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ECG Primer: The Leads of a 12-lead ECG

Created by Diane R. Karius, Ph.D.
 

The Leads

As with skeletal muscle, electrical activation of the heart is required for the mechanical activation of the heart.  The electrical activation of the heart can be picked up (detected) using skin electrodes strategically placed on the body.  The placement of these  electrodes gives us the leads we use in recording an ECG.    You can move through this part of the tutorial as written or you can use the links below to jump to certain parts of the description. 

The standard limb (bipolar) leads
The augmented unipolar leads
The chest leads

The first sets of leads developed were the standard limb leads, also known as the bipolar leads.   In the standard limb leads, the electrodes were placed on the wrists and ankles of the limbs (a good bony place- that's nice because there's no muscle there to generate action potentials that obscure the heart's activity).  Both arms and the left leg were used as active leads (i.e. they became one of the poles in our recording), while the right leg was used as a ground (and still is).  With time, we recognized that the same recording could be made by placing the electrode at the shoulders and hips (and with less opportunity for noise to interfere with our recordings).  The placement of the electrodes for the standard limb leads is shown in this figure.

lead1.GIF (16990 bytes)

Lead I is constructed by comparing the left arm (as positive) to the right arm's electrode (as negative) (as shown in the next figure).   The zero point is in the center of  the lead (indicated by the hash mark).   Any current flowing to the left (towards the left arm's electrode) produces a positive deflection on the ECG, while any current flowing to the right produces a negative deflection.  In essence, we are viewing the heart as if from the zero point (indicated by the little man on the picture).  Lead I gives us a very good view of what is going on from left to right in the heart, but a poor view of events moving up or down (perpendicular to the lead I axis). 

lead2.GIF (17983 bytes) 

There are two additional bipolar leads (the name "bipolar" refers to the fact that one electrode is defined as positive, while a second electrode provides the negative).  Lead II (pictured below, on left) connects the left leg as positive to the right arm's negative.  Lead III connects the left leg as positive to the negative left arm (pictured below, right).   Each of these provides a  view of the heart essentially from the zero point, as with lead I. 

lead3.GIF (18790 bytes)  lead4.GIF (18476 bytes)

There are several ways in which you can remember the different bipolar leads (both what connects to what and which is positive vs. negative.  These are summarized in the table:

Which is which - + or -?

Remembering the connections

Right arm always - Lead I 1 "L":  Left arm to right arm
Left leg always + Lead II 2 "L's":  Left Leg to right arm
Left arm whatever's left Lead III 3 "L's":  Left Leg to Left arm

The Augmented Unipolar leads

The quality of the signal from the standard limb leads can be quite variable - often the electrical signal from the heart is small when compared to the electrical activity of nearby skeletal muscle.  In addition, the standard limb leads give us only three views of the heart.  The augmented unipolar leads were devised as a way of increasing the size of the signal (the heart's electrical activity) and giving us new views of that activity.  The augmented unipolar leads use the same electrodes as you used for the standard limb leads.  The only thing that changes is how these electrodes are connected.  For each of the augmented unipolar leads (represented by the prefix aV), two of the three electrodes we had used are tied together and brought to ground.  The remaining electrode becomes the exploring or active lead.  In the case of any unipolar lead, current flow heading towards the active (exploring) electrode produces a positive deflection, while current going away from the electrode produces a negative deflection.  The three augmented limb leads are shown in the next picture (the little person indicates the "view" of the heart each electrode is providing): 

lead5.GIF (24808 bytes)

Note that space restrictions did not allow the two remaining electrodes from being tied together etc....

 

The Chest Leads: One of the limitations of either the augmented unipolar leads or the standard limb leads is that they all look at the heart from the same plane (the coronal, to use the anatomist's term).  In order to view the heart from another plane (the transverse or horizontal plane), the six chest leads came into use.   Each of these leads is a unipolar lead (so current moving towards the active electrode produces a positive deflection, away a negative deflection).  By using these six leads, we can now get information about the anterior and posterior parts of the heart (as well as inferior and superior).  These six leads are pictured below: 

Lead6a.gif (27799 bytes)


 To move on (The normal ECG is the "next" step): 

ECG basics:   Conduction through the heart
The normal ECG
Calculations (heart rate; PR interval, etc)
The Mean Electrical Axis