View source for Electrocardiography

{{Short description|Examination of the heart's electrical activity}}
{{Redirect2|ECG|EKG|other uses|ECG (disambiguation)|and|EKG (disambiguation)}}
{{distinguish|text=[[electrography (disambiguation)|other types of electrography]] or with [[echocardiography]]}}
{{Use dmy dates|date=May 2014}}
{{Infobox medical intervention
 | Name        = Electrocardiography
 | Image       = SinusRhythmLabels.svg
 | Caption     = ECG of a heart in normal [[sinus rhythm]]
 | ICD10       = R94.31
 | ICD9        = 89.52
 | MeshID      = D004562
 | MedlinePlus = 003868
 | OtherCodes  =
}}
[[File:Patient lying in hospital bed in intensive care unit in Germany in 2015.jpg|thumb|Use of [[monitoring (medicine)|real time monitoring]] of the heart in an [[intensive care unit]] in a German hospital (2015), the monitoring screen above the patient displaying an electrocardiogram and various values of parameters of the heart like heart rate and blood pressure]]

'''Electrocardiography''' is the process of producing an '''electrocardiogram''' ('''ECG''' or '''EKG'''{{efn|The version with '-K-', more commonly used in [[American English]] than in [[British English]], is an early-20th-century [[loanword]] from the German acronym EKG for 
{{lang|de|Elektrokardiogramm}}
(electrocardiogram),<ref>
{{Cite web 
  |url=https://www.lexico.com/en/definition/ekg 
  |archive-url=https://web.archive.org/web/20200215123359/https://www.lexico.com/en/definition/ekg 
  |url-status=dead 
  |archive-date=15 February 2020 
  |title=Definition of EKG  
  |website=Lexico Dictionaries 
  |language=en 
  |access-date=2020-01-20
}}</ref> which reflects that German physicians were pioneers in the field at the time. Today, [[AMA Manual of Style|AMA style]] and–under its stylistic influence–most American medical publications use ECG instead of EKG.<ref name="AMA_MOS">
{{Citation |title=AMA Manual of Style
  |url=https://www.amamanualofstyle.com/ 
  |publisher=[[American Medical Association]] 
  |section=15.3.1 Electrocardiographic Terms
}}</ref> The German term 
{{lang|de|Elektrokardiogramm}} 
as well as the English equivalent, electrocardiogram, consist of the [[Neo-Latin]]/[[international scientific vocabulary]] elements 
{{lang|de|[[wikt:elektro-#German|elektro-]]}}
(cognate 
{{lang|de|[[wikt:electro-#English|electro-]]}}) and 
{{lang|de|kardi-}}
(cognate 'cardi-'), the latter from Greek 
{{lang|el|kardia}}
(heart).<ref name="MW_Collegiate">
{{Cite encyclopedia 
  |title=Merriam-Webster's Collegiate Dictionary 
  |encyclopedia=[[Merriam-Webster]] 
  |url=https://unabridged.merriam-webster.com/collegiate/ 
  |url-access=subscription
}}</ref> The '-K-' version is more often retained under circumstances where there may be verbal confusion between ECG and EEG ([[electroencephalography]]) due to similar pronunciation.
}}<!--END efn-->), a recording of the heart's electrical activity through repeated [[cardiac cycle]]s.<ref name=Kumar2020>
{{cite book 
  |last1=Bunce 
  |first1=Nicholas H. 
  |last2=Ray 
  |first2=Robin
  |last3=Patel
  |first3=Hitesh 
  |editor1-last=Feather 
  |editor1-first=Adam 
  |editor2-last=Randall 
  |editor2-first=David 
  |editor3-last=Waterhouse 
  |editor3-first=Mona 
  |title=Kumar and Clark's Clinical Medicine 
  |date=2020 
  |publisher=Elsevier|isbn=978-0-7020-7870-5 
  |pages=1033–1038 
  |edition=10th 
  |chapter-url=https://books.google.com/books?id=sl3sDwAAQBAJ&pg=PA1033  
  |language=en 
  |chapter=30. Cardiology
}}</ref> It is an [[electrogram]] of the [[heart]] which is a graph of [[voltage]] versus time of the electrical activity of the heart<ref name="Lilly 2016">
{{Cite book
  |last=Lilly
  |first=Leonard S.
  |title=Pathophysiology of Heart Disease: A Collaborative Project of Medical Students and Faculty, 6th Edition
  |date=2016
  |publisher=Lippincott Williams & Wilkins
  |isbn=978-1-4698-9758-5
  |pages=70–78
  |language=English
  |oclc=1229852550
}}</ref> using [[electrode]]s placed on the skin. These electrodes detect the small electrical changes that are a consequence of [[cardiac muscle]] [[depolarization]] followed by [[repolarization]] during each [[cardiac cycle]] (heartbeat). Changes in the normal ECG pattern occur in numerous cardiac abnormalities, including:

* [[Cardiac rhythmicity|Cardiac rhythm]] disturbances, such as [[atrial fibrillation]]<ref>{{Cite journal|last1=Lyakhov|first1=Pavel|last2=Kiladze|first2=Mariya|last3=Lyakhova|first3=Ulyana|date=January 2021|title=System for Neural Network Determination of Atrial Fibrillation on ECG Signals with Wavelet-Based Preprocessing|journal=Applied Sciences|language=en|volume=11|issue=16|pages=7213|doi=10.3390/app11167213|doi-access=free}}</ref> and [[ventricular tachycardia]];<ref>{{cite journal |last1=Hoyland |first1=Philip |last2=Hammache |first2=Nefissa |last3=Battaglia |first3=Alberto |last4=Oster |first4=Julien |last5=Felblinger |first5=Jacques |last6=de Chillou |first6=Christian |last7=Odille |first7=Freddy |title=A Paced-ECG Detector and Delineator for Automatic Multi-Parametric Catheter Mapping of Ventricular Tachycardia |journal=IEEE Access |date=2020 |volume=8 |pages=223952–223960 |doi=10.1109/ACCESS.2020.3043542 |bibcode=2020IEEEA...8v3952H |doi-access=free}}</ref> 
* Inadequate coronary artery blood flow, such as [[myocardial ischemia]]<ref>{{cite journal |last1=Bigler |first1=Marius Reto |last2=Zimmermann |first2=Patrick |last3=Papadis |first3=Athanasios |last4=Seiler |first4=Christian |title=Accuracy of intracoronary ECG parameters for myocardial ischemia detection |journal=Journal of Electrocardiology |date=January 2021 |volume=64 |pages=50–57 |doi=10.1016/j.jelectrocard.2020.11.018 |pmid=33316551 |doi-access=free }}</ref> and [[myocardial infarction]];<ref>{{cite journal |last1=Prabhakararao |first1=Eedara |last2=Dandapat |first2=Samarendra |title=Myocardial Infarction Severity Stages Classification From ECG Signals Using Attentional Recurrent Neural Network |journal=IEEE Sensors Journal |date=1 August 2020 |volume=20 |issue=15 |pages=8711–8720 |doi=10.1109/JSEN.2020.2984493 |bibcode=2020ISenJ..20.8711P }}</ref> 

