The coronary sinus is the principal venous conduit of the heart. It receives most of the venous blood from the left ventricular myocardium and part of the atrial and right ventricular drainage before opening into the right atrium. It is not simply a passive vascular landmark. Its wall is invested by myocardial sleeves, its tributaries follow variable epicardial courses, and its relations to the atrioventricular junctions, coronary arteries, inferior pyramidal space, and left atrium make it central to electrophysiological mapping, ablation, cardiac resynchronisation, and several emerging therapies. [1-4]

Descriptions should follow the heart in its anatomical position. The coronary sinus lies in the inferior left atrioventricular groove and approaches the right atrium near the crux. The middle cardiac vein occupies the inferior interventricular groove. These terms describe the actual orientation more accurately than the traditional use of posterior for structures located predominantly on the diaphragmatic surface. [1,2]

1  Formation course and ostium of the coronary sinus

The coronary sinus is conventionally considered to begin where the great cardiac vein turns into the left atrioventricular groove and receives the oblique vein of the left atrium. A valve may be present at this transition, often termed the valve of Vieussens. The precise external boundary between vein and sinus is not always sharply defined, and calibre changes or the entry of tributaries may provide more practical landmarks than a discrete line. [3,4]

From its leftward origin, the coronary sinus runs rightward in the inferior atrioventricular groove. It courses on the atrial side of the true mitral junction and may be separated from the hinge of the mural mitral leaflet by a variable amount of atrial myocardium, fat, and connective tissue. This separation is especially relevant when the sinus is used as a fluoroscopic surrogate for the mitral annulus: the venous catheter does not necessarily mark the valvar hinge. [1]

The coronary sinus opens into the inferior right atrium between the inferior caval vein and the tricuspid valvar orifice. Its ostium is related superiorly to the inferior margin of the triangle of Koch and anteriorly to the septal vestibule of the tricuspid valve. A variably developed Thebesian valve guards the opening. This valve may be a thin crescent, a fenestrated membrane, or, rarely, a substantial obstacle to cannulation. The dimensions and orientation of the ostium also vary and should be assessed independently of the apparent catheter position in a single projection. [1,3]

2  The major tributaries

The great cardiac vein begins near the ventricular apex and ascends in the anterior interventricular groove beside the anterior interventricular artery. It then turns leftward around the ventricular base to enter the left atrioventricular groove, where it accompanies the circumflex artery. The relative position of artery and vein may change along their course. Near the left ventricular summit, the great cardiac vein and the adjacent coronary arteries help define which epicardial regions can be approached safely. [3,4]

The middle cardiac vein usually arises close to the apex and runs towards the crux within the inferior interventricular groove, accompanying the inferior interventricular artery. It joins the coronary sinus near its right atrial end. The proximal vein is closely related to the inferior pyramidal space and inferior paraseptal atrioventricular junction. Its wall may be covered by extensions of coronary sinus musculature, providing a potential muscular pathway between atrial and ventricular myocardium. [1,2]

The small cardiac vein courses along the inferior and right atrioventricular groove, often accompanying the right coronary artery, and enters the coronary sinus close to the crux. Its calibre and continuity are variable. The posterior vein of the left ventricle, more accurately understood as a vein of the inferior or inferolateral left ventricular surface according to its actual course, enters the coronary sinus between the middle and great cardiac veins. Left marginal veins may drain independently into the great cardiac vein or coronary sinus. [3,4]

Several left ventricular veins can be present between the great and middle cardiac veins. Their number, diameter, angulation, valves, and relation to the phrenic nerve vary substantially. These features influence the feasibility of advancing a left ventricular pacing lead and the likelihood of obtaining stable capture without phrenic stimulation. Venography remains important because no single branching pattern can be assumed.

3  Myocardial sleeves and electrical connections

The wall of the coronary sinus is surrounded by a cuff of myocardium extending for a variable distance from the ostium. In the anatomical study summarised by Cabrera and Sánchez-Quintana, the myocardial cuff extended on average approximately four centimetres from the ostium. Muscular bundles can leave this sleeve and connect with the left atrial wall. The venous myocardium may also extend over the proximal middle cardiac vein and other tributaries. [1]

These connections create more than one potential route of electrical continuity between the right atrium, coronary sinus musculature, and left atrium. They help explain why recording within the sinus can show atrial activity that is not equivalent to activation at the endocardial mitral hinge, and why completion of a left atrial ablation line may occasionally require treatment within or on the epicardial aspect of the coronary sinus. The venous wall must not, however, be assumed to have the same muscular thickness throughout its length. [1]

4  The oblique vein of the left atrium and vein of Marshall

The oblique vein of the left atrium descends along the inferolateral left atrial wall to enter the distal coronary sinus. It is the venous remnant associated with the embryonic left superior caval vein and is commonly referred to clinically as the vein of Marshall. It lies within the left lateral ridge region between the left atrial appendage and the left pulmonary veins. Its calibre, patency, length, and branching are variable. [1,3]

The vein is accompanied by the ligament of Marshall, myocardial fibres, autonomic nerves, and vascular structures. Muscular connections can link this region with the left atrium and pulmonary venous myocardium, providing an anatomical basis for electrical activity and conduction involving the Marshall bundle. The structure should therefore be depicted as a neurovascular and muscular complex rather than as an isolated small vein. [1]

Cannulation of the vein of Marshall permits selective venography and ethanol delivery in appropriately selected atrial fibrillation procedures. Anatomical variability determines whether the vein can be entered and which atrial regions are reached. The presence of the vein alone does not define an arrhythmogenic mechanism; its relevance must be established by the electrophysiological and procedural context.

5  Other pathways of cardiac venous drainage

Not all cardiac veins drain through the coronary sinus. Anterior cardiac veins cross the right ventricular surface and open directly into the right atrium, sometimes after joining one another. The smallest cardiac veins, traditionally called the venae cordis minimae or Thebesian veins, drain directly into the cardiac chambers through minute openings. These systems are distinct from the Thebesian valve at the coronary sinus ostium. [3]

Venous anomalies include persistent left superior caval vein draining to an enlarged coronary sinus, absence or unroofing of the coronary sinus, unusual connections with the left atrium, and marked variation in the tributary network. An enlarged sinus should therefore prompt assessment of its inflow and surrounding atrial septal anatomy rather than being regarded simply as a dilated normal vessel.

6  Relations relevant to electrophysiology and intervention

The coronary sinus provides access for left atrial and left ventricular recording, mapping of accessory pathways, delivery of cardiac resynchronisation leads, and approaches to ventricular arrhythmias. Yet an electrode within the sinus is separated from the mitral junction by a variable interval, and signals can arise from venous musculature, adjacent atrium, ventricle, or an accessory atrioventricular connection. Anatomical location and signal origin must therefore be interpreted separately. [1,2]

Inferior paraseptal accessory pathways may be associated with the coronary sinus ostium, proximal middle cardiac vein, or a coronary sinus diverticulum containing myocardial fibres. The term posteroseptal is anatomically imprecise for this region. The target lies near the crux, right and left atrioventricular grooves, inferior pyramidal space, atrioventricular nodal artery, and normal conduction tissues. [1,2]

Ablation inside the coronary venous system carries risks of venous injury, perforation, thrombosis, and damage to an adjacent coronary artery. The circumflex artery may be close to the coronary sinus and great cardiac vein along the mitral junction, while the right coronary or inferior interventricular artery may lie near the ostium and middle cardiac vein. Patient-specific coronary imaging is required when arterial proximity is uncertain.