In eccentric activation mapping, which coronary sinus electrodes show early activation, indicating conduction from the left side?

• A. CS 1,2
• B. CS distal
• C. CS 1,2 and CS distal
• D. CS proximal

Answer: (C)

In eccentric activation mapping, you’re looking for where the activation starts and how it spreads. When the conduction source is on the left side, the impulse enters the coronary sinus from the left atrioventricular groove and then propagates along the CS toward both ends. That means you’ll see the earliest activation not just at one spot, but both at the CS near the ostium (proximal electrodes) and at the far end of the CS (distal electrode). Seeing early activation on the proximal pair (CS 1,2) and on the distal CS electrode, together, strongly indicates left-sided conduction entering the CS. This pattern helps localize a left-sided pathway and guide ablation.