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Electrocardiography is the process of producing an electrocardiogram (ECG or EKG). It is an electrogram of the
heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste ...

heart
which is a graph of
voltage Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is the ...

voltage
versus time of the electrical activity of the heart using
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor A semiconductor material has an Electrical resistivity and conductivity, electrical conductivity value falling between tha ...

electrode
s placed on the skin. These electrodes detect the small electrical changes that are a consequence of cardiac muscle
depolarization In biology, depolarization or hypopolarization is a change within a cell (biology), cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolar ...
followed by
repolarization In neuroscience Neuroscience is the science, scientific study of the nervous system. It is a Multidisciplinary approach, multidisciplinary science that combines physiology, anatomy, molecular biology, developmental biology, cytology, computer sc ...
during each cardiac cycle (heartbeat). Changes in the normal ECG pattern occur in numerous cardiac abnormalities, including cardiac rhythm disturbances (such as
atrial fibrillation Atrial fibrillation (AF or A-fib) is an abnormal heart rhythm (arrhythmia) characterized by the rapid and irregular beating of the atrial chambers of the heart. It often begins as short periods of abnormal beating, which become longer or cont ...

atrial fibrillation
and
ventricular tachycardia Ventricular tachycardia (V-tach or VT) is a type of regular, fast heart rate that arises from improper electrical activity in the ventricles of the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pump ...
), inadequate coronary artery blood flow (such as
myocardial ischemia Coronary artery disease (CAD), also called coronary heart disease (CHD), Ischemia, ischemic heart disease (IHD), or simply heart disease, involves the reduction of blood flow to the myocardium, heart muscle due to build-up of plaque (atherosclero ...
and
myocardial infarction A myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow Hemodynamics American and British English spelling differences#ae and oe, or haemodynamics are the Fluid dynamics, dynamics of blood flow. The circulatory sy ...

myocardial infarction
), and electrolyte disturbances (such as
hypokalemia Hypokalemia is a low level of potassium Potassium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and b ...
and
hyperkalemia Hyperkalemia is an elevated level of potassium Potassium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the propertie ...
). Traditionally, "ECG" usually means a 12-lead ECG taken while laying down as discussed below. However, other devices can record the electrical activity of the heart such as a
Holter monitor In medicine Medicine is the Art (skill), art, science, and Praxis (process) , practice of caring for a patient and managing the diagnosis, prognosis, Preventive medicine, prevention, therapy, treatment or Palliative care , palliation of their ...

Holter monitor
but also some models of
smartwatch A smartwatch is a wearable computer in the form of a watch; modern smartwatches provide a local touchscreen interface for daily use, while an associated smartphone app provides for management and telemetry (such as long-term biomonitoring). Whil ...

smartwatch
are capable of recording an ECG (starting with the
Apple Watch Apple Watch is a line of smartwatch A smartwatch is a wearable computer in the form of a watch; modern smartwatches provide a local touchscreen interface for daily use, while an associated smartphone app provides for management and telemetry ...

Apple Watch
in 2018). ECG signals can be recorded in other contexts with other devices. In a conventional 12-lead ECG, ten electrodes are placed on the patient's limbs and on the surface of the chest. The overall
magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...
of the heart's
electrical potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is the amount of work energy needed to move a unit of electric charge Electric charge is the physical property of matter that c ...
is then measured from twelve different angles ("leads") and is recorded over a period of time (usually ten seconds). In this way, the overall magnitude and direction of the heart's electrical depolarization is captured at each moment throughout the
cardiac cycle The cardiac cycle is the performance of the human heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxy ...
. There are three main components to an ECG: the
P wave A P wave (primary wave or pressure wave) is one of the two main types of elastic , called in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any affected location or at ...
, which represents the depolarization of the atria; the
QRS complex The QRS complex is the combination of three of the graphical deflections seen on a typical electrocardiogram (ECG or EKG). It is usually the central and most visually obvious part of the tracing; in other words, it's the main spike seen on an E ...

QRS complex
, which represents the depolarization of the ventricles; and the
T wave Normal T wave In electrocardiography, the T wave represents the repolarization of the Ventricle (heart), 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 (ph ...
, which represents the repolarization of the ventricles. During each heartbeat, a healthy heart has an orderly progression of depolarization that starts with pacemaker cells in the
sinoatrial node The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is a group of Cell (biology), cells known as pacemaker cells, located in the wall of the atrium (heart), right atrium of the heart. These cells can produce an electric ...
, spreads throughout the atrium, and passes through the
atrioventricular node The atrioventricular node or AV node is a part of the electrical conduction system of the heart The electrical conduction system of the heart transmits signals generated usually by the sinoatrial node to cause contraction of the heart muscle. Th ...
down into the
bundle of His Discovered in 1893 by Swiss Swiss may refer to: * the adjectival form of Switzerland *Swiss people Places *Swiss, Missouri *Swiss, North Carolina *Swiss, West Virginia *Swiss, Wisconsin Other uses *Swiss-system tournament, in various games and s ...
and into the
Purkinje fibers The Purkinje fibers ( ; Purkinje tissue or subendocardial branches) are located in the inner Ventricle (heart), ventricular walls of the heart, just beneath the endocardium in a space called the subendocardium. The Purkinje fibers are specializ ...

Purkinje fibers
, spreading down and to the left throughout the ventricles. This orderly pattern of depolarization gives rise to the characteristic ECG tracing. To the trained
clinician A clinician is a health care Healthcare is the maintenance or improvement of health Health is a state of physical, mental and social well-being Well-being, also known as ''wellness'', ''prudential value'' or ''quality of life'', refers to ...
, an ECG conveys a large amount of information about the structure of the heart and the function of its electrical conduction system. Among other things, an ECG can be used to measure the rate and rhythm of heartbeats, the size and position of the
heart chambers The heart is a muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use ...
, the presence of any damage to the heart's muscle cells or conduction system, the effects of heart drugs, and the function of implanted
pacemakers A cardiac pacemaker (or artificial pacemaker, so as not to be confused with the cardiac pacemaker, natural pacemaker of the heart), is a medical device that generates electrical impulses delivered by electrodes to cause the heart muscle chambers ...
.


