Line of block: A simple pacing maneuver to distinguish AV nodal from accessory pathway ventriculoatrial conduction

Septal accessory pathways (APs), including concealed APs can be difficult to diagnose using standard electrophysiology (EP) maneuvers. We introduce a simple technique using the "line of block" concept that can potentially added to the list of standard maneuvers in cases where the presence of a septal AP is suspected.

This study aimed to determine the efficacy of the "line of block" concept among patients with septal APs using a reproducible and simple technique.

An exploratory catheter is used to pace the basal right ventricle, and the stimulus-to-atrial interval at pacing site 1 (SA1) is measured. The exploratory catheter is then moved 2-3 cm deeper into the right ventricle toward the apex, and the stimulus-to-atrial interval at pacing site 2 (SA2) is measured. The results (SA1 > SA2 = nodal response vs SA1 < SA2 = AP response) were determined. The technique can also be done during entrainment of ongoing tachycardia.

24 patients undergoing an EP study fulfilled the criteria for enrollment and were included in the study between October 2023 and April 2025. 10 patients (41.7%) had proven APs, while 14 patients (58.3%) had none. The mean SA1-SA2 for patients with proven AP was 19.4 msec (standard deviation ± 9.2), whereas the mean SA1-SA2 for patients without an AP was 10.5 msec (standard deviation ± 10.5). All patients with AP demonstrated an SA1-SA2 less than 0 msec, whereas the reverse was true among patients with no demonstrable AP.

Using the "line of block" concept by determining the ΔSA1-SA2 allows for easy determination to the presence or absence of an AP. Larger studies and sample sizes are required to allow for universal application in other EP labs.