Narrow QRS tachycardia: ablation of a concealed accessory pathway

Dr. F. Molin 1998

One of the mechanisms responsible of a narrow QRS regular tachycardia is the presence of an accessory pathway. When there is no anterograde conduction through an accessory bundle, the pathway is called concealed because it is invisible on the surface ECG. The mechanism of the arrhythmia is a reentrant circuit : atrium, A/V node, ventricle, accessory pathway (retrogradely) and atrium again. The figure is a 12 lead ECG of a narrow QRS tachycardia mediated through a concealed accessory pathway. The differential diagnosis includes A/V nodal reentry and atrial tachycardia. Establishing the diagnosis is difficult from the 12 lead electrocardiogram.

To identify the presence of retrograde A/V 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 AVnode, this proves the presence of an accessory pathway. The figure shows that during RV pacing, the first atrial depolarization is recorded at the coronary sinus site 3-4, located at the left lateral AV ring, clearly before the atria close to the A/V node(HIS recording) and identifies the accessory pathway.

Ablation of the pathway is performed by applying radio frequency current at the A/V ring, at the site of the earliest retrograde atrial depolarization. In the figure (after ablation) there is no more conduction through the accessory pathway or via the A/V node. The ablation was a successful.