by James W. Grier
Department of Biological Sciences
North Dakota State University
Fargo, ND 58102-3400


Provided for: Individuals who are interested in the subject and students/teachers in college/university-level courses for introductory biology, human biology, human anatomy & physiology, physical education, exercise science, sports medicine, intermediate and upper-level physiology, pre-medical and beginning medical, as well as persons involved in training/workshops/seminars for EMT/emergency and first responders, paramedics, public health, military, fire and rescue, and veterinary science/technology. It may be used either for self study, including by persons who are simply curious or out of concern for their own health, or as a part of actual courses/seminars/workshops. This web site is a gateway to better understanding the heart's electrical patterns. Hearts and ECGs are cool!


2006-2008 James W. Grier
North Dakota State University
originally published 11/2/2006
last revision: 2/8/2008 [links updated 2/22/2010]



Table of Contents

Introduction and purpose of website
A microbit of history about the ECG / EKG
Introduction to the standard 12-lead ECG / EKG (including downloadable PowerPoint)
References and further information: texts and website links
Other resources: equipment and supplies



Introduction and purpose of website

First, a note about the terminology: ECG and EKG are synonymous and used interchangably for recordings of the electrical activity of the heart. “EKG” is from the original term, “electrokardiogram,” introduced by the famous Dutch physiologist, Willem Einthoven in the 1890s and early 1900s. ECG is based on the English version of the word, "electrocardiogram", and is the version used most commonly today. EKG, however, is still commonly used and preferred by some people, partly for historical reasons, perhaps because that is simply the way they learned it, and/or partly to avoid confusion with the similar sounding EEG (from the brain) and EGG (from the reproductive system :). I will mostly use "ECG" from here on, with some "EKG" or both, as is commonly done by others and in the literature (and you'll need to get used to it).

The heart itself is a complex organ and the electrical patterns coming from it can seem very complex, confusing, and even overwhelming. ECGs are usually run in a medical/clinical environment involving abnormalities and problems. Most training and learning of ECGs and the associated terminology and jargon involves medically-related persons. However, most of the billions of hearts in the world keep ticking away day in and day out with their everyday normal patterns. Many biologists, biology teachers and students, and other people find the normal heart and its outputs to be interesting!

For over three decades of teaching a variety of introductory biology and anatomy & physiology courses at North Dakota State University, I included the heart's electrical activity in both lectures and labs, for both medical and non-medical students. Our equipment over this period changed from old, bulky "physiographs" with liquid ink tracing pens to smaller computer-based machines and various software, from primitive to more advanced. We recently switched to standard (clinical type) 12-lead and Holter PC-based units for pre-professional students who will eventually encounter the standard ECG machines as well as other students who might either be just plain interested or else encounter problems with their own hearts and end up getting 12-lead ECGs run on themselves in a clinical environment.

Standard ECGs, however, have a fairly steep learning curve, with most of the training and educational material focused on the medical student. It is possible, however, to introduce the subject to virtually anyone who is interested or needs to understand it to whatever degree.

The purpose of this web site is to open the door to the subject in as plain of language and explanation as possible for as wide of an audience as possible, including both for future medical practioners and the general student/public. It is focused on the normal ECG. (It is with all of the various and sundry abnormalities that the subject really gets out of hand!) Disclaimer: the intent of this site is for educational purposes only, NOT diagnostic. ECGs are complex with many subtle variations and can be difficult or misleading to interpret. Full and proper analysis requires much specialized training and experience. Most non-medical instructors (and even some who are medical instructors) do not have the expertise for diagnostic interpretations of abnormal ECGs and might not even be able to detect some abnormal situations. Most, if not all, ECGs that are run on students in a classroom setting or by persons on themselves will be “normal.” However, if anyone encounters something on an ECG that appears like it might be suspicious or not normal, the person should see a physician and have another ECG run under proper clinic or hospital diagnostic conditions and read by someone who is fully trained in ECG interpretation.


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A microbit of history about the ECG / EKG

Our understanding of biological electricity began and gradually increased through the efforts of many persons during the 17th-19th centuries. The term "electrocardiogram" and our modern-day combination of leads started around 1890 with Einthoven (and possibly his immediate predecessors), first with three main limb leads, the addition of the six chest leads during the 1930's, and finally the three augmented limb leads in 1942 to produce the total of 12 leads used up to the present. Some of the earliest equipment and techniques were downright amazing for being able to detect and record the slight amounts of electricity (in millivolts or mV) coming from the heart (and other muscles).