* and electrolyte disturbances, such as [[hypokalemia]].<ref>{{cite journal |last1=Carrizales-Sepúlveda |first1=Edgar Francisco |last2=Vera-Pineda |first2=Raymundo |last3=Jiménez-Castillo |first3=Raúl Alberto |last4=Treviño-García |first4=Karla Belén |last5=Ordaz-Farías |first5=Alejandro |title=Toluene toxicity presenting with hypokalemia, profound weakness and U waves in the electrocardiogram |journal=The American Journal of Emergency Medicine |date=November 2019 |volume=37 |issue=11 |pages=2120.e1–2120.e3 |doi=10.1016/j.ajem.2019.158417 |pmid=31477355 }}</ref>

Traditionally, "ECG" usually means a '''12-lead ECG''' taken while lying down as discussed below.
However, other devices can record the electrical activity of the heart such as a [[Holter monitor]] but also some models of [[smartwatch]] are capable of recording an ECG.
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 (mathematics)|magnitude]] of the heart's [[electrical potential]] 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]].<ref name="LHC">{{Cite web |url=https://www.lifehugger.com/doc/120/ecg-100-steps |title=ECG- simplified |last=Aswini Kumar MD |publisher=LifeHugger |url-status=dead |archive-url=https://web.archive.org/web/20171002213510/https://www.lifehugger.com/doc/120/ecg-100-steps |archive-date=2 October 2017 |access-date=11 February 2010}}</ref>

There are three main components to an ECG:{{sfn|Lilly|2016|pp=80}} 
* The [[P wave (electrocardiography)|P wave]], which represents depolarization of the atria.
* The [[QRS complex]], which represents depolarization of the [[Ventricle (heart)|ventricles]].
* The [[T wave]], which represents 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]], spreads throughout the [[Atrium (heart)|atrium]], and passes through the [[atrioventricular node]] down into the [[bundle of His]] and into the [[Purkinje fibers]], spreading down and to the left throughout the [[Ventricle (heart)|ventricle]]s.{{sfn|Lilly|2016|pp=80}} This orderly pattern of depolarization gives rise to the characteristic ECG tracing. To the trained [[clinician]], an ECG conveys a large amount of information about the structure of the heart and the function of its electrical conduction system.<ref>{{Cite book |title=Basic arrhythmias |last=Walraven |first=Gail |date=2011 |publisher=Brady/Pearson |isbn=978-0-13-500238-4 |edition=7th |location=Boston |pages=1–11 |oclc=505018241}}</ref> 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 presence of any damage to the heart's muscle cells or conduction system, the effects of heart drugs, and the function of implanted [[Artificial pacemaker|pacemakers]].<ref>{{Cite book |title=Heart Disease: A Textbook of Cardiovascular Medicine |date=1997 |publisher=Saunders |isbn=0-7216-5666-8 |editor-last=Braunwald |editor-first=Eugene |edition=5th |location=Philadelphia |page=[https://archive.org/details/heartdiseasetext0000unse_k3g7/page/n313 118] |oclc=32970742 |url=https://archive.org/details/heartdiseasetext0000unse_k3g7 |url-access=limited }}</ref>

==Medical uses==
[[File:Normal 12 lead EKG.jpg|thumb|Normal 12-lead ECG]]
[[File:12 lead ECG of a 26 year old male.jpg|thumb|325px|A 12-lead ECG of a 26-year-old male with an incomplete [[right bundle branch block]] (RBBB)]]

The overall goal of performing an ECG is to obtain information about the electrical functioning 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 [[Indication (medicine)|indications]] for performing an ECG include the following:
* Chest pain or suspected [[myocardial infarction]] (heart attack), such as ST elevated myocardial infarction (STEMI)<ref>{{Cite news |url=https://www.ecgmedicaltraining.com/what-is-a-stemi/ |title=What is a STEMI? - ECG Medical Training |date=2015-06-24 |work=ECG Medical Training |access-date=2018-06-24 |language=en-US}}</ref> or non-ST elevated myocardial infarction (NSTEMI)<ref>{{Cite news |url=https://myheart.net/articles/nstemi/ |title=What is NSTEMI? What You NEED to Know |date=2015-04-30 |work=MyHeart |access-date=2018-06-24 |language=en-US}}</ref>
* Symptoms such as [[shortness of breath]], [[cardiac murmur|murmurs]],<ref name="masters">{{Cite book|last1=Masters|first1=Jo |last2=Bowden|first2=Carole |last3=Martin|first3=Carole|last4=Chandler|first4=Sharon |title=Textbook of veterinary medical nursing|date=2003|page=244|publisher=Butterworth-Heinemann|isbn=978-0-7506-5171-4|location=New York|language=Spanish|oclc=53094318}}</ref> [[Syncope (medicine)|fainting]], [[seizure]]s, funny turns, or [[cardiac dysrhythmia|arrhythmia]]s including new onset [[palpitations]] or monitoring of known cardiac arrhythmias
* Medication monitoring (e.g., [[drug-induced QT prolongation]], [[digoxin toxicity]]) and management of overdose (e.g., [[Tricyclic antidepressant overdose|tricyclic overdose]])
* [[Electrolyte imbalance|Electrolyte abnormalities]], such as [[hyperkalemia]]
* [[Perioperative]] monitoring in which any form of [[anesthesia]] is involved (e.g., [[Intraoperative neurophysiological monitoring|monitored anesthesia care]], [[general anesthesia]]). This includes [[Preoperative care|preoperative assessment]] and intraoperative and postoperative monitoring.
* [[Cardiac stress test]]ing
* [[Computed tomography angiography]] (CTA) and [[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]] is inserted through the [[femoral vein]] 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,<ref>{{Cite journal|last1=Drew|first1=B. J.|last2=Califf|first2=R. M.|last3=Funk|first3=M.|last4=Kaufman|first4=E. S.|last5=Krucoff|first5=M. W.|last6=Laks|first6=M. M.|last7=Macfarlane|first7=P. W.|last8=Sommargren|first8=C.|last9=Swiryn|first9=S.|last10=Van Hare|first10=G. F.|date=26 October 2004|title=Practice Standards for Electrocardiographic Monitoring in Hospital Settings|journal=Circulation|volume=110|issue=17|pages=2721–2746|doi=10.1161/01.CIR.0000145144.56673.59|pmid=15505110|s2cid=220573469|doi-access=free}}</ref><ref name=masters/> 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]]s, internal and external [[defibrillator]]s and [[pacemaker]]s, and/or [[biotelemetry]].<ref>{{cite journal |last1=Galli |first1=Alessio |last2=Ambrosini |first2=Francesco |last3=Lombardi |first3=Federico |title=Holter Monitoring and Loop Recorders: From Research to Clinical Practice |journal=Arrhythmia & Electrophysiology Review |date=2016 |volume=5 |issue=2 |pages=136–143 |doi=10.15420/AER.2016.17.2 |pmc=5013174 |pmid=27617093 }}</ref>