Medical uses

The overall goal of performing an ECG is to obtain information about the electrical function of the heart. Medical uses for this information are varied and often need to be combined with knowledge of the structure of the heart and physical examination signs to be interpreted. Some indications for performing an ECG include the following: * Chest pain or suspected
myocardial infarction A myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow Hemodynamics American and British English spelling differences#ae and oe, or haemodynamics are the Fluid dynamics, dynamics of blood flow. The circulatory sy ...

myocardial infarction
(heart attack), such as ST elevated myocardial infarction (STEMI) or non-ST elevated myocardial infarction (NSTEMI) * Symptoms such as
shortness of breath Shortness of breath (SOB), also known as dyspnea (BrE British English (BrE) is the standard dialect A standard language (also standard variety, standard dialect, and standard) is a language variety that has undergone substantial codificat ...
, murmurs,
fainting Syncope, commonly known as fainting, is a loss of consciousness Consciousness, at its simplest, is sentience or awareness of internal and external existence. Despite millennia of analyses, definitions, explanations and debates by philos ...
,
seizure An epileptic seizure, formally known as a seizure, is a period of symptom Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign for example may be a higher or lower tem ...
s, funny turns, or
arrhythmia Arrhythmia, also known as cardiac arrhythmia or heart arrhythmia, is a group of conditions in which the heartbeat Heartbeat or heartbeats may refer to: Physiology *Cardiac cycle, of the heart *Contraction of the cardiac muscle, muscles of the ...
s including new onset
palpitations Palpitations are perceived abnormalities of the heartbeat characterized by awareness of cardiac muscle contractions in the chest, which is further characterized by the hard, fast and/or irregular beatings of the heart. Symptoms include a rapid ...
or monitoring of known cardiac arrhythmias * Medication monitoring (e.g.,
drug-induced QT prolongationQT prolongation is a measure of delayed ventricular repolarisation, which means the heart muscle takes longer than normal to recharge between beats. It is an electrical disturbance which can be seen on an electrocardiogram (ECG). Excessive QT prolong ...
,
Digoxin toxicity Digoxin toxicity, also known as digoxin poisoning, is a type of poisoning Poisoning is a condition or a process in which an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any indiv ...
) and management of overdose (e.g., tricyclic overdose) *
Electrolyte abnormalities Electrolyte imbalance, or water-electrolyte imbalance, is an abnormality in the concentration of electrolytes An electrolyte is a substance that produces an electrically conducting solution when dissolved in a polar solvent, such as water. The di ...
, such as
hyperkalemia Hyperkalemia is an elevated level of potassium Potassium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the propertie ...
*
Perioperative The perioperative period is the time period of a patient's surgical procedure. It commonly includes ward admission, anesthesia Anesthesia or anaesthesia (from Greek "without sensation") is a state of controlled, temporary loss of sensation o ...
monitoring in which any form of
anesthesia Anesthesia or anaesthesia (from Greek "without sensation") is a state of controlled, temporary loss of sensation or awareness that is induced for medical purposes. It may include some or all of Analgesic, analgesia (relief from or prevention o ...
is involved (e.g., monitored anesthesia care,
general anesthesia General anaesthesia or general anesthesia (see American and British English spelling differences, spelling differences) is a medically induced coma with loss of protective reflexes, resulting from the administration of one or more general anaest ...
). This includes preoperative assessment and intraoperative and postoperative monitoring. *
Cardiac stress test A cardiac stress test (also referred to as a cardiac diagnostic test, cardiopulmonary exercise test, or abbreviated CPX test) is a cardiological test that measures the heart The heart is a cardiac muscle, muscular Organ (biology), organ in ...
ing *
Computed tomography angiography Computed tomography angiography (also called CT angiography or CTA) is a computed tomography technique used for angiography — the visualization of arteries and veins — throughout the human body. Using contrast injected into the blood vessels, ...
(CTA) and
magnetic resonance angiography Magnetic resonance angiography (MRA) is a group of techniques based on magnetic resonance imaging Magnetic resonance imaging (MRI) is a medical imaging Medical imaging is the technique and process of imaging Imaging is the representati ...

magnetic resonance angiography
(MRA) of the heart (ECG is used to "gate" the scanning so that the anatomical position of the heart is steady) * Clinical cardiac electrophysiology, in which a
catheter In medicine Medicine is the science Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity, awareness, or understanding of someone or something, such as facts ( descriptive knowledge) ...

catheter
is inserted through the
femoral vein In the human body The human body is the structure of a Human, human being. It is composed of many different types of Cell (biology), cells that together create Tissue (biology), tissues and subsequently organ systems. They ensure homeostasi ...
and can have several electrodes along its length to record the direction of electrical activity from within the heart. ECGs can be recorded as short intermittent tracings or ''continuous'' ECG monitoring. Continuous monitoring is used for critically ill patients, patients undergoing general anesthesia, and patients who have an infrequently occurring cardiac arrhythmia that would unlikely be seen on a conventional ten-second ECG. Continuous monitoring can be conducted by using
Holter monitor In medicine Medicine is the Art (skill), art, science, and Praxis (process) , practice of caring for a patient and managing the diagnosis, prognosis, Preventive medicine, prevention, therapy, treatment or Palliative care , palliation of their ...

Holter monitor
s, internal and external
defibrillator Defibrillation is a treatment for life-threatening cardiac dysrhythmia Arrhythmia, also known as cardiac arrhythmia or heart arrhythmia, is a group of conditions in which the heartbeat is irregular, too fast, or too slow. The heart rate tha ...

defibrillator
s and
pacemaker A cardiac pacemaker (or artificial pacemaker, so as not to be confused with the natural pacemaker of the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pumps blood through the blood vessels of the cir ...

pacemaker
s, and/or
biotelemetry Biotelemetry (or medical telemetry) involves the application of telemetry File:Crocodylus porosus with GPS-based satellite transmitter attached to the nuchal rosette - journal.pone.0062127.g002.png, 300px, A saltwater crocodile with a GPS-based ...
.


Screening

Evidence does not support the use of ECGs among those without symptoms or at low risk of
cardiovascular disease Cardiovascular disease (CVD) is a class of diseases that involve the heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped ...
as an effort for prevention. This is because an ECG may falsely indicate the existence of a problem, leading to misdiagnosis, the recommendation of invasive procedures, and
overtreatment Unnecessary health care (overutilization, overuse, or overtreatment) is health care Health care, health-care, or healthcare is the maintenance or improvement of health via the preventive healthcare, prevention, diagnosis, therapy, treatment, re ...
. However, persons employed in certain critical occupations, such as aircraft pilots, may be required to have an ECG as part of their routine health evaluations.
Hypertrophic cardiomyopathy Hypertrophic cardiomyopathy (HCM) is a condition in which the heart becomes hypertrophy, thickened without an obvious cause. The parts of the heart most commonly affected are the interventricular septum and the ventricles. This results in the hea ...
screening may also be considered in adolescents as part of a sports physical out of concern for
sudden cardiac death Cardiac arrest is a sudden loss of blood flow resulting from the failure of the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pump ...
.