Initially, and yet today with the "resting 12-lead" ECGs, the subject needed/needs to be at rest to minimize electrical signals from other muscles in the body. However, over time and with improved computing algorithms, it became possible to measure ECGs during exercise and "stress" tests. Portable or "Holter" units, named after Dr. N.J. Holter, a Montana physician who initially developed it in 1949, permit recordings over long periods of time during routine daily life including various activities and exercise. The original Holter unit was a 75 pound backpack! Modern Holter units, whether they record with cassette tapes or digitally, are small in size.

For more (much more) on the history, click here.


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Introduction to the standard 12-lead ECG / EKG

For a PowerPoint version of this material, which may be used by teachers or students for classes or one's own repeated review, click here.

At first encounter for someone who has not seen one before, a standard 12-lead ECG is a bewildering and confusing array of squiggles on a piece of paper! What does it mean and how does it relate to the heart?


eHeart-fig-01.jpg (119643 bytes)


Well, let's start with the basic object of interest, the heart, and make sure we know where it is in the body and how it operates. It is in the chest cavity just under the sternum, as shown on the following models.


eHeart-fig-02.jpg (71293 bytes)

Contractions of skeletal (“voluntary”) muscles are initiated directly from the nervous system whereas contractions of heart muscle are initiated internally, in the muscle fibers themselves, and are only speeded up or slowed down by the nervous system (and a variety of chemical factors). The process involves movements of ions in and out of the muscle cells and cellular depolarization and repolarization. For more on all of that, refer to a basic biology textbook.

For our interest, electrical activity from the muscular contractions spreads throughout the body and can be detected on the surface on the skin. Contacts on the skin are called electrodes or "leads" which connect to the recording/measuring devices, or ECG machines.


eHeart-fig-03.jpg (49851 bytes)


eHeart-fig-04.jpg (37787 bytes)


eHeart-fig-05.jpg (31699 bytes)


eHeart-fig-06.jpg (81070 bytes)


The main, typical waves of an ECG  are identified as P, Q, R, S, and T (the symbols A, B, C, ... and X, Y, Z etc. had already been used for other physiologically-related items at the time when the system was first developed). Note: all of the waves do not appear on all recordings and there are also some other waves (with other names) that sometimes show up. The following recording, for example, does not show a "Q" wave, a downward wave just before the R wave, although the position of a Q wave (when present) is shown.

eHeart-fig-07.jpg (37465 bytes)

The pattern can be broken down separately for the atria and ventricles as follow:

eHeart-fig-08.jpg (42760 bytes)

Note: atrial repolarization is obscured by ventricular depolarization.

eHeart-fig-09.jpg (47156 bytes)


eHeart-fig-10.jpg (59726 bytes)

The specific appearance of a tracing depends on the location of the electrode (position on the body and, for those leads within a few centimeters on the heart, on the chest, the distance from the heart) and what the heart's electrical activity is doing (resting or active, normal vs various abnormalities, etc.).

Healthy hearts vary (somewhat like fingerprints) in their performance and output but produce a normal range of values. When something in the heart isn’t working correctly as a result of disease, accidents, or genetic and developmental malformations, the signals are different or abnormal (and require a trained physician or specialist to diagnose).


In addition to Lead I which was shown above, there are several other leads that provide different views of the electrical activity. Their polarities and measurements are all manipulated by the ECG machine. The other leads are shown in the following figures.

Note that even though there are "12 leads", there are only 10 actual contacts on the body. There are 9 recording leads (3 on the limbs [right arm, left arm, and left leg] plus 6 on the chest over various parts of the heart) and a 10th neutral or ground lead attached to the right leg. The remaining 3 leads representing the limbs are called "augmented" leads, which are derived from vectored combinations of the other 3 actual limb leads and provide different angles of view.

eHeart-fig-11.jpg (34983 bytes)


eHeart-fig-12.jpg (35142 bytes)


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The various limb leads together provide a frontal view of the heart. (Always keep in mind that left and right orientation are relative to the subject, not the observer.)


The remaining 6 leads are located across the chest and provide a cross-sectional view of the heart. They go by several names: "precordial" ("in front of the heart"), "pericardial" ("around the heart"), or simply the "chest" leads.


