Electrocardiography
Intervention
Image showing a patient connected to the 10 electrodes necessary for a 12-lead ECG
ICD-9-CM	89.52
MeSH	D004562

12 Lead EKG of a 26-year-old male.

Electrocardiography (ECG or EKG from the German Elektrokardiogramm) is a transthoracic (across the thorax or chest) interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body.^[1] The recording produced by this noninvasive procedure is termed as electrocardiogram (also ECG or EKG). An electrocardiogram (ECG) is a test that records the electrical activity of the heart.

ECG is used to measure the rate and regularity of heartbeats, as well as the size and position of the chambers, the presence of any damage to the heart, and the effects of drugs or devices used to regulate the heart, such as a pacemaker. See also stress test and Holter monitor (24h).

The etymology of the word is derived from the Greek electro, because it is related to electrical activity, kardio, Greek for heart, and graph, a Greek root meaning "to write".

Most ECGs are performed for diagnostic or research purposes on human hearts, but may also be performed on animals, usually for diagnosis of heart abnormalities or research.

Function[link]

The ECG device detects and amplifies the tiny electrical changes on the skin that are caused when the heart muscle depolarizes during each heartbeat. At rest, each heart muscle cell has a negative charge, called the membrane potential, across its cell membrane. Decreasing this negative charge towards zero, via the influx of the positive cations, Na⁺ and Ca⁺⁺, is called depolarization, which activates the mechanisms in the cell that cause it to contract. During each heartbeat, a healthy heart will have an orderly progression of a wave of depolarisation that is triggered by the cells in the sinoatrial node, spreads out through the atrium, passes through the atrioventracular node and then spreads all over the ventricles. This is detected as tiny rises and falls in the voltage between two electrodes placed either side of the heart which is displayed as a wavy line either on a screen or on paper. This display indicates the overall rhythm of the heart and weaknesses in different parts of the heart muscle.

Usually more than two electrodes are used, and they can be combined into a number of pairs (For example: left arm (LA), right arm (RA) and left leg (LL) electrodes form the three pairs LA+RA, LA+LL, and RA+LL). The output from each pair is known as a lead. Each lead looks at the heart from a different angle. Different types of EKGs can be referred to by the number of leads that are recorded, for example 3-lead, 5-lead or 12-lead ECGs (sometimes simply "a 12-lead"). A 12-lead EKG is one in which 12 different electrical signals are recorded at approximately the same time and will often be used as a one-off recording of an ECG, traditionally printed out as a paper copy. 3- and 5-lead ECGs tend to be monitored continuously and viewed only on the screen of an appropriate monitoring device, for example during an operation or whilst being transported in an ambulance. There may or may not be any permanent record of a 3- or 5-lead ECG, depending on the equipment used.

This is the best way to measure and diagnose abnormal rhythms of the heart,^[2] particularly abnormal rhythms caused by damage to the conductive tissue that carries electrical signals, or abnormal rhythms caused by electrolyte imbalances.^[3] In a myocardial infarction (MI), the ECG can identify if the heart muscle has been damaged in specific areas, though not all areas of the heart are covered.^[4] The ECG cannot reliably measure the pumping ability of the heart, for which ultrasound-based (echocardiography) or nuclear medicine tests are used. It is possible for a human or other animal to be in cardiac arrest but still have a normal ECG signal (a condition known as pulseless electrical activity).

History[link]

Alexander Muirhead is reported to have attached wires to a feverish patient's wrist to obtain a record of the patient's heartbeat while studying for his Doctor of Science (in electricity) in 1872 at St Bartholomew's Hospital.^[5] This activity was directly recorded and visualized using a Lippmann capillary electrometer by the British physiologist John Burdon Sanderson.^[6] The first to systematically approach the heart from an electrical point-of-view was Augustus Waller, working in St Mary's Hospital in Paddington, London.^[7] His electrocardiograph machine consisted of a Lippmann capillary electrometer fixed to a projector. The trace from the heartbeat was projected onto a photographic plate which was itself fixed to a toy train. This allowed a heartbeat to be recorded in real time. In 1911 he still saw little clinical application for his work.

An early commercial ECG device (1911)

An initial breakthrough came when Willem Einthoven, working in Leiden, Netherlands, used the string galvanometer that he invented in 1903.^[8] This device was much more sensitive than both the capillary electrometer that Waller used and the string galvanometer that had been invented separately in 1897 by the French engineer Clément Ader.^[9] Rather than using today's self-adhesive electrodes Einthoven's subjects would immerse each of their limbs into containers of salt solutions from which the EKG was recorded.

