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12 Lead ECG of a 26-year-old male.
Image showing a patient connected to the 10 electrodes necessary for a 12-lead ECG

Electrocardiography (ECG or EKG) is a transthoracic interpretation of the electrical activity of the heart over time captured and externally recorded by skin electrodes.[1] It is a noninvasive recording produced by an electrocardiographic device. The etymology of the word is derived from electro, because it is related to electrical activity, cardio, Greek for heart, and graph, a Greek root meaning "to write".

Electrical impulses in the heart originate in the sinoatrial node and travel through the intrinsic conducting system to the heart muscle. The impulses stimulate the myocardial muscle fibers to contract and thus induce systole. The electrical waves can be measured at electrodes placed at specific points on the skin. Electrodes on different sides of the heart measure the activity of different parts of the heart muscle. An ECG displays the voltage between pairs of these electrodes, and the muscle activity that they measure, from different directions, can also be understood as vectors. This display indicates the overall rhythm of the heart and weaknesses in different parts of the heart muscle. It 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.



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 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]

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

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.[11]

ECG graph paper

One second of ECG graph paper

Timed interpretation of an ECG was once incumbent to a stylus and paper speed. Computational analysis now allows considerable study of heart rate variability. A typical electrocardiograph runs at a paper speed of 25 mm/s, although faster paper speeds are occasionally used. Each small block of ECG paper is 1 mm2. 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 diagnostic quality 12 lead ECG is calibrated at 10 m/V, so 1 mm translates into 0.1 mV. A calibration signal should be included with every record. A standard signal of 1 mV must move the stylus vertically 1 cm, that is two large squares on ECG paper.

Filter selection

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.[12]


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

In electrocardiography, the word lead may refer to either the electrodes attached to the patient, or, properly, (in which case, it is pronounced /lid/) to the voltage between two electrodes. The electrodes are attached to the patient's body, usually with very sticky circles of thick tape-like material (the electrode is embedded in the center of this circle), onto which cables clip.[13] ECG leads use different combinations of electrodes to produce various signals from the heart.


Placement of electrodes

Ten electrodes are used for a 12-lead ECG. 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 Health Association. 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]
12 leads
Electrode label (in the USA) Electrode placement
RA On the right arm, avoiding bony prominences.
LA In the same location that RA was placed, but on the left arm this time.
RL On the right leg, avoiding bony prominences.
LL In the same location that RL was placed, but on the left leg this time.
V1 In the fourth intercostal space (between ribs 4 & 5) just to the right of the sternum (breastbone).
V2 In the fourth intercostal space (between ribs 4 & 5) just to the left of the sternum.
V3 Between leads V2 and V4.
V4 In the fifth intercostal space (between ribs 5 & 6) in the mid-clavicular line (the imaginary line that extends down from the midpoint of the clavicle (collarbone).
V5 Horizontally even with V4, but in the anterior axillary line. (The anterior axillary line is the imaginary line that runs down from the point midway between the middle of the clavicle and the lateral end of the clavicle; the lateral end of the collarbone is the end closer to the arm.)
V6 Horizontally even with V4 and V5 in the midaxillary line. (The midaxillary line is the imaginary line that extends down from the middle of the patient's armpit.)

Limb leads

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

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

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) 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 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).

Augmented limb leads

Leads aVR, aVL, and aVF are augmented limb 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" or a unipolar lead. 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.
  • 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.
  • 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.

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.

\begin{align} aVR &= -\frac{I + II}{2}\ aVL &= I - \frac{II}{2}\ aVF &= II - \frac{I}{2} \end{align}

Precordial leads

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

Schematic representation of normal ECG
Animation of a normal ECG wave.

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.