Electrocardiograph machines

Electrocardiograms are recorded by machines that consist of a set of electrodes connected to a central unit. Early ECG machines were constructed with
analog electronics Analogue electronics ( en-US, analog electronics) are electronic systems with a continuously variable signal, in contrast to digital electronics where signals usually take only two levels. The term "analogue" describes the proportional relations ...
, where the signal drove a motor to print out the signal onto paper. Today, electrocardiographs use
analog-to-digital converter In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a Digital signal (signal processing), digit ...
s to convert the electrical activity of the heart to a
digital signal A digital signal is a signal In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such as audio signal processing, sound, image processing, image ...
. Many ECG machines are now portable and commonly include a screen, keyboard, and printer on a small wheeled cart. Recent advancements in electrocardiography include developing even smaller devices for inclusion in fitness trackers and smart watches. These smaller devices often rely on only two electrodes to deliver a single lead I. Portable six-lead devices are also available. Recording an ECG is a safe and painless procedure. The machines are powered by
mains power Mains electricity (Commonwealth English The use of the English language English is a of the , originally spoken by the inhabitants of . It is named after the , one of the ancient that migrated from , a peninsula on the (not to ...
but they are designed with several safety features including an earthed (ground) lead. Other features include: *
Defibrillation Defibrillation is a treatment for life-threatening cardiac dysrhythmia Arrhythmia, also known as cardiac arrhythmia or heart arrhythmia, is a group of conditions in which the heartbeat is irregular, too fast, or too slow. The heart rate tha ...
protection: any ECG used in healthcare may be attached to a person who requires defibrillation and the ECG needs to protect itself from this source of energy. *
Electrostatic discharge Electrostatic discharge (ESD) is the sudden flow of electricity Electricity is the set of physical Physical may refer to: *Physical examination, a regular overall check-up with a doctor *Physical (album), ''Physical'' (album), a 1981 album b ...
is similar to defibrillation discharge and requires voltage protection up to 18,000 volts. * Additionally, circuitry called the right leg driver can be used to reduce
common-mode interference Common-mode signal is the voltage common to both input terminals of an electrical device. In telecommunication Telecommunication is the transmission of information by various types of technologies over wire A wire is a single usually cyl ...
(typically the 50 or 60 Hz mains power). * ECG voltages measured across the body are very small. This low voltage necessitates a low
noise Noise is unwanted sound In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and t ...
circuit,
instrumentation amplifier An instrumentation (or instrumentational) amplifier (sometimes shorthanded as In-Amp or InAmp) is a type of differential amplifier A differential amplifier is a type of electronic amplifier that amplifies the difference between two input volt ...

instrumentation amplifier
s, and
electromagnetic shielding In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics The field of electronics is a branch of ...
. * Simultaneous lead recordings: earlier designs recorded each lead sequentially, but current models record multiple leads simultaneously. Most modern ECG machines include automated interpretation
algorithm In and , an algorithm () is a finite sequence of , computer-implementable instructions, typically to solve a class of problems or to perform a computation. Algorithms are always and are used as specifications for performing s, , , and other ...

algorithm
s. This analysis calculates features such as the
PR intervalImage:SinusRhythmLabels.svg, Schematic representation of normal EKG In electrocardiography, the PR interval is the period, measured in milliseconds, that extends from the beginning of the P wave (electrocardiography), P wave (the onset of Atrium (h ...
,
QT interval The QT interval is a measurement made on an electrocardiogram Electrocardiography is the process of producing an electrocardiogram (ECG or EKG). It is a graph of voltage Voltage, electric potential difference, electromotive force emf, ...

QT interval
, corrected QT (QTc) interval, PR axis, QRS axis, rhythm and more. The results from these automated algorithms are considered "preliminary" until verified and/or modified by expert interpretation. Despite recent advances, computer misinterpretation remains a significant problem and can result in clinical mismanagement.


Cardiac monitors

Besides the standard electrocardiograph machine, there are other devices capable of recording ECG signals. Portable devices have existed since the
Holter monitor In medicine Medicine is the Art (skill), art, science, and Praxis (process) , practice of caring for a patient and managing the diagnosis, prognosis, Preventive medicine, prevention, therapy, treatment or Palliative care , palliation of their ...

Holter monitor
was produced in 1962. Traditionally, these monitors have used electrodes with patches on the skin to record the ECG, but new devices can stick to the chest as a single patch without need for wires. Implantable devices such as the
artificial cardiac pacemaker A cardiac pacemaker (or artificial pacemaker, so as not to be confused with the natural pacemaker of the heart The heart is a muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Nor ...
and
implantable cardioverter-defibrillator An implantable cardioverter-defibrillator (ICD) or automated implantable cardioverter defibrillator (AICD) is a device implantable inside the body, able to perform cardioversion Cardioversion is a medical procedure A medical procedure is a ...

implantable cardioverter-defibrillator
are capable of measuring a "far field" signal between the leads in the heart and the implanted battery/generator that resembles an ECG signal (technically, the signal recorded in the heart is called an electrogram, which is interpreted differently). Advancement of the Holter monitor became the implantable loop recorder that performs the same function but in an implantable device with batteries that last on the order of years. Additionally,
smartwatch A smartwatch is a wearable computer in the form of a watch; modern smartwatches provide a local touchscreen interface for daily use, while an associated smartphone app provides for management and telemetry (such as long-term biomonitoring). Whil ...

smartwatch
devices are capable of recording an ECG signal as well, such as with the 4th generation
Apple Watch Apple Watch is a line of smartwatch A smartwatch is a wearable computer in the form of a watch; modern smartwatches provide a local touchscreen interface for daily use, while an associated smartphone app provides for management and telemetry ...

Apple Watch
.


Electrodes and leads

Electrodes are the actual conductive pads attached to the body surface. Any pair of electrodes can measure the
electrical potential difference The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of energy needed to move a unit of from a reference point to the specific point in an electric field. ...
between the two corresponding locations of attachment. Such a pair forms ''a lead''. However, "leads" can also be formed between a physical electrode and a ''virtual electrode,'' known as Wilson's central terminal (WCT), whose potential is defined as the average potential measured by three limb electrodes that are attached to the right arm, the left arm, and the left foot, respectively. Commonly, 10 electrodes attached to the body are used to form 12 ECG leads, with each lead measuring a specific electrical potential difference (as listed in the table below). Leads are broken down into three types: limb; augmented limb; and precordial or chest. The 12-lead ECG has a total of three ''limb leads'' and three ''augmented limb leads'' arranged like spokes of a wheel in the
coronal plane A coronal plane (also known as the frontal plane) is any vertical plane that divides the body into ventral Standard anatomical terms of location deal unambiguously with the of s, including s. Terms used generally derive from or roots and use ...
(vertical), and six ''precordial leads'' or ''chest leads'' that lie on the perpendicular
transverse plane The transverse plane or axial plane (also called the horizontal plane or transaxial plane) is an imaginary plane that divides the body into superior and inferior parts. It is perpendicular to the coronal plane A coronal plane (also known as the ...
(horizontal). In medical settings, the term ''leads'' is also sometimes used to refer to the electrodes themselves, although this is technically incorrect. The 10 electrodes in a 12-lead ECG are listed below. Two types of electrodes in common use are a flat paper-thin sticker and a self-adhesive circular pad. The former are typically used in a single ECG recording while the latter are for continuous recordings as they stick longer. Each electrode consists of an
electrically conductive Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that quantifies how strongly it resists electric current An electric current is a stream of charged particles, ...
electrolyte gel and a silver/silver chloride conductor. The gel typically contains
potassium chloride Potassium chloride (KCl, or potassium salt) is a metal halide Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. Covalently bonded metal halides may be discrete m ...