===Screening===
[[File:ECGfemaleathlete.jpg|thumb|A patient undergoing an ECG]]
For adults, evidence does not support the use of ECGs among those without symptoms or at low risk of [[cardiovascular disease]] as an effort for prevention.<ref>{{Cite journal |last1=US Preventive Services Task |first1=Force. |last2=Curry |first2=SJ |last3=Krist |first3=AH |last4=Owens |first4=DK |last5=Barry |first5=MJ |last6=Caughey |first6=AB |last7=Davidson |first7=KW |last8=Doubeni |first8=CA |last9=Epling JW |first9=Jr |last10=Kemper |first10=AR |last11=Kubik |first11=M |date=12 June 2018 |title=Screening for Cardiovascular Disease Risk With Electrocardiography: US Preventive Services Task Force Recommendation Statement |journal=JAMA |volume=319 |issue=22 |pages=2308–2314 |doi=10.1001/jama.2018.6848 |pmid=29896632 |last12=Landefeld |first12=CS |last13=Mangione |first13=CM |last14=Silverstein |first14=M |last15=Simon |first15=MA |last16=Tseng |first16=CW |last17=Wong |first17=JB|doi-access=free }}</ref><ref name="Annals2012">{{Cite journal |last=Moyer VA |date=2 October 2012 |title=Screening for coronary heart disease with electrocardiography: U.S. Preventive Services Task Force recommendation statement |journal=Annals of Internal Medicine |volume=157 |issue=7 |pages=512–518 |doi=10.7326/0003-4819-157-7-201210020-00514 |pmid=22847227|doi-access=free }}</ref><ref name="whenyouneedEKGs">{{Citation |last1=Consumer Reports |title=EKGs and exercise stress tests: When you need them for heart disease – and when you don't |date=April 2012 |url=https://consumerhealthchoices.org/wp-content/uploads/2012/04/ChoosingWiselyEKGAAFP2.pdf |work=Choosing Wisely |publisher=[[Consumer Reports]] |access-date=14 August 2012 |last2=American Academy of Family Physicians |last3=ABIM Foundation |author-link=Consumer Reports |author-link2=American Academy of Family Physicians |author-link3=ABIM Foundation |archive-date=20 December 2013 |archive-url=https://web.archive.org/web/20131220223008/http://consumerhealthchoices.org/wp-content/uploads/2012/04/ChoosingWiselyEKGAAFP2.pdf |url-status=dead }}</ref> This is because an ECG may falsely indicate the existence of a problem, leading to [[misdiagnosis]], the recommendation of invasive procedures, and [[overtreatment]]. However, persons employed in certain critical occupations, such as aircraft pilots,<ref name="FAA Medical Standards 2006">{{Cite web |url=https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/aam/ame/guide/media/synopsis.pdf |title=Summary of Medical Standards |year=2006 |publisher=U.S. Federal Aviation Administration |access-date=27 December 2013}}</ref> may be required to have an ECG as part of their routine health evaluations. [[Hypertrophic cardiomyopathy]] screening may also be considered in adolescents as part of a [[Preparticipation physical evaluation|sports physical]] out of concern for [[sudden cardiac death]].<ref>{{Cite journal |last1=Corrado |first1=D. |last2=Basso |first2=C. |last3=Schiavon |first3=M. |last4=Thiene |first4=G. |date=1998-08-06 |title=Screening for hypertrophic cardiomyopathy in young athletes |journal=The New England Journal of Medicine |volume=339 |issue=6 |pages=364–369 |doi=10.1056/NEJM199808063390602 |pmid=9691102 |doi-access=free }}</ref>

==Electrocardiograph machines==
[[File:EKG sensor.jpg|thumb|220x220px|An ECG electrode]]

Electrocardiograms are recorded by machines that consist of a set of electrodes connected to a central unit.<ref>{{cite web|title=Electrocardiograph, ECG|publisher=World Health Organization|url=https://www.who.int/medical_devices/innovation/electrocardiograph.pdf|access-date=1 August 2020}}</ref> Early ECG machines were constructed with [[analog electronics]], where the signal drove a motor to print out the signal onto paper. Today, electrocardiographs use [[analog-to-digital converter]]s to convert the electrical activity of the heart to a [[digital signal]]. 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 [[Smartwatch|smart watches]].<ref>{{Cite news|url=https://www.technologyreview.com/lists/technologies/2019/|title=How we'll invent the future, by Bill Gates |work=MIT Technology Review|access-date=2019-04-01|language=en-US}}</ref> These smaller devices often rely on only two electrodes to deliver a single lead I.<ref>{{Cite news|url=https://techcrunch.com/2012/12/04/mobile-health-moves-forward-fda-approves-alivecors-heart-monitor-for-the-iphone/|title=FDA approves AliveCor heart monitor|work=Techcrunch|access-date=2018-08-25|language=en-US}}</ref> Portable twelve-lead devices powered by batteries are also available.

Recording an ECG is a safe and painless procedure.<ref>{{Cite web|url=https://stanfordhealthcare.org/medical-tests/e/ekg/risks.html |title=EKG Risks|website=Stanford Health Care|access-date=2019-04-01}}</ref> The machines are powered by [[mains power]] but they are designed with several safety features including an earthed (ground) lead.
Other features include:
* [[Defibrillation]] 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]] is similar to defibrillation discharge and requires voltage protection up to 18,000 volts.
* Additionally, circuitry called the [[Driven right leg circuit|right leg driver]] can be used to reduce [[common-mode interference]] (typically the 50 or 60&nbsp;Hz mains power).
* ECG voltages measured across the body are very small. This low voltage necessitates a low [[Noise (electronics)|noise]] circuit, [[instrumentation amplifier]]s, and [[electromagnetic shielding]].
* Simultaneous lead recordings: earlier designs recorded each lead sequentially, but current models record multiple leads simultaneously.

