Chapter I Physiology of the Heart and Circulation System

Chapter II The Electrocardiogram and the Normal EKG

Chapter III Arrhythmia Determination

Chapter IV Arrhythmias and Select Disease Conditions

Chapter V The 12-Lead EKG



Course Exam

Chapter V The 12-Lead EKG

This chapter presents an introduction to the 12-lead ECG.  The 12-lead ECG gives a tracing from 12 different “electrical positions” of the heart.  Each lead is meant to pick up electrical activity from a different position on the heart muscle.  This allows an experienced interpreter to see the heart from many different angles.  This section is meant only as an introduction to the 12-lead ECG.  It will take much practice of you to be able to interpret a 12-lead ECG tracing.  This section will give you a basic understanding of how to take a 12-lead EKG, how to place the leads, and how to begin to interpret the tracing.

The electrocardiogram is a graphic record of the direction and magnitude of the electrical activity generated by the depolarization and repolarization of the atria and ventricles of the heart.  This electrical activity is readily detected by electrodes attached to the skin.  However, neither the electrical activity that results from the generation and transmission of electrical impulse, nor the mechanical contractions or relaxations of the atria and ventricles appear in the electrocardiogram.

An EKG lead consists of two surface electrodes of opposite polarity (one positive and one negative) or one positive surface electrode and a reference point.  A lead composed of two electrodes of opposite polarity is called bipolar lead. A lead composed of a single positive electrode and a reference point is a unipolar lead.

For a routine analysis of the heart’s electrical activity an ECG recorded from 12 separate leads is used.  A 12-lead ECG consists of three bipolar limb leads (I, II, and III), the unipolar limb leads (AVR, AVL, and AVF), and six unipolar chest leads, also called precordial or V leads, (, , , , , and ).

Limb leads: I, II, III, IV, V, and VI
Lead IV also called AVR
Lead V also called AVL
Lead VI also called AVF

Chest leads: , , , , , and .

Below is a sample of a 12-lead EKG tracing.

It is a normal tracing (shows normal sinus rhythm).  This course is meant to give the nurse a practical education concerning the 12-lead EKG.  We will present the basic clinical aspects of the 12-lead.  We will demonstrate how and where to attach the leads in certain places.  We will also demonstrate the basics of interpreting the results.  Then all you need is practice.

Each 12-lead EKG machine will have its own instructions for use.  Be sure you are familiar with the machines at your facility.  The instructions for the machine will show you how to attach the leads to the patient.  In most instances, the patient electrodes will be attached with the use of either flat elastic straps or by cloth Velcro straps.  In most cases, the metal electrodes will also need to be coated with conductive gel prior to attachment to the patient.  Be sure to clean the electrodes before and after each use, as gel will tend to build up.  Be sure to read the entire instruction manual for the machine including: how to start the machine, how to load the paper, how to calibrate the machine (if needed), and any other pertinent information needed to safely and accurately run the machine.

Once you know how to operate the machine at your facility the next concept to understand is the placement of the leads.  The limb leads are usually first.  As the name implies, the limb leads are attached to the four limbs.  This is usually accomplished by attaching the leads, according to instructions, on each wrist and each ankle.  As mentioned earlier, the electrodes will be sensing the electrical impulses from the heart muscle at ht various locations and with various voltages, either positive or negative.

 The 12-lead EKG tracing below was obtained with universal lead placement.  “Blip Marks” are pointed out on the tracing.  These marks are for the purpose of showing the leads as they are changed.  Every time you see a blip mark, the next lead is being recorded on the tracing. 

The placement is as follows:

Lead I through lead 6             Limb leads
Lead 7 through lead 12           Chest Leads

Location of the Frontal Plane Axis:
In order to accurately interpret the 12-lead EKG, you must have an understanding of the electrical activity of the heart.  The direction in which the impulses flow in the heart is important.  It is also important to understand that 12 different leads pick up those impulses as they travel in many different directions through the heart.


The frontal plane axis is the orientation of the heart’s electrical activity in the frontal plane.

The fontal plane consists of:



Left-to-Right Directions




Inferior-to-Superior Directions


            *See illustration below.

Remember that the normal conduction of the heart begins in the SA node.  The wave of depolarization moves across the atria, through the AV node, into the Bundle of HIS, down the Bundle Branches, and finally through the Purkinje fibers which conduct the electrical impulses throughout the ventricles.

The Frontal Plane Leads:

The activity of the heart produces electrical potentials that can be measured on the surface of the skin.  Using the galvanometer (EKG machine), differences between electrical potentials at different sites of the body can be recorded.  See illustration below:

In picture A above, the negative electrode is on the right arm and the positive electrode is on the left arm.  This is lead I.  Lead I records electrical difference between the left and right arm electrodes.

In picture B above, the negative electrode is on the right arm and the positive electrode is on the left leg (left lower chest). This is lead II.  Lead II records electrical differences between the left leg and right arm electrodes.