potassium chloride
– sometimes
silver chloride Silver chloride is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together b ...

silver chloride
as well – to permit
electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are general ...

electron
conduction from the skin to the wire and to the electrocardiogram. The common virtual electrode, known as Wilson's central terminal (VW), is produced by averaging the measurements from the electrodes RA, LA, and LL to give an average potential of the body: : V_W = \frac(RA+LA+LL) In a 12-lead ECG, all leads except the limb leads are assumed to be unipolar (aVR, aVL, aVF, V1, V2, V3, V4, V5, and V6). The measurement of a voltage requires two contacts and so, electrically, the unipolar leads are measured from the common lead (negative) and the unipolar lead (positive). This averaging for the common lead and the abstract unipolar lead concept makes for a more challenging understanding and is complicated by sloppy usage of "lead" and "electrode". In fact, instead of being a constant reference, VW has a value that fluctuates throughout the heart cycle. It also does not truly represent the center-of-heart potential due to the body parts the signals travel through.


Limb leads

Leads I, II and III are called the ''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 Einthoven's triangle is an imaginary formation of three limb leads in a triangle used in electrocardiography Electrocardiography is the process of producing an electrocardiogram (ECG or EKG). It is a graph of voltage versus time of the electr ...
. * Lead I is the voltage between the (positive) left arm (LA) electrode and right arm (RA) electrode: : I = LA - RA * Lead II is the voltage between the (positive) left leg (LL) electrode and the right arm (RA) electrode: : II = LL - RA * Lead III is the voltage between the (positive) left leg (LL) electrode and the left arm (LA) electrode: : III = LL - LA


Augmented limb leads

Leads aVR, aVL, and aVF are the ''augmented limb leads''. They are derived from the same three electrodes as leads I, II, and III, but they use Goldberger's central terminal as their negative pole. Goldberger's central terminal is a combination of inputs from two limb electrodes, with a different combination for each augmented lead. It is referred to immediately below as "the negative pole". * Lead ''augmented vector right'' (aVR) has the positive electrode on the right arm. The negative pole is a combination of the left arm electrode and the left leg electrode: : aVR = RA - \frac (LA + LL) = \frac 32 (RA - V_W) * Lead ''augmented vector left'' (aVL) has the positive electrode on the left arm. The negative pole is a combination of the right arm electrode and the left leg electrode: : aVL = LA - \frac (RA + LL) = \frac 32 (LA - V_W) * Lead ''augmented vector foot'' (aVF) has the positive electrode on the left leg. The negative pole is a combination of the right arm electrode and the left arm electrode: : aVF = LL - \frac (RA + LA) = \frac 32 (LL - V_W) Together with leads I, II, and III, augmented limb leads aVR, aVL, and aVF form the basis of the
hexaxial reference system The hexaxial reference system, better known as the Cabrera system, is a convention to present the extremity leads of the 12 lead electrocardiogram Electrocardiography is the process of producing an electrocardiogram (ECG or EKG). It is a graph ...

hexaxial reference system
, which is used to calculate the heart's electrical axis in the frontal plane. Older versions of the nodes (VR, VL, VF) use Wilson's central terminal as the negative pole, but the amplitude is too small for the thick lines of old ECG machines. The Goldberger terminals scale up (augments) the Wilson results by 50%, at the cost of sacrificing physical correctness by not having the same negative pole for all three.


Precordial leads

The ''precordial leads'' lie in the transverse (horizontal) plane, perpendicular to the other six leads. The six precordial electrodes act as the positive poles for the six corresponding precordial leads: (V1, V2, V3, V4, V5, and V6). Wilson's central terminal is used as the negative pole. Recently, unipolar precordial leads have been used to create bipolar precordial leads that explore the right to left axis in the horizontal plane.


Specialized leads

Additional electrodes may rarely be placed to generate other leads for specific diagnostic purposes. ''Right-sided'' precordial leads may be used to better study pathology of the right ventricle or for
dextrocardia Dextrocardiaa disease (from Latin ''dexter'', meaning "right," and Greek ''kardia'', meaning "heart") is a rare congenital condition in which the apex of the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, wh ...

dextrocardia
(and are denoted with an R (e.g., V5R). ''Posterior leads'' (V7 to V9) may be used to demonstrate the presence of a posterior myocardial infarction. A ''
Lewis lead A Lewis Lead (also called the S5 lead) is a modified ECG lead used to detect atrial flutter Atrial flutter (AFL) is a common abnormal heart rhythm that starts in the atrial chambers of the heart The heart is a muscle, muscular Organ (an ...

Lewis lead
'' (requiring an electrode at the right sternal border in the second intercostal space) can be used to study pathological rhythms arising in the right atrium. An ''esophogeal lead'' can be inserted to a part of the
esophagus The esophagus (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United State ...

esophagus
where the distance to the posterior wall of the
left atrium The atrium (Latin ātrium, “entry hall”) is the upper chamber through which blood enters the Ventricle (heart), ventricles of the heart. There are two atria in the human heart – the left atrium receives blood from the pulmonary (lung) circula ...
is only approximately 5–6 mm (remaining constant in people of different age and weight). An esophageal lead avails for a more accurate differentiation between certain cardiac arrhythmias, particularly
atrial flutter Atrial flutter (AFL) is a common abnormal heart rhythm that starts in the atrial chambers of the heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the c ...
,
AV nodal reentrant tachycardia AV-nodal reentrant tachycardia (AVNRT) is a type of abnormal fast heart rhythm. It is a type of supraventricular tachycardia (SVT), meaning that it originates from a location within the heart above the bundle of His. AV nodal reentrant tachycar ...

AV nodal reentrant tachycardia
and orthodromic
atrioventricular reentrant tachycardia Atrioventricular reentrant tachycardia (AVRT), or atrioventricular reciprocating tachycardia, is a type of abnormal fast heart rhythm and is classified as a type of supraventricular tachycardia (SVT). AVRT is most commonly associated with Wolf ...
. It can also evaluate the risk in people with Wolff-Parkinson-White syndrome, as well as terminate
supraventricular tachycardia Supraventricular tachycardia (SVT) is an umbrella term for arising from the . This is in contrast to the other group of fast heart rhythms - , which start within the . There are four main types of SVT: , , (PSVT) and . The symptoms of SVT incl ...
caused by
re-entry (MER) aeroshell, artistic rendition Atmospheric entry is the movement of an object from outer space Outer space is the expanse that exists beyond Earth and between astronomical object, celestial bodies. Outer space is not completely empt ...
. An intracardiac electrogram (ICEG) is essentially an ECG with some added ''intracardiac leads'' (that is, inside the heart). The standard ECG leads (external leads) are I, II, III, aVL, V1, and V6. Two to four intracardiac leads are added via cardiac catheterization. The word "electrogram" (EGM) without further specification usually means an intracardiac electrogram.


