The structure and components of the atrial chambers
Cardiac Unit, Institute of Child Health, University College, 30 Guilford Street, London WC1 N 1EH, UK
* Corresponding author. Tel: +44 207 905 2295; fax: +44 207 905 2324. E-mail address: r.anderson{at}ich.ucl.ac.uk
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We discuss the implications of accurate knowledge of the human atrial chambers for those seeking to model atrial structure, and correlate the muscular activity with electrical signals. We stress first the importance of describing atrial components in attitudinally appropriate fashion, a feature sadly ignored by generations of morphologists. When considered relative to the body, the right atrium is positioned anteriorly relative to its alleged left-sided counterpart. We then described how each atrium possesses a venous component, an appendage, a vestibule, these parts being supported by the body of the atrium, and how the two chambers are separated by the septum. We extend this information by describing the detailed structure of each atrium, and then emphasise that it is only the floor of the oval fossa, and its antero-inferior rim, that are true septal structures. The so-called ‘septum secundum’ is the superior interatrial fold. Emphasis is then given to the muscular connections between the atriums, these unions obviously underscoring the potential for interatrial conduction. We then continue by discussing the structure of the atrial walls, which vary markedly in their thickness. It is the alignment of the myocytes within these walls that determines the velocity of conduction through them. In this setting, we also discuss the morphological features that distinguish between working myocytes and the myocytes of the conduction system, stressing the importance of rules established almost 100 years ago.
Key Words: Attitudinally appropriate nomenclature, Atrial appendages, Sinus node, Conduction tissues
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It is intuitive that those seeking to model atrial structure so as to correlate muscular activity with electrical signals should seek to use the most accurate anatomic information available. Although new diagnostic techniques are now permitting the shape of the atrial cavities to be reconstructed with exquisite accuracy, and are revealing remarkably variable arrangements of structure such as the pulmonary veins,1
as far as we are aware there have been relatively few attempts to incorporate such information into models, with one of the best examples using a data set that is now several years old.2 If the greatest value is to be gained from the information now becoming available from the new diagnostic modalities, it is necessary also to understand the arrangement of the myocytes aggregated together to form the atrial walls, since this arrangement is far from isotropic.3,4 At the same time, it is necessary to know how the myocardial walls of the two atrial chambers are joined together, and to appreciate the location of the sinus node, the initiator of atrial activation.5 A review of the anatomy of the atrial chambers, therefore, can put all of this information into an appropriate morphological context, and also cast light on ongoing disputes as to the ‘specialized’ nature of the muscular sleeves of the pulmonary veins. We seek here to provide such an overview, but we begin by emphasizing the importance of attitudinally appropriate descriptions, since unless atrial structures are described and modelled as they are positioned within the body,6 it will never be possible to make accurate correlations with electrocardiographic tracings.
Attitudinally appropriate nomenclature
It is a time-honoured convention that all structures within the human body are described using the anatomical position, in which the subject stands upright and faces the observer. Structures positioned towards the head are then described as being superior, whereas structures closer to the feet are said to be inferior. Within the chest, the structures described as being anterior are those which are closest to the sternum, whereas those closer to the spine are said to be posterior. Such statements would be considered elementary by the medical student beginning a course of human anatomy, yet surprisingly they have been ignored by a generation of cardiac anatomists and pathologists, ourselves included.7 They are still ignored by a recent consensus group of nuclear cardiologists,8 which has suggested that the antonym of inferior when describing the ventricular walls should be anterior, rather than superior! The use of attitudinally appropriate nomenclature has now been recommended by a consensus group of electrophysiologists and arrhythmologists,9 albeit that not all are yet adopting the appropriate terms.
When we consider the atriums in attitudinally appropriate orientation, it is immediately evident that their names are far from accurate. The so-called right atrium is positioned in front of its allegedly left-sided counterpart. Only the tip of the left atrial appendage is visible when the cardiac silhouette is viewed in the frontal projection (Figure 1). The ventricles, furthermore, are positioned largely to the left of their respective atriums, again with the so-called right ventricle being in front of its purported left-sided counterpart.