Einthoven assigned the letters P, Q, R, S and T to the various deflections,^[10] and described the electrocardiographic features of a number of cardiovascular disorders. In 1924, he was awarded the Nobel Prize in Medicine for his discovery.^[11]

Though the basic principles of that era are still in use today, there have been many advances in electrocardiography over the years. The instrumentation, for example, has evolved from a cumbersome laboratory apparatus to compact electronic systems that often include computerized interpretation of the electrocardiogram.^[12]

ECG graph paper[link]

One second of ECG graph paper

The output of an ECG recorder is a graph (or sometimes several graphs, representing each of the leads) with time represented on the x-axis and voltage represented on the y-axis. A dedicated ECG machine would usually print onto graph paper which has a background pattern of 1mm squares (often in red or green), with bold divisions every 5 mm in both vertical and horizontal directions.

It is possible to change the output of most ECG devices but it is standard to represent each mV on the y axis as 1 cm and each second as 25 mm on the x-axis (that is a paper speed of 25 mm/s). Faster paper speeds can be used, for example, to resolve finer detail in the ECG. At a paper speed of 25 mm/s, one small block of ECG paper translates into 40 ms. Five small blocks make up one large block, which translates into 200 ms. Hence, there are five large blocks per second. A calibration signal may be included with a record. A standard signal of 1 mV must move the stylus vertically 1 cm, that is, two large squares on ECG paper.

Layout[link]

By definition, a 12-lead ECG will show a short segment of the recording of each of the 12-leads. This is often arranged in a grid of 4 columns by three rows, the first columns being the limb leads (I,II and III), the second column the augmented limb leads (aVR, aVL and aVF) and the last two columns being the chest leads (V1-V6). It is usually possible to change this layout so it is vital to check the labels to see which lead is represented. Each column will usually record the same moment in time for the three leads and then the recording will switch to the next column which will record the heart beats after that point. It is possible for the heart rhythm to change between the columns of leads.

Each of these segments is short, perhaps 1–3 heart beats only, depending on the heart rate and it can be difficult to analyse any heart rhythm that shows changes between heart beats. To help with the analysis it is common to print one or two "rhythm strips" as well. This will usually be lead II (which shows the electrical signal from the atrium, the P-wave, well) and shows the rhythm for the whole time the ECG was recorded (usually 5–6 seconds). Some ECG machines will print a second lead II along the very bottom of the paper in addition to the output described above. This printing of Lead II is continuous from start to finish of the process.

The term "rhythm strip" may also refer to the whole printout from a continuous monitoring system which may show only one lead and is either initiated by a clinician or in response to an alarm or event.

Leads[link]

The term "lead" in electrocardiography causes much confusion because it is used to refer to two different things. In accordance with common parlance the word lead may be used to refer to the electrical cable attaching the electrodes to the ECG recorder. As such it may be acceptable to refer to the "left arm lead" as the electrode (and its cable) that should be attached at or near the left arm. There are usually ten of these electrodes in a standard "12-lead" ECG.

Alternatively (and some would say properly, in the context of electrocardiography) the word lead may refer to the tracing of the voltage difference between two of the electrodes and is what is actually produced by the ECG recorder. Each will have a specific name. For example "Lead I" (lead one) is the voltage between the right arm electrode and the left arm electrode, whereas "Lead II" (lead two) is the voltage between the right limb and the feet. (This rapidly becomes more complex as one of the "electrodes" may in fact be a composite of the electrical signal from a combination of the other electrodes (see later). Twelve of this type of lead form a "12-lead" ECG

To cause additional confusion the term "limb leads" usually refers to the tracings from leads I, II and III rather than the electrodes attached to the limbs.

Placement of electrodes[link]

Ten electrodes are used for a 12-lead ECG. The electrodes usually consist of a conducting gel, embedded in the middle of a self-adhesive pad onto which cables clip. Sometimes the gel also forms the adhesive.^[13] They are labeled and placed on the patient's body as follows:^[14][15]

Proper placement of the limb electrodes, color coded as recommended by the American Heart Association (a different colour scheme is used in Europe). Note that the limb electrodes can be far down on the limbs or close to the hips/shoulders, but they must be even (left vs right).^[16]

. * Note that when exercise stress tests are performed, limb leads may be placed on the trunk to avoid artifacts while ambulatory (arm leads moved sub-clavicularly and leg leads medial to and above the iliac crest).

