The superior and inferior caval veins are more than passive conduits returning systemic venous blood to the right atrium. Their cavoatrial junctions are continuous with the muscular architecture of the venous component of the atrium, and the superior caval vein commonly contains myocardial extensions capable of electrical activity. The arrangement of these extensions, the terminal crest and its precaval segment, and the neighbouring sinus node and right phrenic nerve creates an anatomically complex region of direct relevance to mapping, ablation, and transvenous procedures. [1-5]

1  Superior caval vein and superior cavoatrial junction

The superior caval vein descends on the right side of the ascending aorta and enters the superior aspect of the right atrium. Its lower segment is intrapericardial. Posteriorly, the right pulmonary artery and the right superior pulmonary vein lie close to the cavoatrial junction, whereas the right atrial appendage is situated anteriorly and rightward. The superior caval orifice has no valve, and its transition into the smooth-walled venous component of the right atrium is not marked by a complete anatomical ring.

Internally, the terminal crest arches anterior to the orifice and then turns along the lateral wall. Externally, the terminal groove indicates the transition between the smooth venous component and the trabeculated appendage. The superior cavoatrial junction should therefore be regarded as a three-dimensional transitional zone rather than as a flat circular ostium. [1,2]

2  Inferior caval vein and inferior cavoatrial junction

The inferior caval vein traverses the central tendon of the diaphragm and enters the inferior aspect of the right atrium. Its atrial orifice is directed superiorly and posteriorly within the inferior venous component. In contrast to the superior junction, its anterior and medial margin is frequently guarded by a variable Eustachian valve. The lateral end of this valve continues into the Eustachian ridge, while its medial fibrous continuation contributes to the tendon of Todaro.

The terminal crest curves beneath the inferior caval orifice and breaks up into trabeculations towards the inferior right atrial wall. These relationships explain why the inferior cavoatrial junction, the Eustachian structures, and the cavotricuspid isthmus appear continuous during catheter manipulation, although they are anatomically distinct. [1,2]

3  Myocardial architecture of the caval veins

Working atrial myocardium extends for variable distances onto the caval veins. These extensions form discontinuous sleeves or fascicles rather than a uniform cylindrical layer. Histological studies have demonstrated myocardial extensions in both caval veins, but they are generally more consistent and extensive in the superior caval vein. Their thickness, longitudinal reach, circumferential distribution, and degree of fibrosis vary markedly between individuals. [4,5]

At the superior cavoatrial junction, bundles may run longitudinally, obliquely, or circumferentially and can be separated by connective tissue and adipose tissue. This non-uniform architecture provides a substrate for anisotropic conduction and helps explain why superior caval activity may be recorded as multiple or fractionated potentials. The inferior caval vein more often has sparse and shorter myocardial extensions; the transition to the atrial wall may therefore be electrically less continuous. Statements about exact sleeve length should be made cautiously because published measurements depend on sampling and on how the cavoatrial boundary is defined. [4,5]

4  Precaval band and superior intercaval region

The term precaval band or precaval bundle describes the superior segment of the terminal crest as it passes anterior to the superior caval orifice. It is not part of the venous wall itself. In the study by Sánchez-Quintana and colleagues, this muscular component could be identified as a distinct structure in most, but not all, specimens. Epicardially, it corresponds to the crest of the right atrial appendage. Laterally it becomes continuous with the vertically oriented terminal crest, while medially its fibres approach the anterior interatrial groove and the region from which Bachmann bundle arises. [2]

This arrangement links three muscular territories: the trabeculated appendage, the smooth venous component, and the anterior interatrial musculature. The precaval band may therefore influence preferential spread of activation around the superior caval orifice. It should not, however, be presented as a discrete insulated conduction tract. Its prominence and boundaries are variable, and it forms part of the broader atrial myocardial continuum. [2]

5  Sinus venarum Eustachian valve and Chiari network

The sinus venarum is the smooth-walled posterior component of the right atrium receiving the two caval veins and the coronary sinus. The terminal crest forms its principal lateral boundary against the trabeculated appendage. This smooth appearance does not indicate absence of myocardium: the wall contains atrial muscle of variable thickness and is crossed by fibre bundles connecting neighbouring regions.

The Eustachian valve is a remnant of the embryonic right venous valve and varies from an inconspicuous fold to a broad mobile membrane. Fenestration and excessive persistence may produce a Chiari network, a delicate reticular structure attached near the inferior caval orifice and other right atrial landmarks. These variants may redirect wires and catheters or become entangled with intracardiac devices. A prominent Eustachian ridge can also impede stable contact near the inferior isthmus. [1]

6  Relations with the sinus node and right phrenic nerve

The sinus node is usually subepicardial near the superior cavoatrial junction, related to the terminal groove and the superior part of the terminal crest. It is not a compact point structure. Histological reconstructions show an irregular nodal body with extensions towards the superior caval vein, terminal crest, intercaval region, and surrounding working myocardium. Its long axis may descend for a variable distance along the terminal groove. [3]

Outside the fibrous pericardium, the right phrenic nerve descends between the pericardial sac and mediastinal pleura. It passes close to the superior cavoatrial junction and subsequently along the right atrial and inferior caval regions. Consequently, ablation near the superior caval vein can threaten both sinus-node tissue and the phrenic nerve. Their location cannot be predicted reliably from the angiographic outline alone. [1,3]

7  Electrophysiological relevance and anatomical variants

Myocardial sleeves within the superior caval vein can harbour focal activity and participate in non-pulmonary venous triggers of atrial tachyarrhythmias. Electrical isolation is therefore sometimes performed at the superior cavoatrial junction. The target cannot be defined solely by the change in vessel calibre: mapping should account for the irregular muscular boundary, sinus-node extensions, and the course of the right phrenic nerve. Energy delivery too far within the vein may also increase the risk of venous narrowing. [2-5]

Relevant variants include duplication or interruption of the right superior caval vein, persistence of a left superior caval vein, an enlarged azygos continuation, and variable drainage of a persistent left caval vein to the coronary sinus or, less commonly, to the left atrium. These patterns alter transvenous access and may enlarge the coronary sinus. They deserve recognition here, but their embryology and procedural management should be developed only to the level required by the atlas, avoiding displacement of the main topic: the anatomy and myocardial architecture of the normal caval junctions.