In AVRT, which activation pattern characterizes left lateral pathways?

• A. HRA earliest concentric activation
• B. CS distal early eccentric activation
• C. HIS earliest concentric activation
• D. CS proximal late activation

Answer: (B)

The concept you’re testing is how the location of an accessory pathway in AVRT shows up on activation maps during retrograde conduction. For left lateral (left free-wall) pathways, the pathway inserts on the left atrial side of the mitral annulus. Because of that anatomical location, the retrograde atrial activation begins at the distal coronary sinus, which runs in the left AV groove and is closest to the left lateral insertion. This gives an activation pattern that is early at the distal CS and spreads eccentrically to the rest of the atria, rather than spreading in a concentric fashion from the His region.

So, the earliest atrial signal being at the distal coronary sinus with an eccentric activation pattern best identifies a left lateral pathway. If activation were earliest at the His region or proximal CS, that would point toward other, more septal or nodal locations, not the left lateral pathway. Similarly, proximal CS sites showing late activation do not match the left free-wall location.