In picture C, the negative electrode is on the left arm and the positive electrode is on the left leg (left lower chest).  Picture C depicts lead III.  Lead III records electrical difference between the left leg and the left arm electrodes. 

The above illustration shows Leads I, II, and III, their placement and the electrical potential on these three leads. 

They are summarized as follows:

Lead I: Right arm-negative, Left arm-positive

Records electrical differences between the left and right arm electrodes.

Lead II: Right arm-negative, Left leg-positive

Records electrical difference between the left leg and right arm electrodes.

Lead III: Left arm-negative, Left leg-positive

Records electrical differences between the left leg and left arm electrodes.

The other three frontal plane limb leads are called the augmented Vector leads.  The Galvanometer (EKG machine) records potential differences and, therefore, the technique is Bipolar (potential site A minus potential site B). However, if the potential of B is zero the recorder records only the potential site A.  this means that these next three electrodes, for all practical purposes have a zero potential and do not change during the cardiac cycle. They became known as the V electrodes, and all three leads became known as the V electrodes or UNIPOLAR leads.

As mentioned earlier, unipolar leads measure the electric impulses at only one point, instead of across two points, as the first three leads.  With these V leads, the second site is -0- so there is noneed to measure from two pointes, only one point is needed.  To obtain the measurements from these V leads, you simply turn the dial on the EKG machine to aVR, aVL, and aVF, respectively. The machine automatically makes the needed connection to measure the voltage from these areas. 

The illustrations below show where these measurements take place. 

They are summarized here:

  • Lead aVR Augmented Vector Right, positive electrode right shoulder.
  • Lead aVL Augmented Vector Left, positive electrode left shoulder.
  • Lead aVF Augmented Vector Foot, positive electrode on Food.

aVR means augmented Vector Right; the positive electrode is on the right shoulder.

 aVL means augmented Vector Left; the positive electrode is on the left shoulder.

aVF means augmented Vector Foot; the positive electrode is on the foot.

NOTE: although the F stands for foot, please conceptualize the positive electrode of aVF as being at the umbilicus.

Now combine the three limb leads I, II, III


And the three augmented Vector leads aVR, aVL, aVF

and this combination creates the Hexaxial Reference System


As the above illustrations point out, the six limb leads measure the electrical activity of the heart from the frontal plan.  The frontal plane only manes that the patient is in anatomical position and facing you. Therefore, the patient’s left side is on your right side.   

The six limb leads measure a copulate circle or 360 degrees around the heart.  They measure the electrical activity of the heart from every possible angle. The reason for this is obvious. By measuring he heart from different angles, you will be able to pinpoint the location of any conduction deft in the heart. 

 If you refer to the previous illustration you will notice that 306 degrees of the front of the heart are completely covered by the six limb leads. This six fontal plane leads placed across the heart form the hex axial reference system.  This system is the means by which we communicate the location of the frontal plane axis.

The angles are as follows;


Is located at 0 degrees


(+) (-) 180 degrees


Is located at +60 degrees


-120 degrees


Is located at +120 degrees


-60 degrees


Is located at +30 degrees


-150 degrees


Is located at -30 degrees


+150 degrees


Is located at +90 degrees


-90 degrees

*NOTE: the (+) positive and (-) negative designation on these degrees of each lead DO NOT

RELATE TO THE (+) AND (-) ELECTRODES. Note also that all the positive (+) degrees are on the inferior surface of the hexaxial figure. All the negative (-) degrees are on the superior surface of the hexaxial figure.  In the future, you may wish to study in more detail the interpretation of the tracings made in each of these six leads.  In this course we have gone into detail of the lead II interpretation only.

The Chest Leads (or Precordial Leads)

We will now discuss the remaining six leads of the 12-lead EKG. These next six leads are called the Chest leads or the Precordial leads.  This part may also become confusing because these precordial leads are six additional leads that use the same V technique we used with the 3 limb leads.  Therefore, do not confuse these Precordial V leads with the three V limb leads (aVR, aVL, aVF).

The precordial (chest leads) leads each consist of a positive electrode strategically placed on the chest of the patient. The positions of the positive electrode for the six precordial leads are very important for a valid tracing to be made on the EKG machine.

These positions are: (INSERT NUMBER with V)  


Fourth intercostals space, right sterna border.


Fourth intercostals space, left sterna border.


One-half way between V and V in straight line with them.


Fifth intercostals space, left midclavicular line.


Fifth intercostals space, left anterior auxiliary line.


Fifth intercostals space, left midauxiliary line.

These positions are critical for interpretation of the EKG. At fort it may seem complicated to position these electrodes.  After some practice it is relatively easy to place the leads properly. The below illustration will show the correct positions across the chest.  One of the main reasons that the precordial leads are important is that these leads show the R Wave Progression.  From V (NUMBER) through V (NUMBER) the R Wave becomes progressively larger.  The experienced interpreter of these leads will be able to rule out many different cardiac disorders by attaching the R wave and other configurations in the pericardial leads. 

V - Lead (Precordial Leads) Placement on the chest.


The normal ECG morphology of the complex in the V leads:

Next: Conclusion