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. ^{,  ,}  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; Figure 1, Figure 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.