Lead locations on an ECG report

A standard 12-lead ECG report (an electrocardiograph) shows a 2.5 second tracing of each of the twelve leads. The tracings are most commonly arranged in a grid of four columns and three rows. The first column is the limb leads (I, II, and III), the second column is the augmented limb leads (aVR, aVL, and aVF), and the last two columns are the precordial leads (V1 to V6). Additionally, a rhythm strip may be included as a fourth or fifth row. The timing across the page is continuous and not tracings of the 12 leads for the same time period. In other words, if the output were traced by needles on paper, each row would switch which leads as the paper is pulled under the needle. For example, the top row would first trace lead I, then switch to lead aVR, then switch to V1, and then switch to V4, and so none of these four tracings of the leads are from the same time period as they are traced in sequence through time.


Contiguity of leads

Each of the 12 ECG leads records the electrical activity of the heart from a different angle, and therefore align with different anatomical areas of the heart. Two leads that look at neighboring anatomical areas are said to be ''contiguous''. 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.


Electrophysiology

The study of the conduction system of the heart is called
cardiac electrophysiology Cardiac electrophysiology is the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes knowledge in the form of Testability, testabl ...
(EP). An EP study is performed via a right-sided
cardiac catheterization Cardiac catheterization (heart cath) is the insertion of a catheter In medicine Medicine is the science Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity, awareness, or understan ...
: a wire with an electrode at its tip is inserted into the right heart chambers from a peripheral vein, and placed in various positions in close proximity to the conduction system so that the electrical activity of that system can be recorded. Standard catheter positions for an EP study include "high right atrium" or hRA near the
sinus node The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is a group of Cell (biology), cells located in the wall of the atrium (heart), right atrium of the heart. These Cell (biology), cells have the ability to spontaneously ...
, a "His" across the septal wall of the tricuspid valve to measure
bundle of His Discovered in 1893 by Swiss Swiss may refer to: * the adjectival form of Switzerland *Swiss people Places *Swiss, Missouri *Swiss, North Carolina *Swiss, West Virginia *Swiss, Wisconsin Other uses *Swiss-system tournament, in various games and s ...
, a "coronary sinus" into the
coronary sinus The coronary sinus is a collection of vein Veins are blood vessels The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system ...
, and a "right ventricle" in the apex of the right ventricle.


Interpretation

Interpretation of the ECG is fundamentally about understanding the
electrical conduction system of the heart The electrical conduction system of the heart transmits signals generated usually by the sinoatrial node to cause contraction of the heart muscle. The Cardiac pacemaker, pacemaking signal generated in the sinoatrial node travels through the right ...

electrical conduction system of the heart
. Normal conduction starts and propagates in a predictable pattern, and deviation from this pattern can be a normal variation or be
pathological Pathology is the study of the causesCauses, or causality, is the relationship between one event and another. It may also refer to: * Causes (band), an indie band based in the Netherlands * Causes (company), an online company See also * Cau ...
. An ECG does not equate with mechanical pumping activity of the heart, for example,
pulseless electrical activity Pulseless electrical activity (PEA) refers to cardiac arrest in which the electrocardiogram shows a heart rhythm that should produce a pulse, but does not. Pulseless electrical activity is found initially in about 55% of people in cardiac arrest. U ...
produces an ECG that should pump blood but no pulses are felt (and constitutes a
medical emergency A medical emergency is an acute Acute may refer to: Science and technology * Acute angle ** Acute triangle ** Acute, a leaf shape in the glossary of leaf morphology#acute, glossary of leaf morphology * Acute (medicine), a disease that it is o ...

medical emergency
and
CPR Cardiopulmonary resuscitation (CPR) is an emergency procedure An emergency procedure is a plan of actions to be conducted in a certain order or manner, in response to a specific class of reasonably foreseeable emergency, a situation that pose ...
should be performed).
Ventricular fibrillation Ventricular fibrillation (V-fib or VF) is an abnormal heart rhythm in which the Ventricle (heart), ventricles of the heart Fibrillation, quiver. It is due to disorganized electrical conduction system of the heart, electrical activity. Ventricular ...

Ventricular fibrillation
produces an ECG but is too dysfunctional to produce a life-sustaining cardiac output. Certain rhythms are known to have good cardiac output and some are known to have bad cardiac output. Ultimately, an
echocardiogram An echocardiography, echocardiogram, cardiac echo or simply an echo, is an Medical ultrasound, ultrasound of the heart. It is a type of medical imaging of the heart, using standard ultrasound or Doppler echocardiography, Doppler ultrasound. Echoc ...

echocardiogram
or other anatomical imaging modality is useful in assessing the mechanical function of the heart. Like all medical tests, what constitutes "normal" is based on
population studiesIn the various fields of healthcare, a population study is a study of a group of individuals taken from the general population In biology, a population is a number of all the organisms of the same group or species In biology, a species i ...
. The heartrate range of between 60 and 100 beats per minute (bpm) is considered normal since data shows this to be the usual resting heart rate.


Theory

Interpretation of the ECG is ultimately that of pattern recognition. In order to understand the patterns found, it is helpful to understand the theory of what ECGs represent. The theory is rooted in
electromagnetics Electromagnetism is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics ...
and boils down to the four following points: * depolarization of the heart ''towards'' the positive electrode produces a positive deflection * depolarization of the heart ''away'' from the positive electrode produces a negative deflection * repolarization of the heart ''towards'' the positive electrode produces a negative deflection * repolarization of the heart ''away'' from the positive electrode produces a positive deflection Thus, the overall direction of depolarization and repolarization produces positive or negative deflection on each lead's trace. For example, depolarizing from right to left would produce a positive deflection in lead I because the two vectors point in the same direction. In contrast, that same depolarization would produce minimal deflection in V1 and V2 because the vectors are perpendicular, and this phenomenon is called isoelectric. Normal rhythm produces four entities – a P wave, a QRS complex, a T wave, and a U wave – that each have a fairly unique pattern. * The P wave represents atrial depolarization. * The QRS complex represents ventricular depolarization. * The T wave represents ventricular repolarization. * The U wave represents papillary muscle repolarization. Changes in the structure of the heart and its surroundings (including blood composition) change the patterns of these four entities. The U wave is not typically seen and its absence is generally ignored. Atrial repolarisation is typically hidden in the much more prominent QRS complex and normally cannot be seen without additional, specialised electrodes.