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All that is seen of the left ventricle in the frontal projection is the small strip that extends to the left border beyond the site of the anterior interventricular artery. The other essential feature of attitudinally appropriate anatomy, key to the understanding of ventricular relationships, is that the aorta is positioned posteriorly and to the right of the pulmonary trunk, even though it emerges from the left ventricle (Figure 1). For those seeking to correlate morphology with electrocardiographic tracings, it is essential that the heart be assessed in its attitudinally appropriate position.
Components of the atrial chambers
Although the atriums differ markedly in their shape, they possess the same basic components. Thus, each atrium is made up of a venous component, an appendage, and a vestibule, with the chambers separated one from the other by the septum. These various components themselves are supported by the bodies of the atriums. The body is much more obvious in the left than in the right atrium, and is derived from medisatinal myocardium. The appendages balloon on each side from the primary atrial component of the linear heart tube. The venous components of the two chambers are then derived from different embryonic sources, with the entirety of the embryonic systemic venous sinus being incorporated into the morphologically right atrium (Figure 2). The pulmonary venous component, in contrast, is derived from mediastinal myocardium, as are the components of the atrial septum.10 The vestibules of both atrial chambers are the remnants of the embryonic atrioventricular canal, which are sequestrated on the atrial side of the insulating tissues subsequent to separation of the atrial from the ventricular muscle masses.11 As already discussed, the body of the right atrium is difficult to discern in the postnatal heart. It is the part of the atrial chamber between the site of the left venous valve and the septum. In the postnatal heart, however, it is not usually possible to recognize the site of the left venous valve. Hence, it is difficult to distinguish the body of the right atrium from the systemic venous sinus.
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Structure of the morphologically right atrium
The anatomy of the morphologically right atrium, positioned anteriorly relative to its alleged left-sided counterpart with the heart in attitudinally appropriate position, is dominated by its appendage. The appendage is readily distinguished from the remainder of the atrium because of its ridged walls, the ridges representing the pectinate muscles, which take their origin from the prominent terminal crest (Figures 3 and 4). Contrary to conventional wisdom, which has us believe that the appendage is no more than the tip of the right atrial chamber, in reality the appendage forms the entirety of the anterior wall of the chamber (Figure 3). Indeed, the pectinate muscles, and hence the appendage, extend all round the vestibule of the tricuspid valve, reaching to the septum in the area of the sub-Thebesian sinus. This sinus is often described as being sub-Eustachian, since when the heart is positioned on its apex, the sinus lies directly beneath the fibrous flap that in many hearts is found adjacent to the orifice of the inferior caval vein, and is described as the Eustachian valve. This solecism is yet another example of the usual, albeit incorrect, practice of describing the heart as if positioned on its apex. When viewed in attitudinally appropriate position, the sinus is seen to be beneath the Thebesian valve, which is the remnant of the valve of the embryonic venous sinus adjacent to the mouth of the coronary sinus. It is not beneath the Eustachian valve, which as explained is related to the mouth of the inferior caval vein (Figure 3). The rightward part of the right atrium is occupied by the systemic venous sinus. The superior caval vein opens to the top of this part, and the inferior caval vein to the bottom, with the coronary sinus also opening to the inferior part of the chamber, but to the right of the mouth of the inferior caval vein (Figure 2). An extension of the wall of the appendage turns upwards between the opening of the inferior caval vein and the coronary sinus. The Eustachian valve is attached to the rightward border of this triangular area, and the Thebesian valve takes its origin from the leftward border (Figure 3). The two valves come together at the apex of this triangle, and insert themselves as the fibrous tendon of Todaro into the posterior wall of the atrium (Figure 4). The entirety of the systemic venous sinus is smooth-walled, with the mouth of the inferior caval vein looking towards the floor of the oval fossa.
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It is the oval fossa that dominates the posterior atrial wall. It has extensive rims surrounding the floor, which is occupied by the flap valve of the septum (discussed later). As we will see, although these rims, best formed superiorly, inferiorly, and to the right, seem also to be part of the septum, for their most part they are infoldings between the right and left atrial walls (discussed later). The vestibule of the tricuspid valve forms the leftward margin of the right atrium, the musculature inserting into the leaflets of the tricuspid valve, and forming the terminal components of the atrial muscle mass. On the posterior atrial wall, however, the vestibule turns superiorly to become the triangle of Koch, the vestibule itself forming the septal isthmus, between the mouth of the coronary sinus and the line of attachment of the septal leaflet of the tricuspid valve (Figure 4).
