Leads informs us about how positive charges are moving in the heart. Based on our knowledge of the sequence of cardiac activation and recovery and the cardiac action potential, we can use leads to gauge how the heart is depolarizing or repolarizing. However, leads can serve another function as well.

Because of their directionality, leads can help localize pathologies to specific regions of the heart. Click through on the 3D model below to visualize this concept.

Types of leads

There are 4 main categories of leads used commonly in clinical practice.

Why do we need so many leads? The more leads we have, the more perspectives from which we can assess the electrical activity of the heart. 

The Posterior Leads

The posterior leads are formed by placing leads on the left upper back. They help us visualize electrical activity in the posterior aspect of the heart along the transverse plane.

The Reverse Precordial Leads

The reverse precordial leads are formed by mirroring the precordial leads about the sternum. They help us visualize activity in the right ventricle along the transverse plane.

In Summary

12-Lead Versus 15-Lead

In practical applications, you may often hear references to “12-lead” or “15-lead” ECGs.

The 12 lead ECG includes the following leads: I, II, III, aVR, aVL, aVF, V1, V2, V3, V4, V5, V6

The 15 lead ECG typically refers to: I, II, III, aVR, aVL, aVF, V1, V2, V3, rV4, V8, V9. 

However, the 15-lead ECG can include any assortment of leads as desired by the clinician, and is usually a mix of the limb leads, as well as a combination of any precordial ± posterior ± reverse precordial leads.

Note that sometimes the 15-lead ECG still only has 12 leads displayed, so the “15-lead” part is a bit of a misnomer. It is so-called because it includes 3 leads not normally found in the regular 12-lead ECG.