Background grid

ECGs are normally printed on a grid. The horizontal axis represents time and the vertical axis represents voltage. The standard values on this grid are shown in the adjacent image at 25mm/sec: * A small box is 1 mm × 1 mm and represents 0.1 mV × 0.04 seconds. * A large box is 5 mm × 5 mm and represents 0.5 mV × 0.20 seconds. The "large" box is represented by a heavier line weight than the small boxes. The standard printing speed in the United States is 25 mm per sec (5 big boxes per second), but it other countries it can be 50 mm per sec. Faster speeds such as 100 and 200 mm per sec are used during electrophysiology studies. Not all aspects of an ECG rely on precise recordings or having a known scaling of amplitude or time. For example, determining if the tracing is a sinus rhythm only requires feature recognition and matching, and not measurement of amplitudes or times (i.e., the scale of the grids are irrelevant). An example to the contrary, the voltage requirements of
left ventricular hypertrophy Left ventricular hypertrophy (LVH) is of the of the left of the , that is, left-sided . Causes While ventricular hypertrophy as a reaction to and , it is most frequently referred to as a pathological reaction to , or . It is one aspect of . ...
require knowing the grid scale.


Rate and rhythm

In a normal heart, the heart rate is the rate in which the
sinoatrial node The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is a group of Cell (biology), cells known as pacemaker cells, located in the wall of the atrium (heart), right atrium of the heart. These cells can produce an electric ...
depolarizes since it is the source of depolarization of the heart. Heart rate, like other
vital signs Vital signs (also known as vitals) are a group of the four to six most important medical signs Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign for example may be a ...
such as blood pressure and respiratory rate, change with age. In adults, a normal heart rate is between 60 and 100 bpm (normocardic), whereas it is higher in children. A heart rate below normal is called "
bradycardia Bradycardia is a condition typically defined wherein an individual has a resting heart rate Heart rate is the speed of the heartbeat Heartbeat or heartbeats may refer to: Physiology *Cardiac cycle, of the heart *Contraction of the cardiac musc ...
" (<60 in adults) and above normal is called "
tachycardia Tachycardia, also called tachyarrhythmia, is a heart rate Heart rate is the speed of the heartbeat Heartbeat or heartbeats may refer to: Physiology *Cardiac cycle, of the heart *Contraction of the cardiac muscle, muscles of the heart, or a per ...
" (>100 in adults). A complication of this is when the atria and ventricles are not in synchrony and the "heart rate" must be specified as atrial or ventricular (e.g., the ventricular rate in
ventricular fibrillation Ventricular fibrillation (V-fib or VF) is an abnormal heart rhythm in which the Ventricle (heart), ventricles of the heart Fibrillation, quiver. It is due to disorganized electrical conduction system of the heart, electrical activity. Ventricular ...

ventricular fibrillation
is 300–600 bpm, whereas the atrial rate can be normal 0–100or faster [100–150]). In normal resting hearts, the physiologic rhythm of the heart is normal sinus rhythm (NSR). Normal sinus rhythm produces the prototypical pattern of P wave, QRS complex, and T wave. Generally, deviation from normal sinus rhythm is considered a cardiac arrhythmia. Thus, the first question in interpreting an ECG is whether or not there is a sinus rhythm. A criterion for sinus rhythm is that P waves and QRS complexes appear 1-to-1, thus implying that the P wave causes the QRS complex. Once sinus rhythm is established, or not, the second question is the rate. For a sinus rhythm, this is either the rate of P waves or QRS complexes since they are 1-to-1. If the rate is too fast, then it is sinus tachycardia, and if it is too slow, then it is sinus bradycardia. If it is not a sinus rhythm, then determining the rhythm is necessary before proceeding with further interpretation. Some arrhythmias with characteristic findings: * Absent P waves with "irregularly irregular" QRS complexes is the hallmark of
atrial fibrillation Atrial fibrillation (AF or A-fib) is an abnormal heart rhythm (arrhythmia) characterized by the rapid and irregular beating of the atrial chambers of the heart. It often begins as short periods of abnormal beating, which become longer or cont ...

atrial fibrillation
. * A "saw tooth" pattern with QRS complexes is the hallmark of
atrial flutter Atrial flutter (AFL) is a common abnormal heart rhythm that starts in the atrial chambers of the heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the c ...
. * A sine wave pattern is the hallmark of ventricular flutter. * Absent P waves with wide QRS complexes and a fast heart rate is
ventricular tachycardia Ventricular tachycardia (V-tach or VT) is a type of regular, fast heart rate that arises from improper electrical activity in the ventricles of the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pump ...
. Determination of rate and rhythm is necessary in order to make sense of further interpretation.


Axis

The heart has several axes, but the most common by far is the axis of the QRS complex (references to "the axis" imply the QRS axis). Each axis can be computationally determined to result in a number representing degrees of deviation from zero, or it can be categorized into a few types. The QRS axis is the general direction of the ventricular depolarization wavefront (or mean electrical vector) in the frontal plane. It is often sufficient to classify the axis as one of three types: normal, left deviated, or right deviated. Population data shows that a normal QRS axis is from −30° to 105°, with 0° being along lead I and positive being inferior and negative being superior (best understood graphically as the
hexaxial reference system The hexaxial reference system, better known as the Cabrera system, is a convention to present the extremity leads of the 12 lead electrocardiogram Electrocardiography is the process of producing an electrocardiogram (ECG or EKG). It is a graph ...

hexaxial reference system
). Beyond +105° is right axis deviation and beyond −30° is left axis deviation (the third quadrant of −90° to −180° is very rare and is an indeterminate axis). A shortcut for determining if the QRS axis is normal is if the QRS complex is mostly positive in lead I and lead II (or lead I and aVF if +90° is the upper limit of normal). The normal QRS axis is generally ''down and to the left'', following the anatomical orientation of the heart within the chest. An abnormal axis suggests a change in the physical shape and orientation of the heart or a defect in its conduction system that causes the ventricles to depolarize in an abnormal way. The extent of a normal axis can be +90° or 105° depending on the source.


Amplitudes and intervals

All of the waves on an ECG tracing and the intervals between them have a predictable time duration, a range of acceptable amplitudes (voltages), and a typical morphology. Any deviation from the normal tracing is potentially pathological and therefore of clinical significance. For ease of measuring the amplitudes and intervals, an ECG is printed on graph paper at a standard scale: each 1 mm (one small box on the standard ECG paper) represents 40 milliseconds of time on the x-axis, and 0.1 millivolts on the y-axis.