12 leads

Additional electrodes[link]

The classical 12-lead ECG can be extended in a number of ways in an attempt to improve its sensitivity in detecting myocardial infarction involving territories not normally "seen" well. This includes an rV₄ lead which uses the equivalent landmarks to the V₄ but on the right side of the chest wall and extending the chest leads onto the back with a V₇, V₈ and V₉.

The Lewis lead or S5 has the LA elctrode placed in the 2nd intercostal space to the right of the sternum with the RA at the 4th intercostal space. It is read as Lead I and is supposed to demonstrate atrial activity much better to aid in identification of atrial flutter or broad-complex tachycardia.

Limb leads[link]

In both the 5- and 12-lead configuration, leads I, II and III are called limb leads. The electrodes that form these signals are located on the limbs—one on each arm and one on the left leg.^[17][18][19] The limb leads form the points of what is known as Einthoven's triangle.^[20]

• Lead I is the voltage between the (positive) left arm (LA) electrode and right arm (RA) electrode:

Failed to parse (Missing texvc executable; please see math/README to configure.): I = LA - RA.

• Lead II is the voltage between the (positive) left leg (LL) electrode and the right arm (RA) electrode:

Failed to parse (Missing texvc executable; please see math/README to configure.): II = LL - RA.

• Lead III is the voltage between the (positive) left leg (LL) electrode and the left arm (LA) electrode:

Failed to parse (Missing texvc executable; please see math/README to configure.): III = LL - LA.

Simplified electrocardiograph sensors designed for teaching purposes at e.g. high school level are generally limited to three arm electrodes serving similar purposes.^[21]

Unipolar vs. bipolar leads[link]

There are two types of leads: unipolar and bipolar. Bipolar leads have one positive and one negative pole.^[22] In a 12-lead ECG, the limb leads (I, II and III) are bipolar leads. Unipolar leads also have two poles, as a voltage is measured; however, the negative pole is a composite pole (Wilson's central terminal, or WCT) made up of signals from lots of other electrodes.^[23] In a 12-lead ECG, all leads besides the limb leads are unipolar (aVR, aVL, aVF, V₁, V₂, V₃, V₄, V₅, and V₆).

Wilson's central terminal V_W is produced by connecting the electrodes, RA; LA; and LL, together, via a simple resistive network, to give an average potential across the body, which approximates the potential at infinity (i.e. zero):

Failed to parse (Missing texvc executable; please see math/README to configure.): V_W = \frac{1}{3}(RA+LA+LL).

Augmented limb leads[link]

Leads aVR, aVL, and aVF are augmented limb leads (after their inventor Dr. Emanuel Goldberger known collectively as the Goldberger's leads). They are derived from the same three electrodes as leads I, II, and III. However, they view the heart from different angles (or vectors) because the negative electrode for these leads is a modification of Wilson's central terminal. This zeroes out the negative electrode and allows the positive electrode to become the "exploring electrode". This is possible because Einthoven's Law states that I + (−II) + III = 0. The equation can also be written I + III = II. It is written this way (instead of I − II + III = 0) because Einthoven reversed the polarity of lead II in Einthoven's triangle, possibly because he liked to view upright QRS complexes. Wilson's central terminal paved the way for the development of the augmented limb leads aVR, aVL, aVF and the precordial leads V₁, V₂, V₃, V₄, V₅ and V₆.

• Lead augmented vector right (aVR) has the positive electrode (white) on the right arm. The negative electrode is a combination of the left arm (black) electrode and the left leg (red) electrode, which "augments" the signal strength of the positive electrode on the right arm:

Failed to parse (Missing texvc executable; please see math/README to configure.): aVR = RA - \frac{1}{2} (LA + LL).

• Lead augmented vector left (aVL) has the positive (black) electrode on the left arm. The negative electrode is a combination of the right arm (white) electrode and the left leg (red) electrode, which "augments" the signal strength of the positive electrode on the left arm:

Failed to parse (Missing texvc executable; please see math/README to configure.): aVL = LA - \frac{1}{2} (RA + LL).

• Lead augmented vector foot (aVF) has the positive (red) electrode on the left leg. The negative electrode is a combination of the right arm (white) electrode and the left arm (black) electrode, which "augments" the signal of the positive electrode on the left leg:

Failed to parse (Missing texvc executable; please see math/README to configure.): aVF = LL - \frac{1}{2} (RA + LA).

