Para-Hisian pacing: Useful clinical technique to differentiate retrograde conduction between accessory atrioventricular pathways and atrioventricular nodal pathways

Para-Hisian pacing is a useful tool to differentiate between retrograde conduction over an accessory pathway and retrograde conduction over the fast or slow atrioventricular (AV) nodal pathways. 1-3 Para-Hisian pacing uses right ventricular (RV) pacing close to the His bundle or proximal right bundle branch (RBB). As the position of the ventricular pacing catheter changes subtly during respiration (or by changing pacing output), the pacing stimulus changes capture among (1) basal anteroseptal RV plus His bundle or proximal RBB (His bundle-RBB capture); (2) capture of basal anteroseptal RV alone; and (3) His bundle-RBB capture alone. These changes in pacing capture result in abrupt changes in the timing of His bundle activation relative to the timing of ventricular activation. The presence or absence of a change in atrial activation sequence, stimulus-atrial (SA) intervals, and His bundle-atrial (HA) interval identifies whether retrograde conduction is dependent on ventricular activation (retrograde conduction over an accessory pathway) or His bundle activation (retrograde conduction over the AV node) or both (accessory pathway and AV node; Figures 1 and 2). The loss of His bundle-RBB capture is usually identified by the widening of the QRS complex, indicating that some of the ventricular myocardium (farthest from the RV basal septal pacing site) is activated by the His-Purkinje system during both RV and His bundle-RBB capture. Our preferred approach is to position the RV pacing catheter toward the RV outflow tract, 1‐2 cm superior to the His bundle catheter. While pacing at moderate output (5‐10 mA and 2 ms pulse width), the RV catheter is slowly withdrawn toward the His bundle catheter (proximal RBB) until intermittent His bundle-RBB capture occurs because of changes in catheter position during respiration. As the pacing catheter is moved closer to the His bundle, the ventricular potential in the His bundle electrogram becomes earlier and the retrograde His bundle potential becomes later, providing greater separation between the local ventricular and His bundle potentials. The use of closely spaced electrodes (1 mm edge-to-edge) on the His bundle catheter shortens the duration of the local ventricular potential in the His bundle electrogram, preventing the masking of the His bundle potential by the local ventricular potential. Closely spaced electrodes on the pacing catheter may provide a smaller pacing field, facilitating intermittent loss of His bundle-RBB capture with small changes in catheter position during respiration. A deflectable RV pacing catheter is positioned more easily for para-Hisian pacing and subsequent positioning of the pacing catheter close to the location of an accessory pathway (Figure 3A). In patients with proximal RBB block (such as resulting from previous ablation), His bundle-RBB capture requires pacing proximal to the RBB block site and may not be achieved from the RV. Other approaches include changing pacing output to achieve intermittent His bundle-RBB capture and use of the distal electrodes on the His bundle catheter for pacing. 4