Most modern ECG machines include [[Automated ECG interpretation|automated interpretation]] [[algorithm]]s. This analysis calculates features such as the [[PR interval]], [[QT interval]], [[Correction for heart rate|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.<ref>{{Cite journal|last1=Schläpfer|first1=J|last2=Wellens|first2=HJ|date=2017-08-29|title=Computer-Interpreted Electrocardiograms: Benefits and Limitations.|journal=Journal of the American College of Cardiology|volume=70|issue=9|pages=1183–1192|doi=10.1016/j.jacc.2017.07.723|pmid=28838369|doi-access=free}}</ref>

===Cardiac monitors===
Besides the standard electrocardiograph machine, there are other devices capable of recording ECG signals. Portable devices have existed since the [[Holter monitor]] was introduced 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, developed by Zio (Zio XT), TZ Medical (Trident), [[Philips]] (BioTel) and BardyDx (CAM) among many others.
Implantable devices such as the [[artificial cardiac pacemaker]] and [[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, there are available various [[Arduino]] kits with ECG sensor modules and [[smartwatch]] devices that are capable of recording an ECG signal as well, such as with the 4th generation [[Apple Watch]], [[Samsung Galaxy Watch 4]] and newer devices.

==Electrodes and leads==
[[File:Limb leads.svg|thumb|Proper placement of the limb electrodes. The limb electrodes can be far down on the limbs or close to the hips/shoulders as long as they are placed symmetrically.<ref>{{Cite web |url=https://www.scst.org.uk/resources/RESTING_12.pdf |title=Resting 12-Lead Electrode |last1=Macfarlane |first1=P.W. |last2=Coleman |date=1995 |website=Society for Cardiological Science and Technology |access-date=21 October 2017 |archive-date=19 February 2018 |archive-url=https://web.archive.org/web/20180219172709/https://www.scst.org.uk/resources/RESTING_12.pdf |url-status=dead }}</ref>]]
[[File:ECG Electrode Placement.png|thumb|Placement of American-color-coded ECG electrodes (using a 5-electrode yoke)]]
[[File:ECG limb and chest electrodes placement.png|thumb|Placement of European-color-coded ECG electrodes (using a 10-electrode yoke)]]
[[File:Precordial leads in ECG.png|thumb|Placement of the precordial electrodes]]
Electrodes are the actual conductive pads attached to the body surface.<ref name="EMT_EKG">{{Cite web |url=https://www.emtresource.com/resources/ecg/12-lead-ecg-placement/ |title=12-Lead ECG Placement |date=2019-04-27 |website=www.emtresource.com |access-date=2019-05-24 |archive-date=19 January 2022 |archive-url=https://web.archive.org/web/20220119023617/http://www.emtresource.com/resources/ecg/12-lead-ecg-placement/ |url-status=dead }}</ref> Any pair of electrodes can measure the [[electrical potential difference]] 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.{{fact|date=November 2024}}

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).<ref name="12Leads">{{Cite web |url=https://www.emtresource.com/resources/ecg/12-lead-ecg-placement/ |title=12-Lead ECG Placement |date=2014-04-27 |website=www.emtresource.com |access-date=2019-05-27 |archive-date=19 January 2022 |archive-url=https://web.archive.org/web/20220119023617/http://www.emtresource.com/resources/ecg/12-lead-ecg-placement/ |url-status=dead }}</ref>

=== Electrodes applied to patient's body ===
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]] (vertical), and six ''precordial leads'' or ''chest leads'' that lie on the perpendicular [[transverse plane]] (horizontal).<ref name="EKGInterp">{{Cite web |url=https://www.nurseslearning.com/courses/nrp/nrp1619/Section%205/index.htm |title=EKG Interpretation |website=Nurses Learning Network |access-date=2019-05-27}}</ref>

Leads should be placed in standard positions. Exceptions due to emergency or other issues should be recorded to avoid erroneous analysis.<ref>{{cite journal |last1=Jowett |first1=N I |last2=Turner |first2=A M |last3=Cole |first3=A |last4=Jones |first4=P A |title=Modified electrode placement must be recorded when performing 12-lead electrocardiograms |journal=Postgraduate Medical Journal |date=February 2005 |volume=81 |issue=952 |pages=122–125 |doi=10.1136/pgmj.2004.021204 |pmid=15701746 |pmc=1743200 }}</ref>

The 12 standard ECG leads are listed below.{{dubious|reason=the table does not match this reliable ref|date=February 2025}}<ref>{{Cite journal |last=Drew |first=Barbara J. |last2=Califf |first2=Robert M. |last3=Funk |first3=Marjorie |last4=Kaufman |first4=Elizabeth S. |last5=Krucoff |first5=Mitchell W. |last6=Laks |first6=Michael M. |last7=Macfarlane |first7=Peter W. |last8=Sommargren |first8=Claire |last9=Swiryn |first9=Steven |last10=Van Hare |first10=George F. |date=2004-10-26 |title=Practice Standards for Electrocardiographic Monitoring in Hospital Settings: An American Heart Association Scientific Statement From the Councils on Cardiovascular Nursing, Clinical Cardiology, and Cardiovascular Disease in the Young: Endorsed by the International Society of Computerized Electrocardiology and the American Association of Critical-Care Nurses |url=https://www.ahajournals.org/doi/10.1161/01.CIR.0000145144.56673.59 |journal=Circulation |language=en |volume=110 |issue=17 |pages=2721–2746 |doi=10.1161/01.CIR.0000145144.56673.59 |issn=0009-7322}}</ref> All leads are effectively bipolar, with one positive and one negative electrode; the term "unipolar" is not useful.<ref>{{cite journal |last1=Kligfield |first1=Paul |last2=Gettes |first2=Leonard S. |last3=Bailey |first3=James J. |last4=Childers |first4=Rory |last5=Deal |first5=Barbara J. |last6=Hancock |first6=E. William |last7=van Herpen |first7=Gerard |last8=Kors |first8=Jan A. |last9=Macfarlane |first9=Peter |last10=Mirvis |first10=David M. |last11=Pahlm |first11=Olle |last12=Rautaharju |first12=Pentti |last13=Wagner |first13=Galen S. |title=Recommendations for the Standardization and Interpretation of the Electrocardiogram: Part I: The Electrocardiogram and Its Technology: A Scientific Statement From the American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society Endorsed by the International Society for Computerized Electrocardiology |journal=Circulation |date=13 March 2007 |volume=115 |issue=10 |pages=1306–1324 |doi=10.1161/CIRCULATIONAHA.106.180200 |pmid=17322457 }}</ref>