The augmented limb leads aVR, aVL, and aVF are amplified in this way because the signal is too small to be useful when the negative electrode is Wilson's central terminal. Together with leads I, II, and III, augmented limb leads aVR, aVL, and aVF form the basis of the hexaxial reference system, which is used to calculate the heart's electrical axis in the frontal plane. The aVR, aVL, and aVF leads can also be represented using the I and II limb leads:

Failed to parse (Missing texvc executable; please see math/README to configure.): \begin{align} aVR &= -\frac{I + II}{2}\\ aVL &= I - \frac{II}{2}\\ aVF &= II - \frac{I}{2} \end{align}

Precordial leads[link]

The electrodes for the precordial leads (V₁, V₂, V₃, V₄, V₅ and V₆) are placed directly on the chest. Because of their close proximity to the heart, they do not require augmentation. Wilson's central terminal is used for the negative electrode, and these leads are considered to be unipolar (recall that Wilson's central terminal is the average of the three limb leads. This approximates common, or average, potential over the body). The precordial leads view the heart's electrical activity in the so-called horizontal plane. The heart's electrical axis in the horizontal plane is referred to as the Z axis.

Waves and intervals[link]

Schematic representation of normal ECG

Animation of a normal ECG wave.

Detail of the QRS complex, showing ventricular activation time (VAT) and amplitude.

A typical ECG tracing of the cardiac cycle (heartbeat) consists of a P wave, a QRS complex, a T wave, and a U wave which is normally visible in 50 to 75% of ECGs.^[24] The baseline voltage of the electrocardiogram is known as the isoelectric line. Typically the isoelectric line is measured as the portion of the tracing following the T wave and preceding the next P wave.

There were originally four deflections, but after the mathematical correction for artifacts introduced by early amplifiers, five deflections were discovered. Einthoven chose the letters P, Q, R, S, and T to identify the tracing which was superimposed over the uncorrected labeled A, B, C, and D.^[10]

In intracardiac electrocardiograms, such as can be acquired from pacemaker sensors, an additional wave that can be seen is the H deflection, which reflects the depolarization of the bundle of His.^[27] The H-V interval, in turn, is the duration from the beginning of the H deflection to the earliest onset of ventricular depolarization recorded in any lead.^[28]

Vectors and views[link]

Graphic showing the relationship between positive electrodes, depolarization wavefronts (or mean electrical vectors), and complexes displayed on the ECG.

Interpretation of the ECG relies on the idea that different leads (by which we mean the ECG leads I, II, III, aVR, aVL, aVF and the chest leads) "view" the heart from different angles. This has two benefits. Firstly, leads which are showing problems (for example ST segment elevation) can be used to infer which region of the heart is affected. Secondly, the overall direction of travel of the wave of depolarisation can also be inferred which can reveal other problems. This is termed the cardiac axis . Determination of the cardiac axis relies on the concept of a vector which describes the motion of the depolarisation wave. This vector can then be described in terms of its components in relation to the direction of the lead considered. One component will be in the direction of the lead and this will be revealed in the behaviour of the QRS complex and one component will be at 90° to this (which will not). Any net positive deflection of the QRS complex (i.e. height of the R-wave minus depth of the S-wave) suggests that the wave of depolarisation is spreading through the heart in a direction that has some component (of the vector) in the same direction as the lead in question.

Axis[link]

Diagram showing how the polarity of the QRS complex in leads I, II, and III can be used to estimate the heart's electrical axis in the frontal plane.

The heart's electrical axis refers to the general direction of the heart's depolarization wavefront (or mean electrical vector) in the frontal plane. With a healthy conducting system the cardiac axis is related to where the major muscle bulk of the heart lies. Normally this is the left ventricle with some contribution from the right ventricle. It is usually oriented in a right shoulder to left leg direction, which corresponds to the left inferior quadrant of the hexaxial reference system, although −30° to +90° is considered to be normal. If the left ventricle increases its activity or bulk then there is said to be "left axis deviation" as the axis swings round to the left beyond −30°, alternatively in conditions where the right ventricle is strained or hypertrophied then the axis swings round beyond +90° and "right axis deviation" is said to exist. Disorders of the conduction system of the heart can disturb the electrical axis without necessarily reflecting changes in muscle bulk.

The hexaxial reference system showing the orientation of each lead. For example, if the bulk of heart muscle is oriented at +60 degrees with respect to the SA node, lead II will show the greatest deflection and aVL the least.

