Fundamentals of Reading EKGs

Fundamentals >Defining Leads

Electrical Conduction Patterns

The pumping action of the heart, termed contractility, depends on an electrical phenomenon called depolarization.  During depolarization, the cells are temporarily altered, switching from a negatively charged interior to a positively charged interior.  This wave of depolarization spreads throughout the myocardium via a series of conduction pathways, ultimately triggering cardiac myocytes (muscle cells) to contract.  Following depolarization, the cells revert back to their resting state, with the interior of the cell negatively charged.  An electrocardiogram, or EKG,  records these changes in potential energy (voltage) through  a series of carefully placed skin electrodes. 

Constructing the 12 Leads

In previous sections, we have discussed the proper placement of EKG leads.  There, we used 6 precordial leads and 3 limb leads to record a 12-lead EKG.  So now the question remains, how do you get twelve leads from 9 wires?

The first step is to clarify the ambiguous terminology.  Within cardiology, the term “lead” can have several meanings, based on the context in which it is used.  When discussing wires and electrodes, the term lead refers to the physical connection between the electrode and recording machine.  Each wire and electrode combination is termed a lead.

However, after the wires are successfully placed and the electrical activity is recording, the term lead takes on a different connotation.  Now, it refers to the potential difference recorded between any two recording locations (electrodes). 

Each combination of electrodes is assigned a unique name, with one electrode designated as “positive” and the other electrode designated as “negative”.  In this context, positive and negative refers to the recording convention.  With positive leads, electrical current flowing towards the electrode will be recorded as an upward or positive deflection on the graph paper, while current flowing away from a positive lead (or towards a negative lead) will be recorded as a downward or negative deflection.

Limb Leads

The first group of leads are termed the limb leads, and it should come as no surprise that they are created by comparing the three electrodes placed on the arms and left leg.  When visualized, they form an inverted triangle, termed Einthoven's Triangle.  The limb leads measure electrical current in the vertical plane (sometimes referred to as the coronal plane or frontal plane).  The limb leads are broken down into two subgroups, termed standard limb leads and augmented limb leads.

The standard limb leads are Leads I, II, and III, and are formed by comparing the electrodes on the right arm, left arm, and leg.


Lead

Negative Electrode

Positive Electrode

Angle (degrees)

Lead I

RA

LA

Lead II

RA

LL

60°

Lead III

LA

LL

120°

The next set of leads are termed augmented leads and are designated as aVR, aVL, and aVF.  The augmented leads use the same three leads as the standard limb leads, but with a twist.  Instead of comparing two leads with one designated as positive and one designated as negative, all three leads are used at once.  Two leads are established as negative, and one lead is positive.  The two negative leads are mathematically averaged together to form a new common lead that is then compared to the single positive lead. 

The end result of combining the two negative leads is to shift the axis, which provides three more angles from which to view the flow of electrical currents within the heart.  When this technique was first employed, the amplitude of the recording was too low, so the leads were augmented to facilitate interpretation.  This augmentation led to the naming convention aV, which stands for Augmented Voltage (Right, Left, or Foot depending on the location of the positive electrode).

Defining Limb Leads

Again, examing the image above is probably the best way to conceptualize where the leads are going.  However, for those of you who love equations, the augmented limb leads are mathematically represented by:

Lead

Formula

Angle (degrees)

aVR

RA – (LA+LL) / 2

-150°

aVL

LA – (RA+LL) / 2

-30°

aVF

LL – (RA+LA) / 2

90°

*The positive electrode is indicated by the lead name.  In lead aVR, the Right Arm is positive.

Adding the augmented limb leads to the standard limb leads provides 6 views of the heart in the vertical plane with only three wires, and is the reason that 6 + 3 = 12-lead EKG.

While the first inclination is to memorize lead angles in a sequential order such as Lead I, Lead II, it is much easier to think of them anatomically.  Thinking of them as inferior leads (II, III, aVF), left lateral leads (I, aVL), and right lateral leads (aVR) will establish the pattern that will be used to examine EKGs.

The above diagram is frequently redrawn so that the six leads have a common origin, represented by the image below.

Frontal Plane Leads

Precordial Leads

Unlike limb leads that measure electrical activity in the vertical plane, the precordial leads measure activity in the horizontal plane.  Each of the 6 electrodes are set as positive, and a common negative point is calculated for comparison.

Fortunately, there are no angles to remember for precordial leads.  However, there are still anatomical groupings to consider.  Together, V1 – V4 are commonly referred to as the anterior chest leads, while V5 and V6 are grouped with the left lateral leads.  In addition, pairs of leads are used to approximate anatomical structures.

Again, getting in the habit of thinking about leads anatomically will make the transition to the 12-lead much easier.

 

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