In summary: the 12 standard leads are

Limb leads –

I, from the right arm (-) toward the left arm (+)
II, from the right arm toward the left leg    
III, from the left arm toward the left leg   
(taken together, these three form the classic "Einthoven's triangle")
aVR, augmented lead toward the right (arm)
aVL, augmented lead toward the left (arm)
aVF, augmented lead toward the foot
(note: aVR is approximately opposite of I and should essentially mirror the shape of I vertically)

Chest leads –

V1 through V6, starting over the right atrium with V1, and placed in a semi-circle of positions leftwards, to the left side of the left ventricle. V1 and V2, on the right and left sides respectively, are placed just off the sternum at the 4th intercostal spaces (the space between the 4th and 5th ribs, which can be felt through the skin) and the others travel around to V6 under the armpit, as shown in the diagram.

[Detailed instructions for placing the leads:

Limb leads: Arms (RA, LA) anywhere from the upper arm to the wrists
                      Legs (RL, LL) anywhere from the thigh to the ankles
                      (positions of limb leads or their distances from heart are not critical, as long as
                      they are more than 10 cm [4 inches] from the heart)

Chest leads:V1 to the right of the sternum, next to it, in the space between ribs 4 and 5
                     V2 as V1 but to the left of the sternum
                     V3 halfway between V2 and V4 (see next one)
                     V4 below the middle of the clavicle, between ribs 5 and 6
                     V5 left of V4, halfway between V4 and V6 (see next one)
                     V6 on same horizontal line with V4 and V5, below middle of armpit]


eHeart-fig-17.jpg (54138 bytes)


The normal progression of muscular contractions, hence, electrical activity, travels from the upper right part of the atria downward and leftwards to the ventricles, with the left ventricle being the strongest. This leads to a topic of ECG interpretation involving the electrical "axis" of the heart, which is mostly beyond this introduction. However, for a general picture: The pattern on any one lead, and all of the leads taken together, represents a view of the sum of all of the vectors from contractions of the different muscle fibers in the various parts of the heart. The total picture for any given wave (P, R, T) is the heart's electrical axis, which normally is from the upper, posterior right side of the heart (in the right atrium) to the lower point of the ventricles in a left direction. For example, the normal axis should generally be a combination toward the left (that is, upward in lead I) and toward the feet (that is, tracing upward also in aVF). Further details of the axis are determined by the direction of the traces in the other leads.

Various combinations of limb leads and chest leads taken together provide a three-dimensional view into the electrical activity and workings of the heart for anyone who knows how to read an ECG. A heart attack and resulting damaged or dead portions of the heart, for example, can greatly affect the summed vectors, hence, the axes of the various waves.

Various abnormalities including heart attacks, arrhythmias, congenital problems, and a host of diseases and factors that affect the heart will cause sometimes major and sometimes subtle changes to the ECG patterns, which can be interpreted by a trained, experienced observer.

Now we can return to a standard 12-lead ECG print and begin to make sense of it. It shows each of the 12 leads in their own segments on the page. Given even a rudimentary understanding, hopefully as provided above, you can interpret some of the variations among the different leads, as shown in the following figures. The figures also illustrate some of the other components of a typical ECG, including a rhythm trace, calibration boxes, etc. This is an example of a "normal" ECG.









To help further understand this and get some hands-on experience, see the section on References and further information: texts and website links, as well as consider doing some exercises as described in the next section.



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We incorporate ECG topics into a human anatomy and physiology lab as well as a general biology physiology lab in our Department of Biological Sciences at North Dakota State University. We demonstrate both the standard resting 12-lead and Holter ECGs, with students comparing patterns from resting vs exercising (by running up and down a set of stairs while wearing the Holter).

For resting 12-lead recordings in clinical settings, the subject normally lies down flat on the back. For demonstration purposes, however, sitting up quietly on a chair also usually works. Sometimes the recordings show differences between sitting and reclining and sometimes not, but for non-clinical recordings, including on oneself, sitting is more convenient.

To demonstrate the effects (and artifacts) on a resting ECG caused by electrical activity from the skeletal/voluntary muscles in the body, the subject can move his or her arms or flex/tense the body's muscles. That will produce striking results on a resting 12-lead setup!

