Step 1 of 7
Conduction substrate
Pre-existing conduction abnormality at baseline?
None — standard 12-lead
No underlying BBB or pacing
LBBB
Left bundle branch block at baseline
RBBB
Right bundle branch block at baseline
LBBB + LAFB
Superior axis deviation at baseline
LBBB + LPFB
Inferior axis deviation at baseline
Nonspecific IVCD
Widened QRS, non-classic pattern
RV Paced
RV-only pacing — LBBB-pattern paced beats
BiV / CRT Paced
Biventricular pacing
Step 2 of 7
V1 morphology
Dominant QRS pattern in lead V1 during the PVC
LBBB-pattern
Predominantly negative — QS or rS in V1
RBBB-pattern
Predominantly positive — rsR′ or qR in V1
Tall monophasic R
No q, no S — broad upright R in V1
qR pattern
Small q then dominant R in V1
Biphasic RS
Roughly equal positive and negative deflections
Step 3 of 7
Frontal plane axis
Based on II, III, aVF, and lead I during the PVC
Inferior axis
II, III, aVF all positive → superior cardiac origin
Superior-left axis
II, III, aVF negative · Lead I positive
Superior-right axis
II, III, aVF negative · Lead I negative
Normal-left axis
Inferior leads positive · Lead I positive
Step 4 of 7
Precordial transition
Where does R ≥ S in the precordial leads?
V1 – V2 (early)
Right precordial — posterior or LV origin
V3
Intermediate — RVOT free wall vs septal border
V4 (normal zone)
Typical RVOT septal location
V5 – V6 (late)
Anterior or leftward septal origin
No transition
Persistent S throughout all precordial leads
Step 5 of 7
Lead morphology details
Polarity in each lead during the PVC
Lead I
aVR
aVL
Step 6 of 7
Epicardial morphology flags
Berruezo scoring criteria — check all that apply
Pseudo-delta wave
Slurring before QRS onset ≥ 34ms in V1–V3
QRS notching / slurring
Visible notch on upstroke or downstroke
MDI ≥ 0.55
Maximum deflection index — slow initial forces
Intrinsicoid delay ≥ 85ms
V5 or V6 — onset to peak R interval
Shortest RS in V2
V2 has shortest RS interval in precordium
Q waves — inferior leads
Basal epicardial LV pattern
None present
Step 7 of 7
Review & analyze
Confirm inputs before running localization