Cardiac • 12-Lead ECG

12-Lead ECG Interpretation for Paramedics

Learn 12-lead ECG interpretation step by step — from electrode placement and lead orientation to systematic analysis, localisation, ischaemia patterns, conduction abnormalities, and common mimics.

What a 12-Lead ECG Shows

10 electrodes → 12 views

A standard 12-lead ECG provides 12 different “views” of cardiac electrical activity using 10 electrodes: 4 limb electrodes (RA, LA, RL, LL) and 6 precordial (chest) electrodes (V1–V6).

Limb leads provide views in the frontal plane (I, II, III, aVR, aVL, aVF) and precordial leads provide views in the horizontal plane (V1–V6). Interpretation is strongest when you combine waveform analysis with lead-territory localisation.

12-Lead Quick References

Fast visual aids you can open alongside a 12-lead for rapid review.

Red warning flag representing high-risk ECG findings

ECG Red Flags

High-risk ECG patterns to recognise early and correlate with the full clinical picture.

View PDF Image by Freepik

HISAL

A simple memory aid for rapid, structured ECG interpretation.

Quick Reference

Electrical Axis

Quick reference for determining frontal plane axis using limb leads.

Quick Reference

Learning Pathways

Work through the fundamentals, then apply a consistent interpretation method and localisation.

ECG waveform illustration used for 12-lead fundamentals

Getting Started

Paper speed and calibration, lead placement, artefact recognition, and core measurements including PR, QRS and QT/QTc.

Open Getting Started

ECG paper grid used for systematic interval and waveform measurement

Systematic Approach

A repeatable sequence for rate, rhythm, intervals, axis, ST-T assessment, conduction and clinical correlation.

Open Systematic Approach

12-lead ECG territory and lead-orientation quick-reference chart

Lead Territories

Understand which leads view inferior, lateral, septal and anterior regions and how those groups support localisation.

Open Lead Territories

Lead Groups and Localisation

Contiguous leads

Inferior: II, III, aVF
High lateral: I, aVL
Lateral: V5–V6 (often with I, aVL)
Septal: V1–V2
Anterior: V3–V4
Posterior involvement: consider posterior leads (V7–V9) when indicated
Right ventricular involvement: consider right-sided leads (e.g., V4R) when indicated

Ischaemia and Injury Patterns

Interpret in context

ST elevation in contiguous leads suggests an acute injury pattern; interpret in context.
Reciprocal changes (ST depression in opposing leads) can support localisation.
ST depression may reflect subendocardial ischaemia, reciprocal change, or other causes.
T-wave inversion can be ischaemic or non-ischaemic — pattern recognition matters.
Q waves may indicate prior infarction when pathological criteria are met.

Axis and Conduction

Intervals + morphology

Axis assessment using I and aVF (and aVL if needed).
Bundle branch blocks (RBBB/LBBB) change QRS morphology and ST-T appearance.
Fascicular blocks (e.g., LAFB) often produce axis shifts.
QT/QTc recognition and the importance of consistent measurement.

Hypertrophy and Common Mimics

Don’t get baited

LVH/RVH patterns and their effect on ST-T segments (“strain” patterns).
Early repolarisation as a common benign ST elevation mimic.
Pericarditis patterns (diffuse ST elevation with PR depression) — context matters.
Electrolytes/drugs can alter T waves, QRS width, and QT interval (high-level recognition).
Lead placement errors can mimic pathology — always consider technical factors.

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