Leads and Their Views: Different Cameras, Same Heart

Think of each ECG lead like a camera lens, each one aimed at a specific region of the heart. When you record a 12-lead ECG, you’re capturing 12 different angles of the same cardiac event, like “Bullet Time” from The Matrix. Each lead provides unique insight into electrical activity through its “viewpoint,” helping to localise pathology like ischaemia or infarction.
The leads are grouped based on which part of the heart they visualise:

Septal leads (V1, V2 / Blue): These peer through the sternum at the interventricular septum.

Inferior leads (II, III, aVF / Red): These look up from below, viewing the inferior wall of the left ventricle, typically supplied by the right coronary artery.

Lateral leads (I, aVL, V5, V6 / Green): These face the left side of the heart, capturing the lateral wall — often reflecting changes in the circumflex artery territory.

Anterior leads (V3, V4 /Yellow): These take a front-on view of the heart, looking directly at the anterior wall, typically supplied by the left anterior descending (LAD) artery.

(Note that the colours on the diagram do not correlate with the colours of the ECG leads).

Each lead records the electrical impulse as it moves toward or away from the electrode. If the current travels toward a lead, you get a positive deflection, if it travels away, you get a negative one. That’s why understanding lead orientation is key to interpreting abnormalities like ST-elevation, T wave inversion, or pathological Q waves.
Misinterpretation often stems from not appreciating what each lead is “watching.” For instance, ST-elevation in II, III, and aVF, but nowhere else, tells you the issue is inferior and not global. Add reciprocal changes, and your case for an MI strengthens.

Mastering lead localisation refines your differential diagnosis, elevates your confidence, and can fast-track time-critical decisions, especially in pre-hospital or acute settings where “time is myocardium.”