For a patient with a left-sided bypass tract, during ventricular pacing the earliest atrial signal on the CS channel will be in which electrodes?

For a patient with a left-sided bypass tract, during ventricular pacing the earliest atrial signal on the CS channel will be in which electrodes?

• A. Proximal CS electrodes
• B. Distal CS electrodes
• C. Mid CS electrodes
• D. Posterior CS electrodes

Answer: (B)

The main idea is that with a left-sided bypass tract, retrograde conduction from the ventricle takes the quickest route into the left atrium at the tract’s insertion point, which sits along the left atrial wall near the lateral/posterior region. The coronary sinus catheter traces a path from the ostium (proximal) toward the distal CS, which lies closest to that left atrial area. Because the left-sided pathway inserts on the left atrial side, the atrial activation generated via the retrograde conduction reaches the distal part of the coronary sinus first. That’s why the earliest atrial signal during ventricular pacing appears on the distal CS electrodes. Proximal or mid CS electrodes are nearer the right atrial side or not as close to the left atrial insertion, so they don’t show the earliest activation as reliably.

The main idea is that with a left-sided bypass tract, retrograde conduction from the ventricle takes the quickest route into the left atrium at the tract’s insertion point, which sits along the left atrial wall near the lateral/posterior region. The coronary sinus catheter traces a path from the ostium (proximal) toward the distal CS, which lies closest to that left atrial area. Because the left-sided pathway inserts on the left atrial side, the atrial activation generated via the retrograde conduction reaches the distal part of the coronary sinus first. That’s why the earliest atrial signal during ventricular pacing appears on the distal CS electrodes. Proximal or mid CS electrodes are nearer the right atrial side or not as close to the left atrial insertion, so they don’t show the earliest activation as reliably.