{| class="wikitable"
|-
! Type
! Electrode name
! Electrode placement
|-
! rowspan=3 | Limb
| I
| On the right arm, avoiding thick [[muscle]].
|-
| II
| In the same location where RA was placed, but on the left arm.
|-
| III
| On the right{{discuss||Left or right?}} leg, lower end of inner aspect of [[Triceps surae muscle|calf muscle]]. (Avoid bony prominences)
|-
! rowspan=3 | Augmented Limb
| aVL
| 
|-
| aVR
|
|-
| aVF
|
|-
! rowspan=6 | Precordial
| V<sub>1</sub>
| In the fourth [[intercostal space]] (between ribs 4 and 5) just to the right of the [[sternum]] (breastbone)
|-
| V<sub>2</sub>
| In the fourth intercostal space (between ribs 4 and 5) just to the left of the sternum.
|-
| V<sub>3</sub>
| Between leads V<sub>2</sub> and V<sub>4</sub>.
|-
| V<sub>4</sub>
| In the fifth intercostal space (between ribs 5 and 6) in the [[mid-clavicular line]].
|-
| V<sub>5</sub>
| Horizontally even with V<sub>4</sub>, in the left [[anterior axillary line]].
|-
| V<sub>6</sub>
| Horizontally even with V<sub>4</sub> and V<sub>5</sub> in the [[midaxillary line|mid-axillary line]].
|}

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]] electrolyte gel and a [[Silver chloride electrode#Biological electrode systems|silver/silver chloride]] conductor.<ref>{{cite book |doi=10.1007/978-1-4613-2323-5_15 |chapter=Advances in Body Surface Potential Mapping (BSPM) Instrumentation |title=Pediatric and Fundamental Electrocardiography |series=Developments in Cardiovascular Medicine |date=1987 |last1=Kavuru |first1=Madhav S. |last2=Vesselle |first2=Hubert |last3=Thomas |first3=Cecil W. |volume=56 |pages=315–327 |isbn=978-1-4612-9428-3 }}</ref>
The gel typically contains [[potassium chloride]] – sometimes [[silver chloride]] as well – to permit [[electron]] conduction from the skin to the wire and to the electrocardiogram.<ref>{{cite journal |last1=Tsukada |first1=Yayoi Tetsuou |last2=Tokita |first2=Miwa |last3=Murata |first3=Hiroshige |last4=Hirasawa |first4=Yasuhiro |last5=Yodogawa |first5=Kenji |last6=Iwasaki |first6=Yu-ki |last7=Asai |first7=Kuniya |last8=Shimizu |first8=Wataru |last9=Kasai |first9=Nahoko |last10=Nakashima |first10=Hiroshi |last11=Tsukada |first11=Shingo |title=Validation of wearable textile electrodes for ECG monitoring |journal=Heart and Vessels |date=July 2019 |volume=34 |issue=7 |pages=1203–1211 |doi=10.1007/s00380-019-01347-8 |pmid=30680493 |pmc=6556171 }}</ref>

The common virtual electrode, known as Wilson's central terminal (V<sub>W</sub>), is produced by averaging the measurements from the electrodes RA, LA, and LL to give an average potential of the body:
:<math>
V_W = \frac{1}{3}(RA+LA+LL)
</math>
In a 12-lead ECG, all leads except the limb leads are assumed to be unipolar (aVR, aVL, aVF, V<sub>1</sub>, V<sub>2</sub>, V<sub>3</sub>, V<sub>4</sub>, V<sub>5</sub>, and V<sub>6</sub>).
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, V<sub>W</sub> 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.<ref>{{cite journal |last1=Gargiulo |first1=GD |title=True unipolar ECG machine for Wilson Central Terminal measurements. |journal=BioMed Research International |date=2015 |volume=2015 |pages=586397 |doi=10.1155/2015/586397 |pmid=26495303 |pmc=460614|doi-access=free }}</ref>  Because voltage is by definition a bipolar measurement between two points, describing an electrocardiographic lead as "unipolar" makes little sense electrically and should be avoided.  The American Heart Association states "All leads are effectively 'bipolar,' and the term 'unipolar' in description of the augmented limb leads and the precordial leads lacks precision."<ref>{{cite journal |last1=Kligfield |first1=P|last2=Gettes |first2=LS |last3=Bailey |first3=JJ |last4=Childers |first4=R |last5=Deal |first5=BJ |last6=Hancock |first6=EW |last7=van Herpen |first7=G |last8=Kors |first8=J |last9=Macfarlane |first9=P |last10=Mirvis |first10=DM |last11=Pahlm |first11=O |last12=Rautaharju |first12=P |last13=Wagner |first13=GS. |title=Recommendations for the standardization and interpretation of the electrocardiogram: Part I: The electrocardiogram and its technology: A scientific statement from the American Heart Association Electrocardiography and Arrhythmia Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society. |journal=Journal of the American College of Cardiology |date=2007 |volume=49 |issue=10|pages=1109–1127 |doi=10.1016/j.jacc.2007.01.024 |pmid=17349896}}</ref>

===Limb leads===
[[File:Limb leads of EKG.png|600px|thumbnail|right|The limb leads and augmented limb leads (Wilson's central terminal is used as the negative pole for the latter in this representation)]]
[[File:EKG leads.png|300px|thumb]]
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.<ref>{{Cite web |url=https://www.nottingham.ac.uk/nursing/practice/resources/cardiology/function/limb_leads.php |title=Limb Leads – ECG Lead Placement – Normal Function of the Heart – Cardiology Teaching Package – Practice Learning – Division of Nursing – The University of Nottingham |publisher=Nottingham.ac.uk |access-date=15 August 2009}}</ref><ref>{{Cite web |url=https://library.med.utah.edu/kw/ecg/ecg_outline/Lesson1/index.html#orientation |title=Lesson 1: The Standard 12 Lead ECG |publisher=Library.med.utah.edu |url-status=dead |archive-url=https://web.archive.org/web/20090322042804/https://library.med.utah.edu/kw/ecg/ecg_outline/Lesson1/index.html |archive-date=22 March 2009 |access-date=15 August 2009}}</ref> The limb leads form the points of what is known as [[Einthoven's triangle]].<ref>{{cite journal |last1=Jin |first1=Benjamin E. |last2=Wulff |first2=Heike |last3=Widdicombe |first3=Jonathan H. |last4=Zheng |first4=Jie |last5=Bers |first5=Donald M. |last6=Puglisi |first6=Jose L. |title=A simple device to illustrate the Einthoven triangle |journal=Advances in Physiology Education |date=December 2012 |volume=36 |issue=4 |pages=319–324 |bibcode=2012BpJ...102..211J |doi=10.1152/advan.00029.2012 |pmc=3776430 |pmid=23209014 }}</ref>
* Lead I is the voltage between the (positive) left arm (LA) electrode and right arm (RA) electrode:
:<math>
I = LA - RA
</math>
* Lead II is the voltage between the (positive) left leg (LL) electrode and the right arm (RA) electrode:
:<math>
II = LL - RA
</math>
* Lead III is the voltage between the (positive) left leg (LL) electrode and the left arm (LA) electrode:
:<math>
III = LL - LA
</math>

