Accessory Pathway Localization from ECG

Localization of an accessory pathway (AP) relies on analyzing the initial 20–40 milliseconds of the QRS complex — the delta wave. Because this initial deflection represents purely pre-excited myocardium before normal His-Purkinje conduction catches up, the vector of the delta wave points directly away from the pathway’s ventricular insertion site.

Here is the step-wise approach to pinpointing the pathway along the atrioventricular annuli.

1. Left vs. Right (The V1 Discriminator)

Lead V1 sits to the right and anteriorly. It is the primary discriminator for which side of the heart holds the pathway.

V1 Morphology	Pathway Location	Vector Direction
Positive Delta (R > S)	Left-sided (Classic “Type A”)	Travels anteriorly and rightward toward V1
Negative Delta (QS or rS)	Right-sided (Classic “Type B”)	Travels posteriorly and leftward away from V1

2. Anterior vs. Posterior (The Inferior Leads)

Leads II, III, and aVF act as the floor of the heart. They determine the pathway’s position along the superior-inferior axis.

Inferior Leads (II, III, aVF)	Pathway Location	Vector Direction
Negative Delta	Posterior / Posteroseptal	Travels superiorly, away from the inferior leads
Positive Delta	Anterior / Anteroseptal	Travels inferiorly, toward the diaphragmatic surface

3. Septal vs. Lateral Free Wall

Differentiating a septal insertion from a free-wall insertion requires looking at the frontal plane leads (I and aVL) and the precordial transition zone.

• Left Lateral Free Wall: Look for a negative or isoelectric delta wave in leads I and aVL. The impulse is traveling from the far left lateral edge toward the right.
• Right Lateral Free Wall: Typically shows a strongly positive delta wave in lead I. The precordial transition (R > S) is delayed, often not occurring until V3 or V4.
• Septal Pathways: These often present with a very early precordial transition (V1 or V2). Posteroseptal pathways specifically will often mimic an inferior myocardial infarction, presenting with deep Q waves (which are actually negative delta waves) in leads II, III, and aVF.

Clinical Caveat: Accurate localization requires maximal pre-excitation. If the baseline ECG shows minimal pre-excitation due to rapid AV node conduction, the delta wave vector will be blunted or misleading.