Flack (1882–1931). Keith was an anatomist and anthropologist, a Scot from Aberdeen, who made many important contributions. For example, he was the first to understand that typical D-transposition of the great arteries (TGA) is caused by infundibular inversion.


In visceroatrial situs inversus, the location of the SAN is a mirror-image of its location in visceroatrial situs solitus (see Fig. 28.1 , upper right diagram ), that is, to the left of the entering SVC, in the left horn of the sinus venosus.


In the heterotaxy syndrome with visceroatrial situs ambiguus and asplenia (see Fig. 28.1 , left lower diagram ), there often are bilateral SVCs and bilateral SANs.


In the heterotaxy syndrome with visceroatrial situs ambiguus and polysplenia (see Fig. 28.1 , right lower diagram ), the SAN can be absent or difficult to locate.


Heterotaxy is derived from two Greek words. Heteros means “other or different.” Taxis means “arrangement.” The heterotaxy syndromes with asplenia, polysplenia, or a right-sided spleen mean that the situs—the pattern of anatomic organization of the viscera and atria—is different from situs solitus (normal) and from situs inversus (the mirror-image of normal).


Ambiguus is a Latin word meaning “uncertain or going about”; from ambigere, “to wander about.” In anatomy, ambiguus has the connotations of uncertain, unknown, undiagnosed. This is what we mean when we use these terms. Others have a different interpretation. In the heterotaxy syndrome with asplenia, they talk about bilaterally right atria (right atrial isomerism) or about bilaterally RAAs (RAA isomerism). In the heterotaxy syndrome with polysplenia, they talk about bilaterally left atria (left atrial isomerism) or about bilaterally left atrial appendages (LAA isomerism).


We think that the concept of atrial level mirror-imagery (isomerism) is anatomically incorrect. Why? Because bilaterally right atria (RAs) and bilaterally left atria (LAs) have never been documented anatomically, accurately speaking.


Bilaterally RA would have to have bilateral IVCs, bilateral SVCs, bilateral coronary sinus ostia, bilateral superior limbic bands of septum secundum, and a SAN bilaterally. No such case has ever been documented. Bilaterally LA would have to have a septum primum bilaterally, four pulmonary veins bilaterally, and no SAN bilaterally. Again, no such case has ever been documented in a single individual. (Siamese twins are excluded.)


Our friends and colleagues concede these points. But then they may retreat to atrial appendage isomerism. We agree that in the asplenia syndrome both atrial appendages often are broad, triangular, and bilaterally “rightish” in appearance. Similarly, in the polysplenia syndrome, we agree that the atrial appendages both often appear small and narrow, that is, bilaterally “leftish.” We think that the appearances of the atrial appendages—bilaterally “rightish” in asplenia and bilaterally “leftish” in polysplenia—are for hemodynamic reasons.


In asplenia the IVC typically is intact and the atrial septum is often poorly formed because of the presence of common atrioventricular (AV) canal. Consequently, the full systemic venous return often can flow into both atrial appendages, distending both, making both atrial appendages appear like RAAs.


In polysplenia, the IVC often is interrupted—absent between the renal veins and the hepatic veins. The blood flow of the hepatic and suprahepatic IVC is much less than normal, that is, no subhepatic systemic venous return. The systemic venous blood returns to the heart via the azygos vein and by the SVC.


So the systemic venous return flows into the atrial level predominantly in a superior-to-inferior direction, passing behind (dorsal to) the atrial appendages, and failing to dilate the atrial appendages, which therefore are bilaterally undistended and bilaterally “leftish” in appearance.


But what about the SAN data? Remember that the SAN is often just called the sinus node, not the sinoatrial node. The SAN tells you where the sinus venosus and SAN are located. It does not necessarily tell you that all of the other features of the RA are there also. So, a bilateral SAN does not mean that the RA is bilateral. Instead, it only means that the sinus venosus and the SAN are bilateral. Similarly, bilateral absence of the SAN means bilateral absence of the SAN, not that a LA is bilateral.


It also should be mentioned that “partial” isomerism is a conceptual error, such as isomerism of the right or LAAs only. Consider D-glucose and L-glucose. These are real isomers.