===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: <math display="block">
aVR = RA - \frac{1}{2} (LA + LL) = \frac 32 (RA - V_W)
</math>
* 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: <math display="block">
aVL = LA - \frac{1}{2} (RA + LL) = \frac 32 (LA - V_W)
</math>
* 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: <math display="block">
aVF = LL - \frac{1}{2} (RA + LA) = \frac 32 (LL - V_W)
</math>

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.<ref name="Meek 415–418">{{cite journal |last1=Meek |first1=S. |title=ABC of clinical electrocardiography: Introduction. I---Leads, rate, rhythm, and cardiac axis |journal=BMJ |date=16 February 2002 |volume=324 |issue=7334 |pages=415–418 |doi=10.1136/bmj.324.7334.415 |pmid=11850377 |pmc=1122339 }}</ref>

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.<ref>{{cite journal |last1=Madias |first1=JE |title=On recording the unipolar ECG limb leads via the Wilson's vs the Goldberger's terminals: aVR, aVL, and aVF revisited. |journal=Indian Pacing and Electrophysiology Journal |date=2008 |volume=8 |issue=4 |pages=292–297 |pmid=18982138 |pmc=2572021}}</ref>

===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: (V<sub>1</sub>, V<sub>2</sub>, V<sub>3</sub>, V<sub>4</sub>, V<sub>5</sub>, and V<sub>6</sub>). 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.<ref name="McLoughlin2020">{{Cite journal |last=Mc Loughlin |first=MJ |year=2020 |title= Precordial bipolar leads: A new method to study anterior acute myocardial infarction |journal= J Electrocardiol |volume=59 |issue=2 |pages=45–64 |doi=10.1016/j.jelectrocard.2019.12.017|pmid=31986362 |s2cid=210935474 }}</ref>

===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]] (and are denoted with an R (e.g., V<sub>5R</sub>). ''Posterior leads'' (V<sub>7</sub> to V<sub>9</sub>) may be used to demonstrate the presence of a posterior myocardial infarction. The [[Lewis lead]] or S5-lead (requiring an electrode at the right sternal border in the second intercostal space) can be used to better detect atrial activity in relation to that of the ventricles.<ref>{{Cite web|last1=Buttner|first1=Robert|last2=Cadogan|first2=Mike|date=2022-01-29|title=Lewis lead|url=https://litfl.com/lewis-lead-s5-lead/|access-date=2022-02-02|website=Life in the Fast Lane |language=en-US}}</ref>

An ''esophageal lead'' can be inserted to a part of the [[esophagus]] where the distance to the posterior wall of the [[left atrium]] is only approximately 5–6&nbsp;mm (remaining constant in people of different age and weight).<ref name="MeigasKaik2008">{{Cite journal |last1=Meigas |first1=K |last2=Kaik |first2=J |last3=Anier |first3=A |s2cid=42055085 |year=2008 |title=Device and methods for performing transesophageal stimulation at reduced pacing current threshold |journal=Estonian Journal of Engineering |volume=57 |issue=2 |page=154 |doi=10.3176/eng.2008.2.05}}</ref> An esophageal lead avails for a more accurate differentiation between certain cardiac arrhythmias, particularly [[atrial flutter]], [[AV nodal reentrant tachycardia]] and orthodromic [[atrioventricular reentrant tachycardia]].<ref name="PehrsonBlomströ-LUNDQVIST1994" /> It can also evaluate the risk in people with [[Wolff-Parkinson-White syndrome]], as well as terminate [[supraventricular tachycardia]] caused by [[Cardiac arrhythmia#Re-entry|re-entry]].<ref name="PehrsonBlomströ-LUNDQVIST1994">{{Cite journal |last1=Pehrson |first1=Steen M. |last2=Blomströ-Lundqvist |first2=Carina |last3=Ljungströ |first3=Erik |last4=Blomströ |first4=Per |year=1994 |title=Clinical value of transesophageal atrial stimulation and recording in patients with arrhythmia-related symptoms or documented supraventricular tachycardia-correlation to clinical history and invasive studies |journal=Clinical Cardiology |volume=17 |issue=10 |pages=528–534 |doi=10.1002/clc.4960171004 |pmid=8001299|doi-access=free }}</ref>

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, V<sub>1</sub>, and V<sub>6</sub>. Two to four intracardiac leads are added via cardiac catheterization. The word "electrogram" (EGM) without further specification usually means an intracardiac electrogram.<ref>{{cite journal |last1=Zhang |first1=Yongan |last2=Banta |first2=Anton |last3=Fu |first3=Yonggan |last4=John |first4=Mathews M. |last5=Post |first5=Allison |last6=Razavi |first6=Mehdi |last7=Cavallaro |first7=Joseph |last8=Aazhang |first8=Behnaam |last9=Lin |first9=Yingyan |title=RT-RCG: Neural Network and Accelerator Search Towards Effective and Real-time ECG Reconstruction from Intracardiac Electrograms |journal=ACM Journal on Emerging Technologies in Computing Systems |date=30 April 2022 |volume=18 |issue=2 |pages=1–25 |doi=10.1145/3465372 |pmid=35765469 |pmc=9236221 }}</ref>

===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 (V<sub>1</sub> to V<sub>6</sub>).
Additionally, a rhythm strip may be included as a fourth or fifth row.<ref name="Meek 415–418"/>

The timing across the page is continuous and notes 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 V<sub>1</sub>, and then switch to V<sub>4</sub>, and so none of these four tracings of the leads are from the same time period as they are traced in sequence through time.<ref name="Conquering the ECG">{{Cite book |last1=Ashley |first1=Euan A. |url=https://www.ncbi.nlm.nih.gov/books/NBK2214/ |title=Conquering the ECG |last2=Niebauer |first2=Josef |date=2004 |publisher=Remedica |language=en}}</ref>

==={{anchor|Lead groups}}Contiguity of leads===
[[File:Contiguous leads.svg|300px|thumb|Diagram showing the contiguous leads in the same color in the standard 12-lead layout]]
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''.<ref name="Meek 415–418"/>

{| class="wikitable"
|-
! Category
! Leads
! Activity
|-
| Inferior leads
| Leads II, III and aVF
| Look at electrical activity from the vantage point of the [[wikt:inferior|inferior]] surface ([[diaphragmatic surface of heart]])
|-
| Lateral leads
| I, aVL, V<sub>5</sub> and V<sub>6</sub>
| Look at the electrical activity from the vantage point of the [[lateral (anatomy)|lateral]] wall of left [[ventricle (heart)|ventricle]]
|-
| Septal leads
| V<sub>1</sub> and V<sub>2</sub>
| Look at electrical activity from the vantage point of the [[septal]] surface of the heart ([[interventricular septum]])
|-
| Anterior leads
| V<sub>3</sub> and V<sub>4</sub>
| Look at electrical activity from the vantage point of the [[anterior]] wall of the right and left ventricles ([[Sternocostal surface of heart]])
|}

In addition, any two precordial leads next to one another are considered to be contiguous. For example, though V<sub>4</sub> is an anterior lead and V<sub>5</sub> is a lateral lead, they are contiguous because they are next to one another.

