ECG Leads

First Time Seeing an ECG, ever?
I highly recommend a slightly more "didactic" approach to looking at ECGs - which thanks to the many free material out there, they aren't hard to come by. For the basics of electrophysiology and the ECG, the following are good introductions to the subject matter:

YouTube: Electrocardiography (ECG/EKG) - basics [OsmosisMed]
CVPhysiology: Electrocardiogram (EKG, ECG)

As described in Electrophysiology Basics, the appearance and size of the waves can depend on the direction that we are viewing the heart. These different views, or "leads", can give different snapshots of the electrical activity throughout the heart.

Cardiac Monitoring with Limb Leads

For the purpose of monitoring arrhythmias, the limb leads are probably the most important. They view the heart in the two-dimensional frontal plane from each of the "limb" directions (more-or-less).

aVF

Since lead II follows the same direction as the overall electrical direction of the heart, the waves in that lead tend to be more visible compared to the other limb leads. Therefore, it's often used as the primary lead for monitoring arrhythmias.

A combination of leads can be used to look at a specific view of the heart:

  • Inferior View: Leads II, III, and aVF
  • Lateral View: Leads I and aVL (of the limb leads)

Complete Picture with Precordial Leads

The precordial leads add a third-dimensional view of the heart, primarily looking at the heart from the central chest wall and sweeping to the left-hand side.

Like the limb leads, a combination of leads can be used to look at a specific view of the heart:

  • Septal View: Leads V1 and V2
  • Anterior View: Leads V3 and V4
  • Lateral View: Leads V5 and V6 (of the precordial leads)

12-Lead ECG

The combination of the limb leads and precordial leads gives us a complete view of the heart: the 12-lead ECG. While the focus of the first part of this reference is on cardiac monitoring arrhythmias (specifically with lead II), it's important to appreciate that the 12-lead ECG is used to assist diagnosing a wide variety of other cardiac condtions, including an MI.

The standard 12-lead ECG is almost always arranged as above.

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