ECG Level 1 Tutorial: Basic Electric Stuff
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Tutorial: Basic Electric Stuff
This module will teach the basics of electrical physiology in the heart.
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Tutorial: Basic Electric Stuff The Limb Leads
Times Practiced
Cases Completed
1h 24m
Total Time spent
1m 24s
Average Time
The Limb Leads
One ECG lead is comprised of 2 electrodes, a positive one and a negative one.

We can place these 2 electrodes at different locations on the body to allow us to measure the direction of electrical movement in different directions. Each ECG lead will show a slightly different deflection, depending on whether the electrical charge is moving toward it, away from it, sideways to it, or at an angle to it. Therefore, at any given moment in time, each ECG lead can show something slightly different.

It is important to memorize the ECG leads to help you visualize how the electrical charge moves through the heart. There are 12 leads in total (and by getting a little bit fancy, we can make even more leads, but we will start at 12). These 12 leads are divided into 2 categories: the limb leads and the precordial leads (next lesson).

The following images will describe the 6 limb leads. Remember that to define one lead, you need to describe 2 electrodes and then draw a line between the 2 electrodes to define the direction of the lead. Every ECG lead has a "positive" end which is the location of the positive electrode.

To create 6 limb leads, we only use 3 electrodes as shown in the diagram on the right. We will label these electrodes A, B, and C.

In real life, the "A" and "B" ECG stickers could be placed on the wrists or the shoulders (or really, anywhere along the arm). It does not make any difference "electrically". Also, for the "C" electrode, we often place stickers on both ankles and use them together. Placing the stickers on wrists and ankles makes sticker placement more consistent. For simplicity in trying to explain the concepts of the different leads, we have drawn the ECG electrodes on the chest.

Pay close attention to the direction of the arrow. It always points to the "positive" electrode. Many years ago, the electrophysiologists made arbitrary decisions which electrode would be the "positive" ones.

Lead I
Lead I

Lead II
Lead II

Lead III

We have now defined half of the limb leads. These leads measure movement in a right-left (or left-right) direction on the body and also in an up-down (or head to toe) direction. Because all the leads are placed on the front of the body, they cannot measure any electrical movement in a front-back direction.

To define the remaining 3 limb leads, what the electrophysiologist did many years ago was to take the "average" of 2 electrodes and compare to a third electrode. In other words, the two negative electrodes are combined, creating a "virtual" electrode that is positioned halfway between the 2 original electrodes. In the first example below (Lead aVF), electrodes A and B are "electrically" averaged and compared to electrode C. You can see how the direction of this lead is different than the above 3 leads and the start of the arrow is midway between electrodes A and B.

Historical note: these leads have a little "a" in the name to mean "augmented". The word "augmented" arose because originally the active electrode was compared to an average of all three electrodes. When they removed the active electrode from the "averaged electrodes" the electrical deflection became greater and thus "augmented" (thank you Dr. Zane Farina for explaining this to me). 

Lead aVR   (R = points to right arm)
Lead aVR

Lead aVF   (F = points to feet)
Lead aVF

Lead aVL  (L = points to left arm)
Lead aVL

The limb leads measure electrical activity in the "coronal" plane, as indicated by the diagram below. Limb leads measure activity that is left-right (or right-left) or up-down, but not front-back.

The next lesson will show the precordial leads which measure activity in a different plane. Yes, I know, you are very excited.