Into which chamber of the heart does the coronary sinus (CS) drain directly?

The Coronary Sinus Drains Directly into the Right Atrium

The coronary sinus drains directly into the right atrium (option A). This is the correct anatomical arrangement of the normal cardiac venous drainage system.

Anatomy of the Coronary Sinus

The coronary sinus is the main venous drainage system of the heart, collecting approximately 85% of the cardiac venous blood. It has several key anatomical characteristics:

• Located in the posterior atrioventricular groove of the heart
• Formed by the confluence of the great cardiac vein and the oblique vein of the left atrium (Marshall's vein)
• Receives blood from multiple cardiac veins including:
  • Great cardiac vein
  • Middle cardiac vein
  • Small cardiac vein
  • Posterior left ventricular vein
  • Oblique vein of the left atrium

The coronary sinus terminates by opening directly into the right atrium, specifically at its posterior aspect between the inferior vena cava opening and the tricuspid valve orifice 1.

Clinical Significance

Understanding the normal drainage of the coronary sinus is important for several clinical scenarios:

1. Cardiac catheterization procedures: During electrophysiology studies and certain interventional procedures, catheters may be placed into the coronary sinus via the right atrium.

2. Congenital heart anomalies: Variations in coronary sinus anatomy can occur, including:

   • Unroofed coronary sinus (a rare form of atrial septal defect where the coronary sinus communicates with the left atrium) 2
   • Coronary sinus orifice atresia (where the connection to the right atrium is absent) 3
   • Anomalous pulmonary venous connections to the coronary sinus 4

3. Heart failure assessment: Dilatation of the coronary sinus can be an echocardiographic sign of right heart failure and systemic venous congestion 5.

Pathological Variants

While the normal anatomical arrangement is for the coronary sinus to drain into the right atrium, several pathological variants exist:

• In some congenital heart defects, the coronary sinus may be "unroofed," creating a communication between the coronary sinus and left atrium 3, 2
• In rare cases, anomalous pulmonary veins may drain into the coronary sinus 4
• Persistent left superior vena cava often drains into the right atrium via the coronary sinus 6

These variants are important to recognize in cardiac imaging and during surgical planning for congenital heart disease repair.

Conclusion

The coronary sinus serves as the principal venous drainage system of the heart and empties directly into the right atrium. This anatomical arrangement allows deoxygenated blood from the cardiac tissue to return to the right side of the heart for subsequent pulmonary circulation.