Isomerism applies to complex structures such as the atria (see Fig. 28.1 , left upper and right upper diagrams ), and like D-glucose and L-glucose molecules, in which all asymmetrical groups are mirror-images. There are four asymmetrical groups. If only one or two asymmetrical groups were mirror-images, but the other asymmetrical groups were not mirror-images, these two molecules would not be mirror-images (isomers). Isomerism is an all-or-nothing phenomenon. “Partial” isomerism—of the appendages only, but not the whole atria—is a mistake. Mirror-imagery (isomerism) applies to the whole structures, like the whole D- and L-glucose molecules, not to just some, but not all, of the asymmetrical parts of these molecules, or of other structures such as cardiac atria.


The Internodal and Interatrial Conduction Pathways


There are three internodal conduction pathways ( Fig. 28.2 ) between the SAN and the AVN: the anterior, middle, and posterior internodal pathways. There is one interatrial conduction pathway between the RA and the LA: the Bachmann bundle.




Fig. 28.2


Diagrammatic presentation of the three internodal conduction pathways and of the interatrial conduction pathway. The three internodal conduction pathways from the sinoatrial node (close to the superior vena cava) to the atrioventricular node (close to the tricuspid valve) are: the anterior (A) internodal pathway, the middle (M) internodal pathway, and the posterior (P) internodal pathway. The interatrial conduction pathway from the right atrium to the left atrium is the Bachmann bundle (BB).

Reproduced with permission from Anderson RH, Arnold R, Wilkinson JL. The conducting system in congenitally corrected transposition. Lancet 1973;1:1286.


The Atrioventricular Node and the Atrioventricular Bundle


Now that we have briefly considered the variable locations of the SAN in visceroatrial situs solitus, in visceroatrial situs inversus, and in visceroatria situs ambiguus (see Fig. 28.1 ), it is now time to consider the variations in the location of the AVN and the AVB.


Where is the AVN? Mentally, draw a line between the right atrial ostium of the coronary sinus and a point slightly below the anteroseptal commissure of the tricuspid valve adjacent to the atrial septum. I say draw this line mentally, not with a sucker in the operating room, because a sucker can get stuck on this line, and pulling it off this line can damage the invisible AVN and/or the invisible AVB. For convenient brevity, we call this line the coronary sinus–membranous septum line. Others use the triangle of Koch to localize the invisible AVN and AV (or His) bundle. The triangle of Koch is formed by (1) the origin of the septal leaflet of the tricuspid valve, (2) the Thebesian valve of the coronary sinus, and (3) the Eustachian valve of the IVC and its anterior extension, the tendon of Todaro. The AVN is close to the apex of the triangle of Koch.


Because the Thebesian valve of the coronary sinus and the Eustachian valve of the IVC are variable in their morphology and can be absent, and because the tendon of Todaro is difficult to visualize grossly (but is well seen histologically), we prefer to use the coronary sinus–membranous septum line to localize the AVN and the AVB.


Historical Note


Wilhelm His, Jr. (1863–1934) described the AVB in 1893; it is now widely known as the His bundle, or the AVB of His. Born in Basel, Switzerland, he moved to Leipzig, Germany as a child. He became a German citizen and a physician. In 1907, he became the director of the first medical clinic at the Charité Hospital in Berlin. He served in the army in World War I, during which he saw and described trench fever. He had a long-standing interest in gout and diet.


Walter Karl Koch (1880–1962) was a German physician from Berlin. He was a cardiologist, surgeon, and pathologist who described the triangle of Koch in which the AVN is sometimes referred to as Koch’s node.


Adam Christian Thebesius (1686–1732) was a German anatomist who worked at Leiden, the Netherlands. He described the small veins of the heart, the Thebesian veins. He also described the coronary sinus and its valve, the Thebesian valve, in 1708.


Bartolomeo Eustachio (1524–1574) was a professor in Rome at the Collegia della Sapienza. In 1552, he completed a superb set of anatomical plates, Tabulae Anatomicae, drawn by himself. These plates remained unprinted in the Papal Library for 162 years. Finally, Pope Clement XI presented the engraved plates to his physician Lancisi. On the advice of Morgagni, Lancisi published these plates with his own notes in 1714. They were the first anatomic plates on copper. These plates were more accurate in delineation than those of Vesalius. These plates revealed that Eustachius (Latin spelling of his name) had discovered the Eustachian valve of the IVC, the Eustachian tube, the thoracic duct, the adrenal glands in 1563 and the abducens (sixth) cranial nerve. Eustachio (Italian spelling of his name) also described the origins of the optic nerves, the cochlea, the pulmonary veins, and the muscles of the throat and neck. He also gave the first correct picture of the uterus, and he wrote the best description of his time of the structure of the teeth, giving the nerve and blood supply in 1563. Eustachio is regarded as a genius of discovery.


