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Electrocardiography
Image showing a patient connected to the 10 electrodes necessary for a 12-lead ECG
ICD-9-CM	89.52
MeSH	D004562
MedlinePlus	003868
[edit on Wikidata]

Electrocardiography (ECG or EKG from Template:Lang-de) 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. The recording produced by this noninvasive procedure is termed as electrocardiogram (also ECG or EKG). An ECG test 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.

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

An ECG is the best way to measure and diagnose abnormal rhythms of the heart, particularly abnormal rhythms caused by damage to the conductive tissue that carries electrical signals, or abnormal rhythms caused by electrolyte imbalances. 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. 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).

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. Three- 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.

History

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".

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. This activity was directly recorded and visualized using a Lippmann capillary electrometer by the British physiologist John Burdon Sanderson. 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. 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, the Netherlands, used the string galvanometer he invented in 1901. This device was much more sensitive than both the capillary electrometer Waller used and the string galvanometer that had been invented separately in 1897 by the French engineer Clément Ader. 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, and described the electrocardiographic features of a number of cardiovascular disorders. In 1924, he was awarded the Nobel Prize in Medicine for his discovery.

Though the basic principles of that era are still in use today, many advances in electrocardiography have been made 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.

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

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 four 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 one to three 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 sec). 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

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. Usually, 10 of these electrodes are standard in a "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" is the voltage between the right arm electrode and the left arm electrode, whereas "Lead II" is the voltage between the right arm 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). 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

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. They are labeled and placed on the patient's body as follows:

Additional electrodes

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 electrode placed in the second intercostal space to the right of the sternum with the RA at the fourth 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

In both the 5- and 12-lead configurations, 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. The limb leads form the points of what is known as Einthoven's triangle.

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

${\displaystyle I=LA-RA}$

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

${\displaystyle II=LL-RA}$

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

${\displaystyle III=LL-LA}$

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

Unipolar vs. bipolar leads

The two types of leads are unipolar and bipolar. Bipolar leads have one positive and one negative pole. 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. 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):

${\displaystyle V_{W}={\frac {1}{3}}(RA+LA+LL)}$

Augmented limb leads

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:

${\displaystyle aVR=RA-{\frac {1}{2}}(LA+LL)}$

• 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:

${\displaystyle aVL=LA-{\frac {1}{2}}(RA+LL)}$

• 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:

${\displaystyle aVF=LL-{\frac {1}{2}}(RA+LA)}$

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:

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

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

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. 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.

Originally, four deflections were noted, but after the mathematical correction for artifacts introduced by early amplifiers, a fifth deflection was 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.

In intracardiac electrocardiograms, such as can be acquired from pacemaker sensors, an additional wave can be seen, the H deflection, which reflects the depolarization of the bundle of His. 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.

Vectors and views

Interpretation of the ECG relies on the idea that different leads (meaning 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 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

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.

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

Clinical lead groups

Of the 12 leads in total, each records the electrical activity of the heart from a different perspective, which also correlates 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. The relevance of this is in determining whether an abnormality on the ECG is likely to represent true disease or a spurious finding.

In addition, any two precordial leads next to one another are considered to be contiguous. For example, 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).

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

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 artifacts 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.

Indications

Medical societies do not recommend either the ECG or any other cardiac imaging procedure as a routine screening procedure in patients without symptoms and who are at low risk for coronary heart disease. This is because overuse of the procedure is more likely to supply incorrect supporting evidence for a nonexistent problem than to detect a true problem. Tests which falsely indicate the existence of a problem are likely to lead to misdiagnosis, the recommendation of invasive procedures, or overtreatment, and the risks associated with managing false information are usually more troublesome than not using ECG results to make a health recommendation in low-risk individuals.

Symptoms generally indicating use of electrocardiography include:

• Symptoms of myocardial infarction (See section below)
• Symptoms of pulmonary embolism (See section below)
• Cardiac murmurs
• Syncope or collapse
• Seizures
• Perceived cardiac dysrhythmias

It is also used to assess patients with systemic disease, as well as monitoring during anesthesia and critically ill patients.

Myocardial infarction

Characteristic changes seen on electrocardiography in myocardial infarction is included in the WHO criteria as revised in 2000. According to these, a cardiac troponin rise accompanied by either typical symptoms, pathological Q waves, ST elevation or depression or coronary intervention are diagnostic of myocardial infarction.