If one is set up to run stress-tests, that could be used for comparisons with resting ECGs as an alternative to using Holter units.

The measurements done in our introductory biology lab are mostly just of the basic ECG pattern and simple resting versus exercise differences (and similarities). In the anatomy and physiology lab, however, we go more into the clinically-related aspects of the pattern, such as PR and QT intervals. For that, I have put together a table of normal values, as shown below. (For a downloadable / printable version, click here. Feel free to make copies of it and/or revise and incorporate it into other versions for your own purposes. [My "copyright" is simply to insure my own free access to this stuff, so I don't end up having to get permission or pay for my own things from someone else who takes and copyrights it.])

eHeart-normal-values.jpg (208103 bytes)

For an additional link to a normal ECG which also includes (by scrolling down to it) a table of normal boundaries and possible abnormalities (and links) outside of those boundaries, click here.

If interested in our lab manual chapters that incorporate the ECG topics, additional information and instructions for the class teaching assistants, or any related questions, please send an e-mail to me at:


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References and further information: texts and website links


These books can be ordered through any book store or online, such as Note: Also be sure to see the relevant sections on the heart in whatever basic college biology or human anatomy & physiology textbook that you happen to be using.

The following is only a selection of ECG / EKG books currently available, but I have found these to be very helpful and useful. I usually like to have more than one book on any particular topic. If one were to have only one introductory book for ECGs, I would recommend Dubin 2000, Garcia and Holtz 2003, or Thaler 2003.  These three are very different from each other, but all are good as first or only introductory books for learning how to interpret ECGs. If you want to go deeper into the subject and have a lot more actual ECG examples to look at and puzzle over, then Garcia and Holtz 2001, Garcia and Miller 2004, and Green and Chiaramida 2003, are good; they are more appropriate as subsequent books, not first or only ones. There are also many more books available, as can be seen on the link above to

Dubin, D. 2000. (6th edition) Rapid Interpretation of EKG's. COVER Publ. Co. Tampa, FL.368 pp. ISBN 0-912912-06-5. A classic and very popular book for learning ECG's. It uses a fairly unique interactive, page by page fill-in-the-blanks self-test system throughout the book.

T.B. Garcia and N.E. Holtz. 2001. 12-lead ECG: The Art of Interpretation. Jones & Bartlett Publ. Sudbury, MA.236 pp. ISBN 0-7637-1284-1. (Also see the next two.) This book is a fairly complete teaching and reference text that includes three levels of difficulty: basic, intermediate, and advanced. The levels of difficulty are mixed together by topic but color coded so the reader can cover what is appropriate to their level and not get lost in more difficult topics. It is profusely illustrated and includes many examples of actual ECGs. The book has a horizontal, rather than standard vertical format, which makes it handy for including full-scale ECGs, but it is awkward to read.

T.B. Garcia and N.E. Holtz. 2003. Introduction to 12-lead ECG: The Art of Interpretation. Jones & Bartlett Publ. Sudbury, MA.536 pp. ISBN 0-7637-1961-7. Essentially the basic material from their more complete 2001 book. The first 72 pages, all basic material, of both books are the same. After the first 72 pages, this book then distills the subsequent material with reworking and additional illustrations. It is a less bulky book, at less than half the thickness and weight of the 2001 book. It shares with the 2001 book the same horizontal and awkward format (but also many of the full-scale ECGs).

T.B. Garcia and G.T. Miller. 2004. Arrhythmia Recognition: The Art of Interpretation. Jones & Bartlett Publ. Sudbury, MA.633 pp. ISBN 0-7637-2246-4. This book is by the same first author as the two above, but it is focused on arrhythmia topics. It begins with much of the same basic material, so any of these three books would be equally useful for introductory topics, but even that material has been reworked and improved slightly (it is good even in the 2001 book, but even better here). I would regard this book as a companion to either of the other two, although it would probably be best with the more thorough 2001 version for the more advanced topics (of which much in the arrhythmia book would be).

J.M. Green and A.J. Chiaramida. 2003. 12-lead EKG Confidence: Step-by-Step to Mastery. Lippincott Williams & Wilkins. Philadelphia, PA. 435 pp. ISBN 0-7817-3921-7. A useful book including many examples with their interpretations. The writing is perhaps less readable at the introductory level than the others in this list and I would not recommend it as the first or only book. But it is good for the examples and a somewhat different approach, after using one or more of the other books to begin with.

