To identify the presence of retrograde conduction through a concealed accessory pathway, pacing is performed from the apex of the right ventricle while recording in the coronary sinus. If the earliest atrial depolarization is far from the A/V node, this proves the presence of an accessory pathway. The tracing shows that during RV pacing, the first atrial depolarization is recorded at the coronary sinus site 3-4 (black recording), located at the left lateral part of the A/V ring. Clearly this is before the atrial deflection close to the A/V node (HIS recording in yellow) identifying the presence of the accessory pathway.

Dr. F. Molin 1998