Observations During paraHisian Entrainment
During paraHisian entrainment, there are 3 competing and interacting waves, namely the AV reentrant tachycardia circuit, His bundle‐right bundle (HB‐RB) capture circuit, and right ventricular (RV) paraHisian myocardial capture circuit. The retrograde atrial (A) activation sequence (RAAS) during tachycardia, HB‐RB capture, and RV capture are the same with earliest atrial electrogram being recorded at CS distal dipole. On the other hand, the net stim to A (S‐A) interval prolonged after loss of His capture. This is best explained by the fact that the His pacing has engaged the AV circuit orthodromically probably via the His and left bundle branch (LBB). Loss of His capture left only advancement of the AV circuit via the RV paraHisian myocardial site, a longer access to the circuit and hence with a longer S‐A. This also suggests the LBB is part of the AV circuit.
The dissimilar local V‐A intervals at the site of earliest atrial activation with HB‐RB capture and RV capture may actually represent obliquity of the accessory pathway. With an oblique course, a ventricular wave front propagating from the direction of the ventricular end of the accessory pathway (concurrent direction) produces an artificially short local V‐A interval at the site of earliest atrial activation as in the case of HB‐RB capture with subsequent left anterior fascicle activation. Reversing the direction of the ventricular wave front (countercurrent direction) increases the local V‐A interval all along the accessory pathway, because the ventricular wave front passes the accessory pathway before reaching and activating its ventricular end. This was demonstrated here subsequent to loss of HB‐RB capture following which the wave front captures the ventricular myocardium and moves in a clockwise fashion across the mitral annulus toward the ventricular insertion.