Venous Drainage of the Heart

1. The Coronary Sinus

• The coronary sinus is the main venous channel of the heart, lying in the posterior part of the coronary (atrioventricular) sulcus.
• It is a wide, short (~2-3 cm) venous channel, roughly tubular, with an ostial diameter of 5-15 mm.
• It opens into the posterior wall of the right atrium, immediately above the tricuspid valve, between the inferior vena cava orifice and the atrioventricular septum (guarded by the valve of Thebesius).
• It receives most of the tributaries draining the left ventricle and left atrium.
• Venous blood flow into the coronary sinus peaks during late systole, because the contracting LV compresses major venous drainage channels.
• Approximately 85% of total coronary blood flow returning from the LV empties into the coronary sinus.

2. Tributaries of the Coronary Sinus

Great Cardiac Vein (largest tributary)

• Begins as the anterior interventricular vein, ascending alongside the left anterior descending (LAD) artery in the anterior interventricular sulcus.
• Turns at the apex to course in the left atrioventricular groove (alongside the left circumflex artery) and continues into the coronary sinus.
• Its transition to the great cardiac vein proper is marked by the valve of Vieussens (present in 80-90% of hearts) and the junction with the oblique vein of Marshall.
• It collects tributaries from the anterior LV, lateral (obtuse marginal) veins, and inferior LV veins - these lateral/posterolateral branches are important targets for LV pacing leads in cardiac resynchronization therapy.

Middle Cardiac Vein

• Starts at the apex of the heart.
• Courses in the posterior interventricular sulcus alongside the posterior descending artery (PDA).
• Drains into the right end of the coronary sinus (or occasionally directly into the right atrium).
• At the cardiac crux it passes immediately superficial to the right coronary artery.

Small Cardiac Vein

• Begins as the right marginal vein, accompanying the right marginal branch of the RCA.
• Turns posteriorly into the right atrioventricular (coronary) sulcus.
• Drains into the right end of the coronary sinus or occasionally into the middle cardiac vein.
• Receives tributaries from the right atrium and inferior wall of the right ventricle.

Posterior Vein of the Left Ventricle

• Courses on the diaphragmatic surface of the left ventricle, just to the left of the middle cardiac vein.
• Enters either the coronary sinus directly or joins the great cardiac vein.

Oblique Vein of the Left Atrium (Vein of Marshall)

• Courses on the posterior wall of the left atrium.
• Merges with the great cardiac vein to form (or enter) the coronary sinus.
• It is an embryonic remnant of the left superior vena cava (left common cardinal vein).
• Rarely exceeds 3 cm before tapering to a blind end; when wide, it can be used for left atrial ablation.

3. Veins That Bypass the Coronary Sinus

Anterior Cardiac Veins (Anterior Veins of the Right Ventricle)

• 3-4 small veins arising on the anterior surface of the right ventricle.
• Cross the coronary sulcus and enter directly into the anterior wall of the right atrium.
• Drain the anterior portion of the RV.
• The right marginal vein may be part of this group if it does not enter the small cardiac vein.

Smallest Cardiac Veins (Venae Cordis Minimae / Thebesian Veins)

• Tiny vessels draining capillary beds of the myocardium directly into the cardiac chambers.
• Most numerous in the right atrium and right ventricle.
• Occasionally associated with the left atrium; rarely found in the left ventricle.
• The remaining ~15% of LV coronary blood flow (not entering the coronary sinus) drains through these veins, predominantly into the RA > LA and LV.

Summary Table

Clinical Relevance

• The coronary sinus is used as access for retrograde cardioplegia in cardiac surgery, and for placing LV pacing leads in cardiac resynchronization therapy (CRT) via the lateral/posterolateral tributaries.
• The left phrenic nerve runs close to the lateral/posterolateral LV veins, and pacing from these veins can cause phrenic capture.
• The oblique vein of Marshall can be used for alcohol ablation of the left atrial wall in certain arrhythmias.
• The muscular sleeve around the proximal coronary sinus may participate in atrial arrhythmias.
• The thebesian veins create a small physiological right-to-left shunt, contributing to normal arterial desaturation.