Feature Description Duration
P wave During normal atrial depolarization, the main electrical vector is directed from the SA node towards the AV node, and spreads from the right atrium to the left atrium. This turns into the P wave on the ECG. 80ms
PR segment The PR segment connects the P wave and the QRS complex. This coincides with the electrical conduction from the AV node to the bundle of His to the bundle branches and then to the Purkinje Fibers. This electrical activity does not produce a contraction directly and is merely traveling down towards the ventricles and this shows up flat on the ECG. 50 to 120ms
QRS complex The QRS complex is a recording of a single heartbeat on the ECG that corresponds to the depolarization of the right and left ventricles. 70 to 110ms
ST segment The ST segment connects the QRS complex and the T wave. The ST segment represents the period when the ventricles are depolarized. It is isoelectric. 80 to 120ms
T wave The T wave represents the repolarization (or recovery) of the ventricles. The interval from the beginning of the QRS complex to the apex of the T wave is referred to as the absolute refractory period. The last half of the T wave is referred to as the relative refractory period (or vulnerable period). 160ms
PR interval The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. The PR interval reflects the time the electrical impulse takes to travel from the sinus node through the AV node and entering the ventricles. The PR interval is therefore a good estimate of AV node function. 120 to 200ms
ST interval The ST interval is measured from the J point to the end of the T wave. 320ms
QT interval The QT interval is measured from the beginning of the QRS complex to the end of the T wave. A prolonged QT interval is a risk factor for ventricular tachyarrhythmias and sudden death. 300 to 430ms[citation needed]
U wave The U wave is not always seen. It is typically low amplitude, and, by definition, follows the T 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.[25]

Pathophysiological indications of EKG

Shortened QT interval Hypercalcemia, some drugs, certain genetic abnormalities.
Prolonged QT interval Hypocalcemia, some drugs, certain genetic abnormalities.
Flattened or inverted T waves Coronary ischemia, left ventricular hypertrophy, digoxin effect, some drugs.
Hyperacute T waves Possibly the first manifestation of acute myocardial infarction.
Prominent U waves Hypokalemia.

Clinical lead groups

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 the same anatomical area of the heart are said to be contiguous (see color coded chart).

Diagram showing the contiguous leads in the same color
Category Color on chart Leads Activity
Inferior leads Yellow Leads II, III and aVF Look at electrical activity from the vantage point of the inferior surface (diaphragmatic surface of heart).
Lateral leads Green I, aVL, V5 and V6 Look at the electrical activity from the vantage point of the lateral wall of left ventricle.
  • The positive electrode for leads I and aVL should be located distally on the left arm and because of which, leads I and aVL are sometimes referred to as the high lateral leads.
  • Because the positive electrodes for leads V5 and V6 are on the patient's chest, they are sometimes referred to as the low lateral leads.
Septal leads Orange V1 and V2 Look at electrical activity from the vantage point of the septal wall of the ventricles (interventricular septum).
Anterior leads Blue V3 and V4 Look at electrical activity from the vantage point of the anterior surface of the heart (sternocostal surface of heart).

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.

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.


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. 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.

Normal −30° to 90° Normal Normal
Left axis deviation −30° to −90° May indicate left anterior fascicular block or Q waves from inferior MI. Left axis deviation is considered normal in pregnant women and those with emphysema.
Right axis deviation +90° to +180° May indicate left posterior fascicular block, Q waves from high lateral MI, or a right ventricular strain pattern. Right deviation is considered normal in children and is a standard effect of dextrocardia.
Extreme right axis deviation +180° to −90° Is rare, and considered an 'electrical no-man's land'.

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

Electrocardiogram heterogeneity

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.


There are over 350,000 cases of sudden cardiac death (SCD) in the United States each year, and over twenty percent of these cases involve people with no outward signs of serious heart disease. For decades, researchers have been attempting to come up with methods of identifying electrocardiogram (ECG) patterns that reliably precede dangerous arrhythmias. As these methods are found, devices are being created that monitor the heart in order to detect the onset of dangerous rhythms and to correct them before they cause death.


Research being conducted[26] suggests that a crescendo in ECG heterogeneity, both in the R-wave and the T-wave, often signals the start of ventricular fibrillation. In patients with coronary artery disease, exercise increases T-wave heterogeneity, but this effect is not seen in normal patients. These results, when combined with other pieces of emerging evidence, suggest that R-wave and T-wave heterogeneity both have predictive value.