Francesco Todaro (1839–1918) was an Italian physician who described the Todaro tendon.


The Atrioventricular Node and Atrioventricular Bundle Continued


In 1958, Maurice Lev, who was one of my teachers and an outstanding investigator of the conduction system in congenital heart disease, published a paper concerning the conduction system in common AV canal ( Fig. 28.3 ). In a right lateral view of the opened RA, common AV valve, and right ventricle (RV), many of the most important anatomic details are shown, including the ostium of the coronary sinus, the posteroinferior AVN, the penetrating portion of the AVB, and the branching portion of the AVB.




Fig. 28.3


Diagrammatic presentation of the atrioventricular (AV) conduction system in common AV canal, right lateral view of right atrium and right ventricle. 1, superior vena cava; 2, inferior vena cava; 3, limbus of foramen ovale (superior limbic band); 4, patent foramen ovale; 5, cut edge of right atrial appendage; 6, right atrial ostium of coronary sinus; 7, base of tricuspid valve; 8, defect in the septum of the AV canal (between the atrial septum posteriorly and superiorly and the ventricular septum anteriorly and inferiorly), that is, an AV septal defect (AVSD); 9, infundibulum; 10, base of pulmonary valve; 11, muscle of Lancisi, or muscle of Lushka, or papillary muscle of the conus (or infundibulum), beneath which the right bundle branch of the conduction system appears; and 12, transection of the moderator band. The AV bundle runs inferiorly and anteriorly relative to the AVSD, and the top (crest) of the muscular ventricular septum is “scooped out” (concave).

Reproduced with permission from Titus JL. Normal anatomy of the human cardiac conduction system. Mayo Clin Proc. 1973;48:24.


In 1973, Jack Titus published a study of the normal human cardiac conduction system ( Fig. 28.4 ). This is a left posterolateral view. Titus found that in the average adult, the AVN measures 7.5 mm in length. Then the AVB begins. The first 1.5 to 2.0 mm are unbranched. This unbranched portion of the AVB penetrates from the atrial level to the ventricular level. Branching occurs at the ventricular level. The left posterior bundle branch travels to the posteromedial papillary muscle group of the left ventricle (LV). The left anterior bundle branch runs to the anterolateral papillary muscle group of the LV. Some fascicles of the left bundle branch (LBB) travel down the left ventricular septal surface between the anterior and posterior LBBs. The right bundle branch (RBB) is 1 mm wide (diameter) and about 50 mm in length.




Fig. 28.4


Diagrammatic presentation of the normal atrioventricular (AV) conduction system, left posterolateral view. In the average adult, the AV node (AVN) measured 7.5 mm in length. Then the AV bundle (AVB) began. The first 1.5 to 2.0 mm were unbranched. This portion of the AV bundle penetrates from the atrial level to the ventricular level. Branching occurs at the ventricular level: the posterior fascicle (fasc) or posterior left bundle branch (LBB); the anterior fascicle or the anterior LBB; and the right bundle branch (RBB) that was 1 mm wide (diameter) and about 50 mm in length. The normal branching AVB ran beneath the membranous septum and beneath the right coronary and noncoronary leaflets of the normally located aortic valve. The membranous septum and the branching AVB varied between 6.5 and 20 mm in length. MS, membranous septum.

Reproduced with permission, from Mullin MP, Van Praagh R, Walsh EP. Development and anatomy of the cardiac conducting system. In: Walsh EP, Saul JP, Triedman JK, eds. Cardiac Arrhythmias in Children and Young Adults with Congenital Heart Disease. Philadelphia, PA: Lippincott Williams & Wilkins; 2001:3.


The normal branching AVB runs immediately beneath the membranous septum, and beneath the right coronary and noncoronary leaflets of the normal aortic valve. The membranous septum and the branching AVB vary between 6.5 and 20 mm in length.


Surgical Relevance


Now you can see how misplaced surgical sutures (or devices) can produce various kinds of iatrogenic heart block:



  • 1.

    If the misplaced suture is superficial, the result can be RBB block (RBBB).


  • 2.

    If the misplaced suture is deeper, the result can be RBBB and left anterior hemiblock.


