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 Limb Leads
The limb leads help us visualize charge movement along the frontal plane (up/down and left/right). As such, the limb leads are not good for assessing currents moving anteriorly/posteriorly.
Importantly, limb leads help us determine the direction of electrical activity in the frontal plane, which is also known as axis.
The Precordial Leads
The precordial leads form a set of unipolar leads, because they’re technically only formed by using one electrode each. The negative electrode for all precordial leads is a mathematical ground, calculated by summing up the potentials at all of the electrodes used for the limb leads. This mathematical ground, known as Wilson’s central terminal, can be thought of being located in the centre of the thorax.
The precordial leads help us visualize charge movement along the transverse plane (left/right and front/back). As such, the precordial leads are not good for assessing currents moving up/down – not to worry, however, as the limb leads take care of this.
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
An example of 12-lead ECG paper.
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.
An example of 15-lead ECG on a 12-lead paper.