A second, clinically important, isthmus is seen forming the inferior wall of the right atrium. This is the cavo-tricuspid isthmus, a crucial part of the usual circuit for atrial flutter.12 Superiorly, the tendon of Todaro inserts into the membranous septum, with the septal attachment of the tricuspid valve crossing this septum to divide it into atrioventricular and interventricular components. The membranous septum forms the apex of the triangle of Koch. The atrioventricular conduction axis penetrates the insulating plane of the atrioventricular junctions at this point to become the penetrating bundle, or the bundle of His. The superior caval vein enters the roof of the right atrium, with the terminal crest curling round its rightward margin to join the vestibule, a prominent pectinate muscle usually continuing into the tip of the appendage at this point as the so-called ‘septum spurium’.
Structure of the morphologically left atrium
As already discussed, the left atrium differs from the right in that its body is much more obvious. The body forms the central part of the atrium, with the appendage bulging superiorly and leftward as a tube-like structure to pass round the origin of the pulmonary trunk. This is the only part of the left atrium seen on the anterior silhouette (Figure 1). The vestibule of the mitral valve also forms the leftward and anterior atrial border, and the pulmonary venous component forms the atrial roof (Figure 5). When viewed from above, it can be seen that, in most instances, the four pulmonary veins enter the corners of the atrial roof (Figure 6). Experience using magnetic resonance imaging has shown marked variation in the pattern of termination of the pulmonary veins, including some instances where one vein enters directly into the roof.13 The coronary sinus also has an important relationship to the left atrium, even though it drains into the right atrium. The sinus occupies the left atrioventricular groove. Many consider the sinus to originate at the site of the oblique vein of the left atrium, although the lumen of the great cardiac vein is directly continuous with that of the sinus at this point. Because of this continuity, others consider the site of the valve of Vieussens to mark the start of the coronary sinus. This structure is a prominent venous valve usually found within the lumen of the great cardiac vein as it turns into the inferior atrioventricular groove. But, since the sinus is the remnant of the left sinus horn, as is the oblique vein, there is much to be said for regarding their union as the commencement of the sinus.14 The flap valve of the oval fossa forms the anterior wall of the left atrium, this important structure overlapping the infolded rims of the fossa so that there is no potential for interatrial shunting as long as left atrial pressure exceeds right. In up to one-third of the normal population, however, the flap valve is not anatomically fused to the rims of the fossa.15 The situation in which the flap valve overlaps the rims, without anatomic fusion (Figure 7), produces the situation in which a probe can be inserted between the two, hence the term ‘probe-patency of the oval foramen’.
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The structure of the atrial septum
Most textbooks of embryology describe the atrial septum as formed in two steps, with initial growth of a primary septum, and then formation of a second septum, the second structure held to overlap the first so as to form the rims of the oval fossa. In reality, there is no second septum formed superiorly and anteriorly. The real arrangement of the septum is revealed by sections across it (Figure 8). The entirety of the superior and posterior rims of the fossa, along with much of the anterior rim, is no more than infoldings of the atrial walls. The true septum is the flap valve, along with its point of anchorage antero-inferiorly. This antero-inferior rim becomes confluent with the floor of the triangle of Koch, but much of the triangle is a sandwich rather than a septum, since the atrial wall overlaps the crest of the ventricular septum in this area, with an upward extension of the inferior atrioventricular groove separating the atrial from the ventricular muscle masses. Although often described as the ‘posterior’ septum, in reality this is the inferior paraseptal space (Figure 9).
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Muscular connections between the atrial walls
The obvious muscular connections between the atriums, important for conduction between them, are the margins of the oval foramen. As shown in Figure 9, these are no more than infoldings of the walls. The floor of the oval fossa, however, although itself a septal structure, is usually a fibro-collagenous wall in the postnatal heart, so this area does not provide electrical interatrial continuity. The most important muscular interatrial bridge is provided by the insertion of the terminal crest into the atrial roof anterior to the mouth of the superior caval vein. When seen externally, this insertion is directly continuous with the anterior interatrial groove (Figure 10). The myocytes forming the atrial wall are aggregated together in parallel fashion at this point, and continue into the left atrial wall as Bachman’s bundle.16
In addition to the bundle itself, there are often robust connections through the inferior part of the anterior interatrial groove,17 whereas further muscular bridges between the walls of the coronary sinus and left atrium provide for inferior and leftward communications.18 It is the proximity of Bachman’s bundle to the terminal crest, and the site of the sinus node, however, that makes this the most significant electrical interatrial connection.