  • 3.

    If the misplaced surgical suture is even deeper, the result can be complete heart block. These are some of the reasons why a good understanding of the conduction system is so important in the interventional transcatheter management and in the surgical management of congenital heart disease.



The Communicating Atrioventricular Node


Many studies have confirmed that the communicating AVN usually is located posteroinferiorly, quite close to the right atrial ostium of the coronary sinus (see Fig. 28.3 ). By communicating AVN we mean an AVN that is confluent with the AVB and its branches. As we shall soon see, there is such a thing as a noncommunicating AVN, that is, an AVN that communicates with nothing.


The Course of the Atrioventricular Bundle


Many studies have confirmed that the branching portion of the AVB usually runs beneath (or inferior to) (1) the membranous portion of the ventricular septum normally, as in Fig. 28.4 ; or (2) inferior to the usual type of ventricular septal defect (VSD) between the infundibular septum above and the ventricular septum below (an infundibuloventricular type of VSD, as in tetralogy of Fallot); or (3) inferior to an AV septal defect, as in the complete form of common AV canal (see Fig. 28.3 ).


Why are the LBBs so much bigger than the RBB? The LBB enters the LV first and massively. The RBB enters the RV second, as a single, slim bundle (see Fig. 28.4 ). Why is there such a difference between the LBB and the RBB of the conduction system? It is hard to answer questions like this. But it is worth remembering that the RV is only about 36% as old phylogenetically as is the LV—180 million years (RV) as opposed to at least 500 million years (LV). And, of course, the LV is the systemic pump, whereas the RV is the lung pump. Fish and amphibia do not have an RV, whereas higher reptiles (such as alligators and crocodiles), birds, and mammals normally all do. Thus, in the evolution of our phylum Chordata (the Chordates), the RV and the RBB are Johnnies-come-lately compared with the LV and the LBB (see Fig. 28.3 ). The two fascicles, or radiations, of the LBB—the superior (or anterior) and the inferior (or posterior)—are clearly shown (see Fig. 28.3 ).


The Atrioventricular Conduction System in Transposition of the Great Arteries {S,L,L}


One of the most important form of congenital heart disease with AV situs discordance is congenital physiologically corrected TGA {S,L,L}. To the best of my knowledge, the location of the AV conduction system in TGA {S,L,L} was first figured out correctly and presented comprehensibly in 1973 and 1974 by Robert Anderson et al , ( Figs. 28.5 through 28.9 ), a great achievement.




Fig. 28.5


Congenital physiologically corrected transposition of the great arteries in visceroatrial situs solitus, that is, transposition of the great arteries (TGA) {S,L,L}, diagrammatic presentation viewed from above showing the atrioventricular (AV) conduction system. The normal AV node (AVN) is posterior (Posterior node), but it does not connect with either bundle branch; that is, the posterior AVN is noncommunicating. The anterior AVN is communicating; it does connect with the left bundle branch (LBB) and with the right bundle branch (RBB) of the AV conduction system. The anterior AVN is located in the medial wall of the right atrium (Right auricle). The anterior AVN lies to the right of the transposed pulmonary artery (PA) that originates from the right-sided morphologically left ventricle (LV). The right-sided AV valve is a bicuspid mitral valve because the morphology of the AV valves corresponds to that of the ventricle of entry, not to that of the atrium of exit. Thus, the right-sided AV connection (Ravc) between the right-sided right atrium and the right-sided LV is via a right-sided mitral valve. Similarly, the left-sided AV connection (Lavc) between the left-sided left atrium and the left-sided morphologically right ventricle is via a left-sided trileaflet tricuspid valve. The transposed aorta (Ao) is anterior and to the left of the transposed PA. A subpulmonary ventricular septal defect is present (Defect). The anterior AVN gives off an anterior AV bundle (AVB) that runs anteriorly relative to the transposed PA. Then the anterior AVB gives off the LBB that heads down into the right-sided LV. At about the same level, the anterior AVB also branches into the RBB that continues leftward into the left-sided right ventricle.

Reproduced with permission from Anderson RH, Becker AE, Arnold R, Wilkinson JL. The conduction tissues in congenitally corrected transposition. Circulation. 1974;50:911.



