Paradoxical Increase in Stimulus to Atrium Interval During Para‐Hisian Pacing

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A 40‐year‐old gentleman has undergone electrophysiological study for paroxysmal palpitation. Surface electrocardiogram during sinus rhythm showed no preexcitation. The AH and HV intervals during sinus rhythm were 104 and 40 ms, respectively. There was no dual AV node physiology demonstrated. Para‐Hisian pacing was performed to exclude a septal bypass tract which showed an interesting response (Fig. 1).
Para‐Hisian pacing demonstrated relatively broader (104 ms) and narrow (60 ms) QRS morphologies with stimulus to atrium (S–A) intervals of 156 and 175 ms, respectively. The atrial activation sequence with the 2 QRS morphologies is not similar (Fig. 2). The ventricular activation has also changed from the left to the right panels, with the proximal left ventricle (as suggested by the V electrogram [EGM] in the coronary sinus [CS] electrodes 5, 6, 3, 4, 1, and 2) and para‐Hisian right ventricle are relatively early when the QRS is broader and distal LV is relatively earlier (as suggested by the V EGM in the CS electrodes 5, 6–1, 2), when the QRS is narrow. The local V component of the His bundle (HB) EGM on the right panel when the QRS is narrow is delayed.
Delay in the local V component of HB EGM when the QRS is narrow suggests pure Hisian capture. The figures would be readily explained by para‐Hisian on the left panel (relatively broader QRS with QRS duration of 104 ms) with conduction to the atrium via a right‐sided concealed accessory pathway or possibly via a nodoventricular bypass tract. With the loss of para‐Hisian ventricular capture and pure His pacing (on the right panel with narrow QRS), retrograde conduction is now going over the AV node with a longer HA time and hence longer S–A interval with relatively narrower QRS.
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