Pulmonary embolism

In pulmonary embolism, an ECG may show signs of right heart strain or acute cor pulmonale in cases of large PEs — the classic signs are a large S wave in lead I, a large Q wave in lead III and an inverted T wave in lead III (S1Q3T3). This is occasionally (up to 20%) present, but may also occur in other acute lung conditions and has therefore limited diagnostic value. The most commonly seen signs in the ECG is sinus tachycardia, right axis deviation and right bundle branch block. Sinus tachycardia was however still only found in 8–69% of people with PE.

Some pathological patterns which can be seen on the ECG

The following table mentions some pathological patterns that can be seen on electrocardiography, followed by possible causes.

Electrocardiogram heterogeneity

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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 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.

Fetal electrocardiography

Fetal electrocardiography records the electrical activity of a fetus, and when performed as a part of monitoring in childbirth, involves a single electrode being passed through the woman's cervix and attached to the baby's scalp. According to a Cochrane review, monitoring the fetus using ECG plus cardiotocography (CTG) resulted in fewer instances of fetal scalp blood testing, and less surgical assistance with the birth, compared to CTG alone. There was no difference in the number of Caesarean deliveries and little to suggest the babies were in better condition at birth.

See also

References

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38. Amal Mattu; Deepi Goyal; Barrett, Jeffrey W.; Joshua Broder; DeAngelis, Michael; Peter Deblieux; Gus M. Garmel; Richard Harrigan; David Karras; Anita L'Italien; David Manthey (2007). Emergency medicine: avoiding the pitfalls and improving the outcomes. Malden, Mass: Blackwell Pub./BMJ Books. p. 10. ISBN 1-4051-4166-2.{{cite book}}: CS1 maint: multiple names: authors list (link)
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External links

• 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"
• ECGsim - A 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.

v t e Tests and procedures involving the heart
Cardiac surgery Interventional cardiology Cardiology diagnostic tests and procedures Cardiac imaging
Surgery	Heart valves and septa Valve repair Valvulotomy Mitral valve repair Valvuloplasty aortic mitral Valve replacement Aortic valve repair Aortic valve replacement Ross procedure Transcatheter Mitral valve replacement Transcatheter pulmonary valve replacement production of septal defect in heart enlargement of existing septal defect Atrial septostomy Balloon septostomy creation of septal defect in heart Blalock–Hanlon procedure shunt from heart chamber to blood vessel atrium to pulmonary artery Fontan procedure left ventricle to aorta Rastelli procedure right ventricle to pulmonary artery Sano shunt compound procedures for transposition of the great vessels Arterial switch operation Mustard procedure Senning procedure for univentricular defect Norwood procedure Kawashima procedure shunt from blood vessel to blood vessel systemic circulation to pulmonary artery shunt Blalock–Taussig shunt SVC to the right PA Glenn procedure Cardiac vessels CHD Angioplasty Bypass/Coronary artery bypass MIDCAB Off-pump CAB TECAB Coronary stent Bare-metal stent Drug-eluting stent Obstacle removal Endarterectomy Atherectomy Bentall procedure Valve-sparing aortic root replacement LeCompte maneuver Other Pericardium Pericardiocentesis Pericardial window Pericardiectomy Myocardium Cardiomyoplasty Dor procedure Septal myectomy Ventricular reduction Alcohol septal ablation Conduction system Maze procedure (Cox maze and minimaze) Catheter ablation Cryoablation Radiofrequency ablation Pacemaker insertion S-ICD implantation ICD implantation Cardiac resynchronization therapy Left atrial appendage occlusion Cardiotomy Heart transplantation
Tests	Electrophysiology Electrocardiography Vectorcardiography Holter monitor Implantable loop recorder Cardiac stress test Bruce protocol Electrophysiology study Cardiac imaging Angiocardiography Echocardiography TTE TEE Myocardial perfusion imaging Cardiovascular MRI Ventriculography Radionuclide ventriculography Cardiac catheterization/Coronary catheterization Cardiac CT Cardiac PET sound Phonocardiogram
Function tests	Impedance cardiography Ballistocardiography Cardiotocography
Pacing	Cardioversion Transcutaneous pacing
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• Cardiac procedures
• Electrodiagnosis
• Cardiac electrophysiology
• Electrophysiology