==Electrophysiology==
{{main|Cardiac electrophysiology}}

The study of the conduction system of the heart is called [[cardiac electrophysiology]] (EP). An EP study is performed via a right-sided [[cardiac catheterization]]: 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.{{fact|date=November 2024}}

Standard catheter positions for an EP study include "high right atrium" or hRA near the [[sinus node]], a "His" across the septal wall of the tricuspid valve to measure [[bundle of His]], a "coronary sinus" into the [[coronary sinus]], and a "right ventricle" in the apex of the right ventricle.<ref>{{cite journal |last1=Pennoyer |first1=James |last2=Bykhovsky |first2=Michael |last3=Sohinki |first3=Daniel |last4=Mallard |first4=Rachel |last5=Berman |first5=Adam |title=Successful Catheter Ablation of Two Macro-reentrant Atrial Tachycardias in a Patient with Congenitally Corrected Transposition of the Great Arteries: A Case Report |journal=Journal of Innovations in Cardiac Rhythm Management |date=October 2020 |volume=11 |issue=10 |pages=4273–4280 |doi=10.19102/icrm.2020.111005 |pmid=33123416 |pmc=7588239 }}</ref>

==Interpretation==
Interpretation of the ECG is fundamentally about understanding the [[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]].
An ECG does not equate with mechanical pumping activity of the heart; for example, [[pulseless electrical activity]] produces an ECG that should pump blood but no pulses are felt (and constitutes a [[medical emergency]] and [[Cardiopulmonary resuscitation|CPR]] should be performed).
[[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]] or other anatomical imaging modality is useful in assessing the mechanical function of the heart.<ref>{{cite journal |last1=Ewy |first1=Gordon A |title=Defining electromechanical dissociation |journal=Annals of Emergency Medicine |date=September 1984 |volume=13 |issue=9 |pages=830–832 |doi=10.1016/s0196-0644(84)80452-7 |pmid=6476549 }}</ref>

Like all medical tests, what constitutes "normal" is based on [[population studies]]. 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.<ref>{{cite journal |last1=Avram |first1=Robert |last2=Tison |first2=Geoffrey H. |last3=Aschbacher |first3=Kirstin |last4=Kuhar |first4=Peter |last5=Vittinghoff |first5=Eric |last6=Butzner |first6=Michael |last7=Runge |first7=Ryan |last8=Wu |first8=Nancy |last9=Pletcher |first9=Mark J. |last10=Marcus |first10=Gregory M. |last11=Olgin |first11=Jeffrey |title=Real-world heart rate norms in the Health eHeart study |journal=npj Digital Medicine |date=25 June 2019 |volume=2 |issue=1 |page=58 |doi=10.1038/s41746-019-0134-9 |pmid=31304404 |pmc=6592896 }}</ref>

===Theory===
[[File:ECG Vector.svg|230px|thumb|right|QRS is upright in a lead when its axis is aligned with that lead's [[Euclidean vector|vector]]]]
[[File:EKG Complex en.svg|280px|thumb|Schematic representation of a normal ECG]]

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]] and boils down to the four following points:<ref>{{cite journal |last1=Schrepel |first1=Caitlin |last2=Amick |first2=Ashley E. |last3=Sayed |first3=Madeline |last4=Chipman |first4=Anne K. |title=Ischemic ECG Pattern Recognition to Facilitate Interpretation While Task Switching: A Parallel Curriculum |journal=MedEdPORTAL |date=7 September 2021 |volume=17 |page=11182 |doi=10.15766/mep_2374-8265.11182 |pmid=34557588 |pmc=8421424 }}</ref>
* depolarization of the heart ''toward'' the positive electrode produces a positive deflection
* depolarization of the heart ''away'' from the positive electrode produces a negative deflection
* repolarization of the heart ''toward'' 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 V<sub>1</sub> and V<sub>2</sub> because the vectors are perpendicular, and this phenomenon is called isoelectric.

Normal rhythm produces four entities – a [[P wave (electrocardiography)|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 repolarization is typically hidden in the much more prominent QRS complex and normally cannot be seen without additional, specialized 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 (or 40ms per mm):<ref>{{Cite journal |last=Becker |first=Daniel E. |date=2006 |title=Fundamentals of Electrocardiography Interpretation |journal=Anesthesia Progress |volume=53 |issue=2 |pages=53–64 |doi=10.2344/0003-3006(2006)53[53:FOEI]2.0.CO;2 |pmid=16863387 |pmc=1614214 }}</ref>
* A small box is 1&nbsp;mm × 1&nbsp;mm and represents 0.1 mV × 0.04 seconds.
* A large box is 5&nbsp;mm × 5&nbsp;mm and represents 0.5 mV × 0.20 seconds.

The "large" box is represented by a heavier [[Font weight|line weight]] than the small boxes.

[[File:ECG Paper v2.svg|500px|left|Measuring time and voltage with ECG graph paper]]
{{clear}}

The standard printing speed in the United States is 25&nbsp;mm per sec (5 big boxes per second), but in other countries it can be 50&nbsp;mm per sec.
Faster speeds such as 100 and 200&nbsp;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]] require knowing the grid scale.