Limb leads and electrical conduction through the heart

The animation shown to the right illustrates how the path of electrical conduction gives rise to the ECG waves in the limb leads. Recall that a positive current (as created by depolarization of cardiac cells) traveling towards the positive electrode and away from the negative electrode creates a positive deflection on the ECG. Likewise, a positive current traveling away from the positive electrode and towards the negative electrode creates a negative deflection on the ECG. The red arrow represents the overall direction of travel of the depolarization. The magnitude of the red arrow is proportional to the amount of tissue being depolarized at that instance. The red arrow is simultaneously shown on the axis of each of the 3 limb leads. Both the direction and the magnitude of the red arrow's projection onto the axis of each limb lead is shown with blue arrows. Then, the direction and magnitude of the blue arrows are what theoretically determine the deflections on the ECG. For example, as a blue arrow on the axis for Lead I moves from the negative electrode, to the right, towards the positive electrode, the ECG line rises, creating an upward wave. As the blue arrow on the axis for Lead I moves to the left, a downward wave is created. The greater the magnitude of the blue arrow, the greater the deflection on the ECG for that particular limb lead. Frames 1–3 depict the depolarization being generated in and spreading through the Sinoatrial node. The SA node is too small for its depolarization to be detected on most ECGs. Frames 4–10 depict the depolarization traveling through the atria, towards the Atrioventricular node. During frame 7, the depolarization is traveling through the largest amount of tissue in the atria, which creates the highest point in the P wave. Frames 11–12 depict the depolarization traveling through the AV node. Like the SA node, the AV node is too small for the depolarization of its tissue to be detected on most ECGs. This creates the flat PR segment. Frame 13 depicts an interesting phenomenon in an over-simplified fashion. It depicts the depolarization as it starts to travel down the interventricular septum, through the Bundle of His and Bundle branches. After the Bundle of His, the conduction system splits into the left bundle branch and the right bundle branch. Both branches conduct action potentials at about 1 m/s. Interestingly, however, the action potential starts traveling down the left bundle branch about 5 milliseconds before it starts traveling down the right bundle branch, as depicted by frame 13. This causes the depolarization of the interventricular septum tissue to spread from left to right, as depicted by the red arrow in frame 14. In some cases, this gives rise to a negative deflection after the PR interval, creating a Q wave such as the one seen in lead I in the animation to the right. Depending on the mean electrical axis of the heart, this phenomenon can result in a Q wave in lead II as well. Following depolarization of the interventricular septum, the depolarization travels towards the apex of the heart. This is depicted by frames 15–17 and results in a positive deflection on all three limb leads, which creates the R wave. Frames 18–21 then depict the depolarization as it travels throughout both ventricles from the apex of the heart, following the action potential in the
Purkinje fibers The Purkinje fibers ( ; Purkinje tissue or subendocardial branches) are located in the inner Ventricle (heart), ventricular walls of the heart, just beneath the endocardium in a space called the subendocardium. The Purkinje fibers are specializ ...

Purkinje fibers
. This phenomenon creates a negative deflection in all three limb leads, forming the S wave on the ECG. Repolarization of the atria occurs at the same time as the generation of the QRS complex, but it is not detected by the ECG since the tissue mass of the ventricles is so much larger than that of the atria. Ventricular contraction occurs between ventricular depolarization and repolarization. During this time, there is no movement of charge, so no deflection is created on the ECG. This results in the flat ST segment after the S wave. Frames 24–28 in the animation depict repolarization of the ventricles. The epicardium is the first layer of the ventricles to repolarize, followed by the myocardium. The endocardium is the last layer to repolarize. The plateau phase of depolarization has been shown to last longer in endocardial cells than in epicardial cells. This causes repolarization to start from the apex of the heart and move upwards. Since repolarization is the spread of negative current as membrane potentials decrease back down to the resting membrane potential, the red arrow in the animation is pointing in the direction opposite of the repolarization. This therefore creates a positive deflection in the ECG, and creates the T wave.


Ischemia and infarction

Ischemia or NSTEMI, non-ST elevation myocardial infarctions (non-STEMIs) may manifest as ST depression or inversion of
T wave Normal T wave In electrocardiography, the T wave represents the repolarization of the Ventricle (heart), 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 (ph ...
s. It may also affect the HFQRS, high frequency band of the QRS. STEMI, ST elevation myocardial infarctions (STEMIs) have different characteristic ECG findings based on the amount of time elapsed since the MI first occurred. The earliest sign is ''hyperacute T waves,'' peaked T waves due to local
hyperkalemia Hyperkalemia is an elevated level of potassium Potassium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the propertie ...
in ischemic myocardium. This then progresses over a period of minutes to elevations of the ST segment by at least 1 mm. Over a period of hours, a pathologic QRS complex#Q wave, Q wave may appear and the T wave will invert. Over a period of days the ST elevation will resolve. Pathologic Q waves generally will remain permanently. The coronary artery that has been occluded can be identified in an STEMI based on the location of ST elevation. The Anterior interventricular branch of left coronary artery, left anterior descending (LAD) artery supplies the anterior wall of the heart, and therefore causes ST elevations in anterior leads (V1 and V2). The Circumflex branch of left coronary artery, LCx supplies the lateral aspect of the heart and therefore causes ST elevations in lateral leads (I, aVL and V6). The right coronary artery (RCA) usually supplies the inferior aspect of the heart, and therefore causes ST elevations in inferior leads (II, III and aVF).


Artifacts

An ECG tracing is affected by patient motion. Some rhythmic motions (such as shivering or tremors) can create the illusion of cardiac arrhythmia. Artifacts are distorted signals caused by a secondary internal or external sources, such as muscle movement or interference from an electrical device. Distortion poses significant challenges to healthcare providers, who employ various techniques and strategies to safely recognize these false signals. Accurately separating the ECG artifact from the true ECG signal can have a significant impact on patient outcomes and Legal liability, legal liabilities. Improper lead placement (for example, reversing two of the limb leads) has been estimated to occur in 0.4% to 4% of all ECG recordings, and has resulted in improper diagnosis and treatment including unnecessary use of thrombolytic therapy.


Diagnosis

Numerous diagnoses and findings can be made based upon electrocardiography, and many are discussed above. Overall, the diagnoses are made based on the patterns. For example, an "irregularly irregular" QRS complex without P waves is the hallmark of
atrial fibrillation Atrial fibrillation (AF or A-fib) is an abnormal heart rhythm (arrhythmia) characterized by the rapid and irregular beating of the atrial chambers of the heart. It often begins as short periods of abnormal beating, which become longer or cont ...

atrial fibrillation
; however, other findings can be present as well, such as a bundle branch block that alters the shape of the QRS complexes. ECGs can be interpreted in isolation but should be applied – like all diagnostic tests – in the context of the patient. For example, an observation of peaked T waves is not sufficient to diagnose hyperkalemia; such a diagnosis should be verified by measuring the blood potassium level. Conversely, a discovery of hyperkalemia should be followed by an ECG for manifestations such as peaked T waves, widened QRS complexes, and loss of P waves. The following is an organized list of possible ECG-based diagnoses. Rhythm disturbances or arrhythmias: * Atrial fibrillation and
atrial flutter Atrial flutter (AFL) is a common abnormal heart rhythm that starts in the atrial chambers of the heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the c ...
without rapid ventricular response * Premature atrial contraction (PACs) and premature ventricular contraction (PVCs) * Sinus arrhythmia * Sinus bradycardia and sinus tachycardia * Sinus pause and sinoatrial arrest * Sinus node dysfunction and bradycardia-tachycardia syndrome * Supraventricular tachycardia ** Atrial fibrillation with rapid ventricular response ** Atrial flutter with rapid ventricular response **
AV nodal reentrant tachycardia AV-nodal reentrant tachycardia (AVNRT) is a type of abnormal fast heart rhythm. It is a type of supraventricular tachycardia (SVT), meaning that it originates from a location within the heart above the bundle of His. AV nodal reentrant tachycar ...