Fig. 28.6


Transposition of the great arteries (TGA) {S,L,L}, right lateral diagrammatic view of the right-sided morphologically right atrium (RA) and of the right-sided morphologically left ventricle (ML ventricle) showing the atrioventricular (AV) conduction system and a ventricular septal defect (Defect). The normal AV node (AVN) is present and posterior (Posterior node), but it does not communicate with the AV bundle (AVB) or bundle branches. The normal posterior AVN is hypoplastic and has no known function. A second AVN is present. It is anterior (Anterior node). The anterior AVN is located in the medial wall of the RA. It is to the right of the transposed pulmonary valve and slightly posterior to the pulmonary valve. The anterior AVN is the communicating AVN that connects with the AVB. In turn, the AVB connects with the left and right bundle branches. Here one can see that the anterior AVN and AVB connect with the right-sided left bundle branch (LBB) that flows into the right-sided left ventricle. Just as the AV valves correspond to the ventricles of entry, not to the atria of exit, so too the LV and RV bundle branches of the conduction system correspond to the ventricles of entry, not to the atria of exit. These are basic principles. The transposed aorta (Ao) is anterior and to the left of the transposed pulmonary artery (PA); that is, L-TGA is present. This heart diagram is drawn in the vertical heart position (ventricular apex pointing downward). This is the correct cardiac position for many adults. However, infants and children usually have a horizontal heart position (ventricular apex pointing horizontally). As drawn in the vertical heart position, the AVB runs in the anterior rim of the VSD. Now rotate this diagram 90 degrees counterclockwise, putting this heart diagram into a horizontal heart position, and you will see that the AV bundle is running in the horizontal or superior rim of the VSD. In the normal heart, and in most forms of congenital heart disease, there is a posterior communicating AVN, not an anterior communicating AVN as in TGA {S,L,L}. In most forms of congenital heart disease, the AVB runs in the posteroinferior rim of the VSD, not in the anterosuperior rim of the VSD, as in TGA {S,L,L}. Thus congenital physiologically corrected L-TGA typically has anomalies of the AVN and AVB that are associated with AV situs discordance: TGA {S,L,L}, that is, solitus atria and inverted (L-loop) ventricles.

Reproduced with permission from Anderson RH, Becker AE, Arnold R, Wilkinson JL. The conduction tissues in congenitally corrected transposition. Circulation. 1974;50:911.



Fig. 28.7


Transposition of the great arteries {S,L,L}, right lateral view of right-sided right atrium and of right-sided left ventricle. Right-sided mitral valve has been excised to permit a better view of the atrioventricular (AV) conduction system. 1, The location of the normal posterior AV node (AVN) that is hypoplastic and noncommunicating. The posterior AVN is slightly anterior to the right atrial ostium of the coronary sinus. 2, The location of the abnormal anterior AVN, to the right of the transposed pulmonary valve. From 2 to 3 is a black line indicating the location of the AV bundle (AVB) that runs to the right of and anterior to the transposed pulmonary valve. 2, Indicates where the abnormal AVB penetrates from the atrial to the ventricular level. 3, Indicates where the AVB branches. 4, Depicts where the right-sided left bundle branch descends down the left ventricular septal surface into the right-sided left ventricle.

Reproduced with permission from Anderson RH, Arnold R, Wilkinson JL. The conducting system in congenitally corrected transposition. Lancet. 1973;1:1286.



Fig. 28.8


Transposition of the great arteries (TGA) {S,L,L}, photo showing right-sided right atrium (RA), right-sided mitral valve (MV), right-sided morphologically left ventricle (MLV), and pulmonary outflow tract (POT) of right-sided transposed pulmonary artery (PA). Because there is a double alignment discordance in TGA {S,L,L}, at the atrioventricular (AV∗) and ventriculoarterial (VA∗∗) levels, RA and the PA are ipsilateral. Both are right-sided. Similarly, the left atrium (LA) and the aorta are both ipsilateral; that is, both are left-sided. Consequently, TGA {S,L,L} is congenitally physiologically corrected because of the ipsilaterality of the RA and PA (both right-sided) and of the left atrium (LA) and aorta (both left-sided), as long as associated malformations (such as pulmonary stenosis or atresia with a ventricular septal defect [VSD]) do not spoil the potential physiologic correction of the systemic venous and the pulmonary venous circulations. This same photo is used in the next figure to delineate the abnormal course of the AV conduction system in classic corrected TGA {S,L,L}.

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Aug 8, 2022 | Posted by in CARDIOLOGY | Comments Off on The Cardiac Conduction System

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