Understanding Differential Pacing: Unraveling the pitfalls of base vs apex pacing in distinguishing AVNRT from AVRT over a septal APPavel Antiperovitch1, Ahmed Mokhtar2, George Klein1Western University, Department of MedicineDepartment of Medicine, King Abdulaziz University, Jeddah, Saudi ArabiaWord Count : 552Abbreviations :AVNRT: Atypical Atrioventricular Nodal Reentrant Tachycardia, AVRT: Atrioventricular Reentrant Tachycardia, AP: Accessory Pathway, AV: Atrioventricular, CS: Coronary Sinus, PPI-TCL:, Post-Pacing Interval – Tachycardia Cycle Length, RA: Right Atrium, RBB: Right Bundle Branch, RV: Right Ventricle, SVT: Supraventricular Tachycardia, VA: Ventriculo-Atrial, VOP: Ventricular Overdrive PacingThe article “Differential Ventricular Overdrive Pacing During Long RP’ Supraventricular Tachycardia: How Can We Interpret?” by Nakashima et al1provides some reflection on a widely used differential pacing maneuver. The authors present a case of long RP supraventricular tachycardia, where right ventricular (RV) entrainment excluded atrial tachycardia. Following adenosine administration, there was AH prolongation and a block to the RV while the tachycardia persisted, indicating that neither the AH interval nor the RV were part of the reentrant circuit. The authors conclude that the most likely diagnosis is atypical AV nodal reentrant tachycardia (AVNRT). To further explore this, the authors conducted ventricular overdrive pacing (VOP) from both the base and apex of the RV. They observed that entrainment from the RV base resulted in a shorter VA interval and a smaller PPI-TCL compared to the apex. This finding supports the presence of an AV pathway, which contradicts earlier diagnostic assessmentsDifferential pacing is a widely used technique to distinguish between nodal and extranodal ventriculo-atrial (VA) conduction via an accessory pathway.² In this case, why did the standard maneuver fail? The authors suggest that inadvertent His bundle capture was the reason, as indicated by the narrower paced QRS on the surface ECG when pacing close to the base. This issue can be avoided by repositioning the pacing electrode away from the His bundle or adjusting the pacing output. Another common pitfall in differential pacing is the location of the second pacing site. The traditional second pacing site is the apical region, which is a surrogate for approximating the site of the right bundle branch (RBB) exit into the RV. However, identifying the apex on fluoroscopy or a mapping system is subjective and probably poorly reproducible between patients and operators, leading to inconsistent distances of the recording catheter to the RBB terminus .To overcome these issues, we propose a slight shift of focus away from the conventional “apex” and “base” approach to a more reproducible physiological concept that is familiar to every electrophysiologist: the line of block. The atrioventricular (AV) plane of the heart serves as a natural line of block, and any conduction gaps other than the normal AV conduction system along this plane represent AV pathways. The operator’s goal is to test for completeness of the line of block by pacing close to and at a fixed distance from the AV ring (Figure 1). The first pacing site is positioned as close as possible to the annulus, typically in the posteroseptal RV just beyond the coronary sinus (CS) ostium, to reduce the risk of capturing the conduction system or atrium. The second site is placed a few centimeters farther away from the base moving inferiorly. Very simply, moving inferiorly a short distance from the annulus must shorten the SA interval if the “line of block” is complete (i.e. no septal pathway) and lengthen if there is no block (i.e. a septal AP). This approach removes the need to locate the true apex, at best an imprecise surrogate for the RBB exit into the RV, and reframes differential pacing in terms familiar to every electrophysiologist—it’s grounded in the fundamental principles we apply daily in the lab. It also sheds light on the common pitfalls of the maneuver, which are akin to those encountered when testing for gaps along ablation lines.REFERENCES1. Nakashima T, Nagase M, Usui T, et al. Differential ventricular overdrive pacing during long-RP supraventricular tachycardia: How can we interpret?Journal of Cardiovascular Electrophysiology. n/a.2. Martínez-Alday JD, Almendral J, Arenal A, et al. Identification of concealed posteroseptal Kent pathways by comparison of ventriculoatrial intervals from apical and posterobasal right ventricular sites. Circulation. 1994;89:1060–1067.3. Derval N, Skanes AC, Gula LJ, et al. Differential sequential septal pacing: A simple maneuver to differentiate nodal versus extranodal ventriculoatrial conduction. Heart Rhythm. 2013;10:1785–1791.FIGURE 1 - Diagram illustrating the concept of differential pacing/entrainment using the “Line of Block” principle.