Which activation sequence is observed for tachycardia originating from the left lateral pathway?

• A. Distal CS activates before Proximal CS
• B. Proximal CS activates before Distal CS
• C. Simultaneous activation of CS segments
• D. Coronary sinus is not involved

Answer: (A)

Activation timing along the coronary sinus reflects where the retrograde impulse enters the left atrium. For a tachycardia arising from the left lateral pathway, the left lateral mitral annulus is closest to the distal part of the coronary sinus. As the retrograde wavefront spreads from that region into the left atrium, the distal coronary sinus electrodes fire first, with activation then sweeping proximally toward the CS ostium. That distal-to-proximal sequence is the telltale sign of a left lateral/left free-wall pathway.

If the proximal coronary sinus activated first, or if activation were simultaneous, it would not fit the left lateral anatomy. The coronary sinus is typically involved with left-sided pathways, so saying the CS is not involved isn’t correct.