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The structure of the atrial walls
The atrial walls vary markedly in their thickness between the atriums, and also in their structure. For the most part, the walls of the left atrium are thicker than their right atrial counterparts, but parts of the right atrium are also thickened, such as the terminal crest and the pectinate muscles. Within these thickened parts of the right atrium, the myocytes are aligned with their own long axis parallel to the long axis of the muscular bundles. This arrangement potentiates to more rapid conduction along the long axis than parallel to it.19
In similar fashion, it is the parallel arrangement of the myocytes within Bachman’s bundle that potentiates to somewhat more rapid interatrial conduction across the anterior interatrial groove, a mechanism emphasized by Bachman himself in his original description of this muscular bridge. Within the left atrium itself, there is a much more irregular arrangement of the myocytes, the alignment often changing at different depths within the atrial walls. Generally speaking, the myocytes are aligned parallel to the atrioventricular grooves in the vestibule, but transversely across the atrial roof (Figure 11). Much interest has been generated of late in the sleeves of myocardium that continue from the atrial roof to invest the pulmonary veins as they insert into the left atrial cavity. Suggestions have been made that these sleeves are made up of ‘specialized myocytes’.20 Those making such suggestions have ignored completely the excellent criterions established by Monkeberg and Aschoff in 1910 for the recognition of histologically specialized conducting cells.21,22 As indicated by these giants of morphology and pathology, in order to be considered ‘specialized’, cells should be histologically discrete, traceable from section to section, and if considered to form ‘tracts’, should also be insulated from the surrounding working myocardium. Examination of the pulmonary venous sleeves (Figure 12) shows that the myocytes making up the sleeves satisfy none of these criterions. In fact, the myocytes are ordinary working cells. Studies in the developing heart show that they have never had a ‘primary’ lineage.10 It is almost certainly the non-uniform anisotropic arrangement of these myocytes19 that, in the setting of fibrosis and ageing, sets the scene for the focal activity now known to underscore many episodes of atrial fibrillation. In this respect, it cannot be coincidental that the sleeves are known to be longest along the superior pulmonary veins, and longest of all along the left vein.23 These are the veins known to be most frequently involved as the seat of focal activity producing fibrillation.24
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The histologically specialized cells in the atrial walls are found only within the right atrium. They make up the sinus node, positioned laterally and sub-epicardially within the terminal groove, and the atrioventricular node, located at the apex of the triangle of Koch. Limited areas of transitional myocardium form the entrances to the atrioventricular node, but such transitional zones are very short at the margins of the sinus node. Elsewhere within the right atrial walls, rests of histologically specialized tissue are to be found scattered around the vestibule of the tricuspid valve, but unlike the suggestions of Kent,25
these collections of specialized cells do not form atrioventricular muscular connections in the normal heart, albeit that they may form such connections in congenitally malformed hearts, or in hearts with the so-called ‘Mahaim’ pre-excitation.26 Other than these cells, and the cardiac nodes, the remaining atrial walls are made up of working cardiomyocytes. Any preferential conduction across thick muscular components relates only to the ordered packing of these working myocytes.19 |
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It is now possible for the structure of the atriums to be reconstructed with just as much, if not more, accuracy in the clinical setting as can be achieved by the morphologist.1
The recent studies, furthermore, show the structure of the atrial chambers in the setting of the thorax. It is crucial, therefore, that we begin to describe the atrial components in attitudinally appropriate fashion. With this information, and knowing more about the alignment of the aggregated myocytes making up the atrial walls, it should now be possible to make exquisitely accurate models of atrial activation, and relate them to external electrocardiographic recordings. This will not be possible, however, until we adopt the attitudinally appropriate approach, and use appropriate morphological markers to distinguish working myocytes from those which have been specialized to generate and conduct the cardiac impulse.21,22 Conflict of interest: none declared.
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The research on which this review is based was supported by grants from the British Heart Foundation together with the Joseph Levy Foundation. Research at the Institute of Child Health and Great Ormond Street Hospital for Children NHS Trust benefits from R&D funding received from the NHS Executive.
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