Bioelectromagnetism and the 12 Lead ECG System

Publish Date: Aug 24, 2016 | 7 Ratings | 4.00 out of 5 | Print


This tutorial explains the basics of a 12 Lead ECG System. The text was edited or republished from the web-version of the book: Jaakko Malmivuo & Robert Plonsey: Bioelectromagnetism - Principles and Applications of Bioelectric and Biomagnetic Fields, Oxford University Press, New York, 1995.

Table of Contents

  1. Formation of the Electrocardiogram (ECG) Signal
  2. Understanding ECG Limb Leads
  3. Related Documentation
  4. Testing Your ECG (EKG) Products to ANSI/AAMI EC13

1. Formation of the Electrocardiogram (ECG) Signal

One way of describing cardiac process is to plot the sequence of instantaneous wavefront vectors.  An evaluation of the ECG signal sources is achieved by applying generalized equations.  This process assumes that on one side cells are entirely at rest, while on the other cells are entirely in the plateau phase, then the source is zero everywhere except at the wavefront. 

From this it is possible to examine the actual generation of the ECG by taking into account a realistic progression of activation.  Figures 1 to 4 show the electric activation of the heart that has begun at the sinus node.


Figure 1 through 4:  Plots of the sequence of ECG wavefronts


As shown in figure one through four, from the sinus node the vector spreads along the atrial walls. The resultant vector of the atrial electric activity is illustrated with a thick arrow. The projections of this resultant vector on each of the three limb ECG leads I, II and III is positive, and therefore, the measured signals are also positive.

After the depolarization has propagated over the atrial walls, it reaches the AV node. The propagation through the AV junction is very slow and involves negligible amount of tissue; it results in a delay in the progress of activation.  This is a desirable pause which allows completion of ventricular filling.

Once activation has reached the ventricles, propagation proceeds along the Purkinje fibers to the inner walls of the ventricles. The ventricular depolarization starts first from the left side of the interventricular septum, and therefore, the resultant dipole from this activation points to the right. This is what causes a negative signal in ECG leads I and II.

In the next phase, depolarization waves occur on both sides of the septum, and their electric forces cancel. However, early apical activation is also occurring, so the resultant vector points to the apex.  After a while, the depolarization front has propagated through the wall of the right ventricle; when it first arrives at the epicardial surface of the right-ventricular free wall, the event is called breakthrough. Because the left ventricular wall is thicker, activation of the left ventricular free wall continues even after depolarization of a large part of the right ventricle. Because there are no compensating electric forces on the right, the resultant vector reaches its maximum in this phase, and it points leftward. 

The depolarization front continues propagation along the left ventricular wall toward the back. Because its surface area now continuously decreases, the magnitude of the resultant vector also decreases until the whole ventricular muscle is depolarized. The last to depolarize are basal regions of both left and right ventricles. Because there is no longer a propagating activation front, there is no ECG signal either.

Ventricular repolarization begins from the outer side of the ventricles and the repolarization front "propagates" inward. This seems paradoxical, but even though the epicardium is the last to depolarize, its action potential durations are relatively short, and it is the first to recover. Although recovery of one cell does not propagate to neighboring cells, one notices that recovery generally does move from the epicardium toward the endocardium. The inward spread of the repolarization front generates a signal with the same sign as the outward depolarization front. Because of the diffuse form of the repolarization, the amplitude of the ECG signal is much smaller than that of the depolarization wave and it lasts longer.

The normal electrocardiogram is illustrated in Figure 5. The figure also includes definitions for various segments and intervals in the ECG. The deflections in this signal are denoted in alphabetic order starting with the letter P, which represents atrial depolarization. The ventricular depolarization causes the QRS complex, and repolarization is responsible for the T-wave. Atrial repolarization occurs during the QRS complex and produces such a low signal amplitude that it cannot be seen apart from the normal ECG.


Figure 5. The normal electrocardiogram signal


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2. Understanding ECG Limb Leads

In 1908, Willem Einthoven illustrated an important ECG measuring method.  The Einthoven triangle in Figure 6. is an approximate description of the lead vectors associated with ECG limb leads. The lead vectors associated with Einthoven's lead system are conventionally found based on the assumption that the heart is located in an infinite, homogeneous volume conductor (or at the center of a homogeneous sphere representing the torso). It was shown that if the position of the right arm, left arm, and left leg were at the vertices of an equilateral triangle, having the heart located at its center, then the lead vectors form an equilateral triangle.  A simple model results from assuming that the cardiac sources are represented by a dipole located at the center of a sphere representing the torso, hence at the center of the equilateral triangle. With these assumptions, the ECG voltages measured by the three limb leads I, II and II are proportional to the projections of the electric heart vector on the sides of the lead vector triangle.

Figure 6.  The Einthoven triangle

Additional innovation through the years lead to three additional leads VR, VL, and VF which were obtained by measuring the potential between each limb electrode and a point called the Wilson central terminal.  Following this discovery, it was shown that these signals could be augmented. The resulting leads were called augmented leads aVR, aVL, and aVF.  These augmented lead locations are fully redundant with respect to limb leads I,II and III.

In 1944 six additional leads for measuring close to the heart V1, V2, V3, V4, V5 and V6 were introduced.  These leads are located at the fourth intercostal space on the right and left side of the sternum.  The resultant 12 lead system is the one of greatest clinical use.  

Figure 7. Precordial leads of the chest

Of the 12 ECG lead,

aVR, aVL, aVF
V1, V2, V3, V4, V5, V6

the first six are derived from the same three measurement points. Therefore, any two of these six leads include exactly the same information as the other four.  Over 90% of the heart's electric activity can be explained with a dipole source model (Geselowitz, 1964). To evaluate this dipole, it is sufficient to measure its three independent components. In principle, two of the limb leads (I, II, III) could reflect the frontal plane components, whereas one precordial lead could be chosen for the anterior-posterior component. The combination should be sufficient to describe completely the electric heart vector. 

To the extent that the cardiac source can be described as a dipole, the 12-lead ECG system could be thought to have three independent leads and nine redundant leads.  However, in fact, the precordial leads detect also nondipolar components, which have diagnostic significance because they are located close to the frontal part of the heart. Therefore, the 12-lead ECG system has eight truly independent and four redundant leads. 

The lead vectors for each lead based on an idealized (spherical) volume conductor are shown in Figure 8. 

Figure 8. The projections of lead vectors for the 12 lead ECG

The heart's basic operation is also described in a short video presented by a doctor.



Web-version of the book: Jaakko Malmivuo & Robert Plonsey: Bioelectromagnetism - Principles and Applications of Bioelectric and Biomagnetic Fields, Oxford University Press, New York, 1995.

All the material of this Web edition is free for publishing.


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3. Related Documentation

Refer to the following documents to learn more about ECG measurements and the application of Graphical System Design using the NI sbRIO with the TI MDXMDKEK1258 Electrocardiogram (ECG) Analog Front End (AFE) module:

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4. Testing Your ECG (EKG) Products to ANSI/AAMI EC13

Learn how to test and validate any Electrocardiography (ECG) (EKG)  based medical device to ANSI/AAMI EC13.  

In this document, you will learn how to automate and reduce time required to test and validate any ECG based device using NI PXI modular instruments and NI software.

Originally Authored By: Greg Crouch, National Instruments

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