AV nodal reentrant tachycardia
** Atrioventricular reentrant tachycardia ** Junctional ectopic tachycardia ** Atrial tachycardia *** Ectopic atrial tachycardia (unicentric) *** Multifocal atrial tachycardia *** Paroxysmal atrial tachycardia ** Sinoatrial nodal reentrant tachycardia * Torsades de pointes (polymorphic ventricular tachycardia) * Wide complex tachycardia ** Ventricular flutter **
Ventricular fibrillation Ventricular fibrillation (V-fib or VF) is an abnormal heart rhythm in which the Ventricle (heart), ventricles of the heart Fibrillation, quiver. It is due to disorganized electrical conduction system of the heart, electrical activity. Ventricular ...

Ventricular fibrillation
** Ventricular tachycardia (monomorphic ventricular tachycardia) * Pre-excitation syndrome ** Lown–Ganong–Levine syndrome ** Wolff–Parkinson–White syndrome * J wave (Osborn wave) Heart block and conduction problems: * Cardiac aberrancy, Aberration * Sinoatrial block: first, second, and third-degree * AV node ** First-degree AV block ** Second-degree AV block (Mobitz [Wenckebach] I and II) ** Third-degree AV block or complete AV block * Right bundle ** Incomplete right bundle branch block ** Complete right bundle branch block (RBBB) * Left bundle ** Complete left bundle branch block (LBBB) ** Incomplete left bundle branch block ** Left anterior fascicular block (LAFB) ** Left posterior fascicular block (LPFB) ** Bifascicular block (LAFB plus LPFB) ** Trifascicular block (LAFP plus FPFB plus RBBB) * QT syndromes ** Brugada syndrome ** Short QT syndrome ** Long QT syndromes, genetic and drug-induced * Right atrial abnormality, Right and left atrial abnormality Electrolytes disturbances and intoxication: * Digoxin poisoning, Digitalis intoxication * Calcium: hypocalcemia and hypercalcemia * Potassium:
hypokalemia Hypokalemia is a low level of potassium Potassium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and b ...
and
hyperkalemia Hyperkalemia is an elevated level of potassium Potassium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the propertie ...
* Serotonin Syndrome, Serotonin Toxicity Ischemia and infarction: * Wellens' syndrome (LAD occlusion) * de Winter T waves (LAD occlusion) * ST elevation and ST depression * High Frequency QRS changes * Myocardial infarction (heart attack) ** Non-Q wave myocardial infarction ** NSTEMI ** STEMI ** Sgarbossa's criteria for ischemia with a left bundle branch block, LBBB Structural: * Acute pericarditis * Right ventricular hypertrophy, Right and
left ventricular hypertrophy Left ventricular hypertrophy (LVH) is of the of the left of the , that is, left-sided . Causes While ventricular hypertrophy as a reaction to and , it is most frequently referred to as a pathological reaction to , or . It is one aspect of . ...
* Right ventricular strain or S1Q3T3 (can be seen in pulmonary embolism)


History

* In 1872, Alexander Muirhead is reported to have attached wires to the wrist of a patient with fever to obtain an electronic record of their heartbeat. * In 1882, John Burdon-Sanderson working with frogs, was the first to appreciate that the interval between variations in potential was not electrically quiescent and coined the term "isoelectric interval" for this period. * In 1887, Augustus Desiré Waller, Augustus Waller invented an ECG machine consisting of a lippmann electrometer, Lippmann capillary electrometer fixed to a projector. The trace from the heartbeat was projected onto a photographic plate that was itself fixed to a toy train. This allowed a heartbeat to be recorded in real time. * In 1895, Willem Einthoven assigned the letters P, Q, R, S, and T to the deflections in the theoretical waveform he created using equations which corrected the actual waveform obtained by the capillary electrometer to compensate for the imprecision of that instrument. Using letters different from A, B, C, and D (the letters used for the capillary electrometer's waveform) facilitated comparison when the uncorrected and corrected lines were drawn on the same graph. Einthoven probably chose the initial letter P to follow the example set by Descartes in geometry. When a more precise waveform was obtained using the string galvanometer, which matched the corrected capillary electrometer waveform, he continued to use the letters P, Q, R, S, and T, and these letters are still in use today. Einthoven also described the electrocardiographic features of a number of cardiovascular disorders. * In 1897, the string galvanometer was invented by the French engineer Clément Ader. * In 1901, Einthoven, working in Leiden, the Netherlands, used the string galvanometer: the first practical ECG. This device was much more sensitive than the capillary electrometer Waller used. * In 1924, Einthoven was awarded the Nobel Prize in Medicine for his pioneering work in developing the ECG. * By 1927, General Electric had developed a portable apparatus that could produce electrocardiograms without the use of the string galvanometer. This device instead combined amplifier tubes similar to those used in a radio with an internal lamp and a moving mirror that directed the tracing of the electric pulses onto film. * In 1937, Taro Takemi invented a new portable electrocardiograph machine. * In 1942, Emanuel Goldberger increases the voltage of Wilson's unipolar leads by 50% and creates the augmented limb leads aVR, aVL and aVF. When added to Einthoven's three limb leads and the six chest leads we arrive at the 12-lead electrocardiogram that is used today. * In the late 1940s Rune Elmqvist invented an inkjet printer - thin jets of ink deflected by electrical potentials from the heart, with good frequency response and direct recording of ECG on paper - the device, called the Mingograf, was sold by Siemens Elema until the 1990s.


Etymology

The word is derived from the Greek language, Greek ''electro'', meaning related to electrical activity; ''kardia'', meaning heart; and ''graph'', meaning "to write".


See also

* Electrical conduction system of the heart * Electrogastrogram * Electropalatography * Electroretinography * Heart rate * Heart rate monitor * Emergency medicine * Forward problem of electrocardiology


Notes


References


External links


The whole ECG course on 1 A4 paper
fro
ECGpedia
a wiki encyclopedia fo
a course on interpretation of ECG

Wave Maven – a large database of practice ECG questions
provided by Beth Israel Deaconess Medical Center
PysioBank – a free scientific database with physiologic signals (here ecg)

EKG Academy – free EKG lectures, drills and quizzes

ECG Learning Center
created by Eccles Health Sciences Library at University of Utah {{Authority control Cardiac electrophysiology Cardiac procedures Electrodiagnosis Electrophysiology Mathematics in medicine Medical tests Dutch inventions Science and technology in the Netherlands Biology in the Netherlands