The initial model of atrioventricular nodal tachycardia (AVNRT) as a small circuit of reentry within the AV node precluded a catheter-based cure for this arrhythmia without the occurrence of AV conduction block. Landmark observations that the atria were part of the circuit of AVNRT1 and that the arrhythmia could be reset with paced beats placed at sites anatomically distant from the compact AV node2 set the stage for the present expectation of curative ablation without AV block. Despite this understanding of the AVNRT mechanism and the large accrued operator experience during the past 3 decades when ablating this arrhythmia, AV block still occurs in ≈1% to 2.3% as a complication of ablation.3 See Article p 739 In this installment of teaching rounds, Chen et al4 share with us what we can learn from a series of patients with permanent or transient AV block noted during and soon after radiofrequency energy delivery. The authors provide an honest, transparent, and instructive view of their 5 cases. The fact that we are successful ≈99% of the time without complication suggests that the present methods for maintaining safety, including monitoring junctional rhythm during ablation and relating catheter position to the fluoroscopic anatomic approximation of the AV node location are mostly successful. The student of electrophysiology must, however, be aware of how these precautions can fail us. Even closely spaced bipolar electrodes placed over the compact AV node are not able to record electrograms from the node itself. Because we cannot map the structure we need to avoid, we must estimate anatomically its location and correlate this information with our standard fluoroscopic views. ### Utility The compact AV node is located within Koch’s triangle, approximately in its mid and central position. Using the His bundle as a surrogate for the superior vertex and …