Future applications

In the future, researchers hope to simplify the ECG to a larger encyclopedic audience. Technology now allows deployment of temporary and permanent cardiac electrodes in a plurality of anatomic positions capable of novel ECGs unimpeded by the skin or thoracic cage. ECGs can be as variable as fingerprints to a trained observer. Patterns may be appreciated and computational analysis may illuminate the process of heterogeneity detection and to augment the clinical evidence supporting the validity of ECG heterogeneity as a predictor of arrhythmia. The electrocardiogram is fundamentally an interpretative entity but allows interventional measures, see Interventional Cardiology. Someday soon, 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.[27]

See also


  1. ^ "ECG- simplified. Aswini Kumar M.D". LifeHugger. Retrieved 2010-02-11. 
  2. ^ Braunwald E. (Editor), Heart Disease: A Textbook of Cardiovascular Medicine, Fifth Edition, p. 108, Philadelphia, W.B. Saunders Co., 1997. ISBN 0-7216-5666-8.
  3. ^ "The clinical value of the ECG in noncardiac conditions." Chest 2004; 125(4): 1561-76. PMID 15078775
  4. ^ "2005 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care - Part 8: Stabilization of the Patient With Acute Coronary Syndromes." Circulation 2005; 112: IV-89 - IV-110.
  5. ^ Ronald M. Birse, rev. Patricia E. Knowlden [1] Oxford Dictionary of National Biography 2004 (Subscription required) - (original source is his biography written by his wife - Elizabeth Muirhead. Alexander Muirhead 1848 - 1920. Oxford, Blackwell: privately printed 1926.)
  6. ^ Burdon Sanderson J (1878). "Experimental results relating to the rhythmical and excitatory motions of the ventricle of the frog heart". Proc Roy Soc Lond 27: 410–14. doi:10.1098/rspl.1878.0068. 
  7. ^ Waller AD (1887). "A demonstration on man of electromotive changes accompanying the heart's beat". J Physiol (Lond) 8: 229–34. 
  8. ^ "Einthoven's String Electrovanometer". 1927-09-29. Retrieved 2009-08-15. 
  9. ^ Einthoven W. Un nouveau galvanometre. Arch Neerl Sc Ex Nat 1901; 6:625
  10. ^ Cooper J (1986). "Electrocardiography 100 years ago. Origins, pioneers, and contributors". N Engl J Med 315 (7): 461–4. PMID 3526152. 
  11. ^ Mark, Jonathan B. (1998). Atlas of cardiovascular monitoring. New York: Churchill Livingstone. ISBN 0443088918. 
  12. ^ Mark JB "Atlas of Cardiovascular Monitoring." p. 130. New York: Churchill Livingstone, 1998. ISBN 0-443-08891-8.
  13. ^ See images of ECG electrodes here: or here:
  14. ^ "lead_dia". Retrieved 2009-08-15. 
  15. ^
  16. ^ [2]
  17. ^ "Univ. of Maryland School of Medicine Emergency Medicine Interest Group". Retrieved 2009-08-15. 
  18. ^ "Limb Leads - ECG Lead Placement - Normal Function of the Heart - Cardiology Teaching Package - Practice Learning - Division of Nursing - The University of Nottingham". Retrieved 2009-08-15. 
  19. ^ "Lesson 1: The Standard 12 Lead ECG". Retrieved 2009-08-15. 
  20. ^
  21. ^ e.g. Pasco Pasport EKG Sensor PS-2111, Sciencescope ECG Sensor, etc.
  22. ^
  23. ^ "Electrocardiogram Leads". CV Physiology. 2007-03-26. Retrieved 2009-08-15. 
  24. ^ Watch a movie by the National Heart Lung and Blood Institute explaining the connection between an ECG and the electricity in your heart at this site
  25. ^ Hurst JW. Current Perspective: Naming of the Waves in the ECG, With a Brief Account of Their Genesis. Circulation. 1998;98:1937-1942.
  26. ^ In the lab of Richard L. Verrier of Harvard Medical School
  27. ^ Verrier, Richard L. "Dynamic Tracking of ECG Heterogeneity to Estimate Risk of Life-threatening Arrhythmias." CIMIT Forum. September 25, 2007.

External links


Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary




  1. (cardiology) electrocardiogram


  • Anagrams of ceg
  • GCE


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