Thaler, M.S. 2003. The Only EKG Book You'll Ever Need. Lippincott Williams & Wilkins. Philadelphia, PA. 317 pp. ISBN 0-7817-3921-7. A very readable, concise, and helpful introduction. It is well illustrated with segments that pertain to the points at hand, although it lacks examples of complete 12-lead ECGs.


Additional WEB SITES ...

There is a large number of web sites currently available on the topic of ECG / EKG, including many instructional ones and several that provide numerous examples of actual ECGs. A quick search on Google will turn up lots of them! The following selection is only a sample, but these are ones that I have found useful and interesting.

A large collection of ECGs, including normal and with examples of most of the abnormal conditions

ECG Learning Center tutorial -- an extensive, very useful site; designed to accompany the books by Garcia et al. but accessible for everyone

Electrocardiogram Game

Interactive EKG quizzer (starts with a simple quiz; scroll down in it for additional quizzes)

... and there are many more, including links within the sites above. You can continue searching on your own.

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Other resources: equipment and supplies

ECG machines and/or their software components, with a few exceptions and alternatives as discussed below, usually are not cheap. The high price results partly (and legitimately) from their complexity and partly because they are normally used in the medical industry (which tends to inflate costs). For educational purposes and schools, as well as for personal and home use, most new, full-scale 12-lead EKG equipment is not very affordable. There are, however, small, 1-lead units (which can be used with various locations on the body to achieve 12-lead results) now available at low price that provide excellent ECG recordings -- see below.

At least one company,  Nasiff Associates, Inc., will sell their standard, clinical 12-lead and Holter equipment and software for educational purposes at an educational discount. Their units are highly suited for educational purposes: they are PC-computer based, which permits printing via a regular printer (not requiring specialized ECG paper) and the screen display can be projected to a room screen. It is a full-function system designed and normally used in clinical settings, with options for stress tests and Holter units. This is what we ourselves are using and I highly recommend it. The Nasiff ECG Cardio-Card system is also very compact, easy to move and store during the long periods between use, and user friendly. To obtain the educational discount, contact Nasiff Associates at their toll free number (866-627-4332), ask for Roger, tell him that you are seeking an educational discount, and be prepared to verify that you will be using the equipment and software for educational purposes only. The educational discount is not available for clinical or commercial uses.

Other possible sources for clinical 12-lead, Holter, and stress-testing equipment include contacting your local clinics and hospitals to see if they have used equipment they would be willing to give away or sell at a reasonable price for educational purposes, or by browsing the ECG / EKG section of eBay. Caution: be careful of picking up computer-based ECG units that do not include the software to run them. The software is critical to the functioning of the system and is usually the most expensive part! If you get ECG equipment without the software, you might as well have nothing. To get the necessary software might cost as much as a new system to begin with and you would be better off starting with that in the first place, as it will also include warranty, service, and consultation.

There are also low-priced but excellent 1- to 3-lead systems. Many of these are now available at much lower cost than full 12-lead clinical units and they can be used on the various locations of the body to achieve 12-lead results. These are great for personal home use and also for many educational purposes. They provide ECG recordings for lead I, from the right to left limbs, or the electrodes can be positioned at different locations on the limbs and chest for the other leads, one lead at a time. Some have built-in electrodes that work with direct finger of skin contact and do not require cables and adhesive skin electrodes, but they also have the cables and can use electrodes for more stable recordings if desired. I have tested and highly recommend the ones from They are a worldwide distributor of pulse oximeters, handheld ECG-EKG, and fetal Dopplers. Other sources for educational and combination physiological systems include Vernier (their EKG sensor also requires an interface to connect to a computer), and iWorx.

Electrodes can be obtained from an ECG supplier such as Nasiff, a local or online medical supplier, or the eBay link above.

Many, if not most, persons reading ECGs use a divider style caliper to measure the various characteristics of the recordings. Measurements can also be made for most practical educational purposes simply with a metric ruler or measuring caliper such as a dial or electronic caliper. A good, low-priced ECG caliper is the Alvin 5 1/2 inch divider, model 560, available online or check with your local book store, art, or drafting store, and see if they either stock or can order them.


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To contact the author, e-mail:

Author's web page: James W. Grier

Department of Biological Sciences, NDSU

[NDSU home page]