Localizes the accessory pathway by delta-wave polarity in the frontal plane and by the precordial transition. E = side · P = posterior/anterior · M = precordial migration (right-sided pathways only).
Step 1
Step E · side
“E”: how many criteria? (1) negative delta wave in lead I or aVL; (2) R > S in V1
Limitation
An estimate based on surface ECG morphology. It does not replace the electrophysiology study for definitive pathway localization.
A tool to support learning and decision-making. The result does not replace clinical evaluation of the patient.
About: EPM algorithm
The EPM algorithm localizes the accessory pathway in ventricular pre-excitation (Wolff-Parkinson-White pattern) in up to three steps. Each letter is a step: E defines the side, P separates posterior from anterior pathways, and M, the migration of the precordial transition, refines right-sided pathways.
Knowing where the pathway lies helps plan ablation and estimate procedural risk.
How to use
Step E: count the criteria (negative delta wave in lead I or aVL, and R > S in V1). None indicates a right-sided pathway; one, left septal; two, left lateral.
Step P: check delta-wave polarity in lead III. Negative suggests a posterior pathway; positive, anterior.
Step M (right-sided pathways only): check whether the precordial polarity transition occurs by V3 or later.
How to interpret
Left-sided pathways: left posteroseptal, anterolateral or posterolateral.
Right-sided pathways: right posteroseptal, posterolateral, anteroseptal or anterolateral.
Limitations
It works best with maximal pre-excitation. Multiple pathways, subtle pre-excitation or structural heart disease reduce accuracy. Definitive localization is made in the electrophysiology study.
References
de Alencar Neto JN, Sakai MH, de Almeida Neto RG, Scheffer MK, Alencar e Silva GPS, Cirenza C, et al. EPM algorithm: a stepwise approach to accessory pathway localization in ventricular pre-excitation. J Electrocardiol. 2024;84:1–8.