In the setting of right bundle branch block, right or left axis deviation may indicate bifascicular block.

Clinical lead groups[link]

There are twelve leads in total, each recording the electrical activity of the heart from a different perspective, which also correlate to different anatomical areas of the heart for the purpose of identifying acute coronary ischemia or injury. Two leads that look at neighbouring anatomical areas of the heart are said to be contiguous (see color coded chart). The relevance of this is in determining whether an abnormality on the ECG is likely to represent true disease or a spurious finding.

Diagram showing the contiguous leads in the same color

In addition, any two precordial leads that are next to one another are considered to be contiguous. For example, even though V4 is an anterior lead and V5 is a lateral lead, they are contiguous because they are next to one another. A common saying to remember the contiguous leads is "I see all leads". (Inferior, Septal, Anterior and Lateral)

Wiggers diagram, showing a normal ECG curve synchronized with other major events during the cardiac cycle.

Lead aVR offers no specific view of the left ventricle. Rather, it views the inside of the endocardial wall to the surface of the right atrium, from its perspective on the right shoulder.

Filter selection[link]

Modern ECG monitors offer multiple filters for signal processing. The most common settings are monitor mode and diagnostic mode. In monitor mode, the low frequency filter (also called the high-pass filter because signals above the threshold are allowed to pass) is set at either 0.5 Hz or 1 Hz and the high frequency filter (also called the low-pass filter because signals below the threshold are allowed to pass) is set at 40 Hz. This limits artifact for routine cardiac rhythm monitoring. The high-pass filter helps reduce wandering baseline and the low-pass filter helps reduce 50 or 60 Hz power line noise (the power line network frequency differs between 50 and 60 Hz in different countries). In diagnostic mode, the high-pass filter is set at 0.05 Hz, which allows accurate ST segments to be recorded. The low-pass filter is set to 40, 100, or 150 Hz. Consequently, the monitor mode ECG display is more filtered than diagnostic mode, because its passband is narrower.^[29]

Indications[link]

Symptoms generally indicating use of electrocardiography include:

• Cardiac murmurs^[30]
• Syncope or collapse^[30]
• Seizures^[30]
• Perceived cardiac dysrhythmias^[30]
• Symptoms of myocardial infarction. See Electrocardiography in myocardial infarction

It is also used to assess patients with systemic disease as well as monitoring during anesthesia and critically ill patients.^[30]

Some pathological entities which can be seen on the ECG[link]

Electrocardiogram heterogeneity[link]

This section may require cleanup to meet Wikipedia's quality standards. No cleanup reason has been specified. Please help improve this section if you can; the talk page may contain suggestions. (March 2010)

Electrocardiogram (ECG) heterogeneity is a measurement of the amount of variance between one ECG waveform and the next. This heterogeneity can be measured by placing multiple ECG electrodes on the chest and by then computing the variance in waveform morphology across the signals obtained from these electrodes. Recent research suggests that ECG heterogeneity often precedes dangerous cardiac arrhythmias.

In the future, implantable devices may be programmed to measure and track heterogeneity. These devices could potentially help ward off arrhythmias by stimulating nerves such as the vagus nerve, by delivering drugs such as beta-blockers, and if necessary, by defibrillating the heart.^[31]

See also[link]

References[link]

External links[link]

Wikimedia Commons has media related to: ECG

• Electrocardiogram, EKG, or ECG – Explanation of what an ECG is, who needs one, what to expect during one, etc. Written by the National Heart Lung and Blood Institute (a division of the NIH)
• Free ECG tutorial videos
• 12-lead ECG library
• ECG in 100 steps – a PowerPoint presentation
• ECGpedia: Course for interpretation of ECG
• Photographic guide to 12-lead ECG electrode placement on males & females
• ECG Lead Placement – A teaching guide "designed for student nurses who know nothing at all about Cardiology"
• Simulation tool to demonstrate and study the relation between the electric activity of the heart and the ECG
• Minnesota ECG Code
• EKG Review: Arrhythmias – A guide to reading ECGs not written for a university biology (anatomy and physiology) course.

