Background: Accurate mapping of the compact atrioventricular (AV) node is critical during ablation of a range of arrhythmias.

Objective: The purpose of this multicenter prospective study was to test the hypothesis that microelectrode (ME)-embedded catheters more accurately define the near-field compact AV node compared to conventional catheters.

Methods: For the mapping phase, detailed AV junction maps were created in 47 patients using an ME-embedded catheter. His electrograms (EGMs) detected by conventional electrodes (His_c) and by ME (His_μ) were annotated. For the ablation phase, AV nodal ablation (Qmode 50 W) was performed in 10 patients after pacemaker implantation, with initial His_c-only ablation in group 1 (n = 6) and initial His_μ ablation in group 2 (n = 4). For the clinical phase, a prospective registry of parahisian tachycardia using QDOT was obtained.

Results: In the mapping phase, 7.0 ± 5.4 His_c and 8.0 ± 5.6 His_μ points were acquired per map (n = 47). His_μ cloud was smaller and more proximally located than His_c cloud: (99.4 ± 74.7 mm² vs 197.6 ± 110.6 mm²; P = .0008). His_μ EGMs had larger amplitudes than His_c EGMs (0.40 ± 0.38 mV vs 0.16 ± 0.1 mV; P = .0002). In the ablation phase, for group 1: His_c-only ablation never resulted in AV block, whereas His_μ ablation resulted in AV block after limited ablation in all patients (after 13.3 ± 9.2 s); and for group 2: His_μ ablation always resulted in AV block after 1 application (after 14.3 ± 10.3 s). In the clinical phase, a His_μ-avoidance strategy could avoid AV block in a prospective registry of 11 patients.

Conclusion: ME more accurately defines the region of the compact node, and ablation in this region is associated with a high risk for AV block. ME-based mapping has the potential to significantly enhance ablation safety and efficacy.