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Cardiac • 12-Lead ECG

12-Lead ECGs

A structured pathway for interpreting 12-lead ECGs — from setup and lead placement to 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.

PDF High risk

ECG Red Flags

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

View PDF
Image by Freepik
Image Mnemonic

HISAL

A simple memory aid for rapid, structured ECG interpretation.

Quick Reference
Image Limb leads

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.

Basics Setup

Getting Started

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

Open
Method Repeatable

Systematic Approach

A repeatable method: rate → rhythm → axis → intervals → hypertrophy → ischaemia/injury → conduction → comparison to prior ECG.

Open
Localise Walls

Lead Territories

Which leads look at which walls: inferior, lateral, septal/anterior — and how that guides localisation.

Open

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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