Willem Einthoven
Willem Einthoven in 1906
Born	21 May 1860 Semarang
Died	29 September 1927 Leiden, Netherlands
Nationality	Netherlands
Fields	Physiology
Institutions	University of Leiden
Alma mater	University of Utrecht
Known for	Electrocardiogram
Notable awards	Nobel Prize in Medicine in 1924

Willem Einthoven (Semarang, 21 May 1860 – Leiden, 29 September 1927) was a Dutch doctor and physiologist. He invented the first practical electrocardiogram (ECG or EKG) in 1903 and received the Nobel Prize in Medicine in 1924 for it.^[1]

Einthoven was born in Semarang on Java in the Dutch East Indies (now Indonesia). His father, a doctor, died when Einthoven was a child. His mother returned to the Netherlands with her children in 1870 and settled in Utrecht. His parents ancestry was Dutch, his mother's ancestry was Dutch and Swiss^[2][3] In 1885, Einthoven received a medical degree from the University of Utrecht. He became a professor at the University of Leiden in 1886.

Before Einthoven's time, it was known that the beating of the heart produced electrical currents, but the instruments of the time could not accurately measure this phenomenon without placing electrodes directly on the heart. Beginning in 1901, Einthoven completed a series of prototypes of a string galvanometer. This device used a very thin filament of conductive wire passing between very strong electromagnets. When a current passed through the filament, the electromagnetic field would cause the string to move. A light shining on the string would cast a shadow on a moving roll of photographic paper, thus forming a continuous curve showing the movement of the string. The original machine required water cooling for the powerful electromagnets, required 5 people to operate it and weighed some 600 lb. This device increased the sensitivity of the standard galvanometer so that the electrical activity of the heart could be measured despite the insulation of flesh and bones.

An early ECG device

Although later technological advances brought about better and more portable EKG devices, much of the terminology used in describing an EKG originated with Einthoven. His assignment of the letters P, Q, R, S and T to the various deflections is still used. The term "Einthoven's triangle" is named for him. It refers to the imaginary inverted equilateral triangle centered on the chest and the points being the standard leads on the arms and leg.

After his development of the string galvanometer, Einthoven went on to describe the electrocardiographic features of a number of cardiovascular disorders. Later in life, Einthoven turned his attention to the study of acoustics, particularly heart sounds which he researched with Dr. P. Battaerd.

He died in Leiden in the Netherlands and is buried in the graveyard of the Reformed Church at 6 Haarlemmerstraatweg in Oegstgeest.^[4]

See also[link]

• Electrocardiogram
• String galvanometer

References and further reading[link]

1. ^ "Willem Einthoven". IEEE Global History Network. IEEE. http://www.ieeeghn.org/wiki/index.php/Willem_Einthoven. Retrieved 10 August 2011. 
2. ^ http://familytreemaker.genealogy.com/users/w/a/l/Theodorus-J-Walbeek/WEBSITE-0001/UHP-0191.html
3. ^ http://www.rootsweb.ancestry.com/~chevaud/vevay/d0000/g0000021.html#I3651
4. ^ Van Ditzhuijzen, Jeannette (September 9, 2005). Bijna vergeten waren ze, de rustplaatsen van roemruchte voorvaderen. Trouw (Dutch newspaper), p. 9 of supplement.

• Nobel Lectures, Physiology or Medicine 1922-1941, Elsevier Publishing Company, Amsterdam, 1965.
• Snellen, Herman Adrianus (1995), Willem Einthoven (1860-1927) Father of Electrocardiography, Life and Work, Ancestors and Contemporaries, Dordrecht: Kluwer Academic Publishers, ISBN 0-7923-3274-1 
• Shouldice RB, Bass G (2002), "From Bench to Bedside - Developments in Electrocardiology", The Engineers Journal, Institution of Engineers of Ireland 56 (4): 47–9, http://shouldice.net/research/PDF/IEI_Electrocardiology.pdf 

External links[link]

Wikimedia Commons has media related to: Willem Einthoven

• The Einthoven Foundation Cardiology Information Portal Historical pictures
• Nobel prize citation
• Einthoven's triangle
• Bibliography in the Virtual Laboratory of the Max Planck Institute for the History of Science
• Museum Boerhaave Negen Nederlandse Nobelprijswinnaars
• A.M. Luyendijk-Elshout, Einthoven, Willem (1860-1927), in Biografisch Woordenboek van Nederland.
• Biography Willem Einthoven (1860-1927) at the National Library of the Netherlands
• Willem Einthoven (1860–1927): Father of electrocardiography.Moukabary, T. Cardiology Journal. 2007, Vol. 14, No. 3, pp. 316–317 Copyright © 2007 Via Medical ISSN 1897–559.