===Rate and rhythm===
In a normal heart, the heart rate is the rate at which the [[sinoatrial node]] depolarizes since it is the source of depolarization of the heart.
Heart rate, like other [[vital signs]] 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.<ref>{{cite journal |last1=Fleming |first1=Susannah |last2=Thompson |first2=Matthew |last3=Stevens |first3=Richard |last4=Heneghan |first4=Carl |last5=Plüddemann |first5=Annette |last6=Maconochie |first6=Ian |last7=Tarassenko |first7=Lionel |last8=Mant |first8=David |title=Normal ranges of heart rate and respiratory rate in children from birth to 18 years of age: a systematic review of observational studies |journal=The Lancet |date=March 2011 |volume=377 |issue=9770 |pages=1011–1018 |doi=10.1016/S0140-6736(10)62226-X |pmc=3789232 |pmid=21411136 }}</ref>
A heart rate below normal is called "[[bradycardia]]" (<60 in adults) and above normal is called "[[tachycardia]]" (>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]] is 300–600 bpm, whereas the atrial rate can be normal [60–100] or faster [100–150]).{{fact|date=November 2024}}

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.<ref name="Conquering the ECG"/>

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 are the hallmark of [[atrial fibrillation]].
* A "saw tooth" pattern with QRS complexes is the hallmark of [[atrial flutter]].
* A [[sine wave]] pattern is the hallmark of [[ventricular flutter]].
* Absent P waves with wide QRS complexes and a fast heart rate are [[ventricular tachycardia]].

Determination of rate and rhythm is necessary in order to make sense of further interpretation.

===Axis===
[[File:Rapid Axis Vector.svg|thumb|Diagram showing how the polarity of the QRS complex in leads I, II, and III can be used to estimate the heart's electrical axis in the frontal plane.]]
The heart 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.<ref>{{Cite web |title=Sample records for qrs complex relationship |url=https://www.science.gov/topicpages/q/qrs+complex+relationship}}</ref>

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]]).<ref>{{Cite book |title=Chou's electrocardiography in clinical practice : adult and pediatric |url=https://archive.org/details/chouselectrocard00macc |url-access=limited |last1=Surawicz |first1=Borys |last2=Knillans |first2=Timothy |date=2008 |publisher=Saunders/Elsevier |isbn=978-1416037743 |edition=6th |location=Philadelphia, PA |page=[https://archive.org/details/chouselectrocard00macc/page/n277 12]}}</ref>
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).<ref>{{Citation |last1=Kashou |first1=Anthony H. |title=Electrical Right and Left Axis Deviation |date=2022 |url=http://www.ncbi.nlm.nih.gov/books/NBK470532/ |work=StatPearls |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=29262101 |access-date=2022-10-28 |last2=Basit |first2=Hajira |last3=Chhabra |first3=Lovely}}</ref>

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.<ref name="Conquering the ECG"/>

{| class="wikitable"
|-
! Classification
! Angle
! Notes
|-
| Normal
| −30° to 105°
| Normal
|-
| [[Left axis deviation]]
| −30° to −90°
| May indicate [[left ventricular hypertrophy]], [[left anterior fascicular block]], or an old inferior STEMI
|-
| [[Right axis deviation]]
| +105° to +180°
| May indicate [[right ventricular hypertrophy]], [[left posterior fascicular block]], or an old lateral STEMI
|-
| ''Indeterminate axis''
| +180° to −90°
| Rarely seen; considered an 'electrical no-man's land'
|}

The extent of a normal axis can be +90° or 105° depending on the source.

===Amplitudes and intervals===
[[File:ECG principle slow.gif|thumb|Animation of a normal ECG wave]]
[[File:EKG Complex en.svg|210px|thumb|Schematic representation of a normal ECG]]
All of the waves on an ECG tracing and the intervals between them have a predictable time duration, a range of acceptable amplitudes ([[Voltage|voltages]]), and a typical morphology. Any deviation from the normal tracing is potentially pathological and therefore of clinical significance.<ref>{{Cite book |last=Publishing |first=M. D. K. |url=https://books.google.com/books?id=ukQ-CQAAQBAJ&dq=All+of+the+waves+on+an+ECG+tracing+and+the+intervals+between+them+have+a+predictable+time+duration%2C+a+range+of+acceptable+amplitudes+%28voltages%29%2C+and+a+typical+morphology.&pg=PP5 |title=EKGS and ECGS (Speedy Study Guides) |date=2015-04-28 |publisher=Speedy Publishing LLC |isbn=978-1-68185-011-5 |language=en}}</ref>

For ease of measuring the amplitudes and intervals, an ECG is printed on graph paper at a standard scale: each 1&nbsp;mm (one small box on the standard 25mm/s ECG paper) represents 40 milliseconds of time on the x-axis, and 0.1 millivolts on the y-axis.<ref>{{Cite web |title=ECG Study Guide |url=https://www.bannerhealth.com/-/media/files/project/bh/careers/reviewforekgs2016aclsv51.ashx}}</ref>
{| class="wikitable"
|-
!Feature
!Description
!Pathology
!Duration
|-
|[[P wave (electrocardiography)|P wave]]
|The P wave represents depolarization of the atria. Atrial depolarization spreads from the SA node towards the AV node, and from the right [[atrium (anatomy)|atrium]] to the left [[atrium (anatomy)|atrium]].
|The P wave is typically upright in most leads except for aVR; an unusual P wave axis (inverted in other leads) can indicate an [[Ectopic pacemaker|ectopic atrial pacemaker]]. If the P wave is of unusually long duration, it may represent atrial enlargement. Typically a large ''right atrium'' gives a tall, peaked P wave while a large ''left atrium'' gives a two-humped bifid P wave.
|<80 ms
|-
|[[PR interval]]
|The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. This interval reflects the time the electrical impulse takes to travel from the sinus node through the AV node.
|A PR interval shorter than 120 ms suggests that the electrical impulse is bypassing the AV node, as in [[Wolff-Parkinson-White syndrome]]. A PR interval consistently longer than 200 ms diagnoses [[first degree atrioventricular block]]. The PR segment (the portion of the tracing after the P wave and before the QRS complex) is typically completely flat, but may be depressed in [[pericarditis]].
|120 to 200 ms
|-
|[[QRS complex]]
|The QRS complex represents the rapid depolarization of the right and left ventricles. The ventricles have a greater muscle mass proportion compared to the atria, hence the QRS complex usually has a much larger amplitude than the P wave.
|If the QRS complex is wide (longer than 120 ms) it suggests disruption of the heart's conduction system, such as in [[LBBB]], [[RBBB]], or ventricular rhythms such as [[ventricular tachycardia]]. Metabolic issues such as severe [[hyperkalemia]], or [[tricyclic antidepressant overdose]] can also widen the QRS complex. An unusually tall QRS complex may represent [[left ventricular hypertrophy]] while a very low-amplitude QRS complex may represent a [[pericardial effusion]] or [[restrictive cardiomyopathy|infiltrative myocardial disease]].
|80 to 100 ms
|-
|[[J-point]]
|The J-point is the point at which the QRS complex finishes and the ST segment begins.
|The J-point may be elevated as a normal variant. The appearance of a separate ''[[Osborn wave|J wave]]'' or ''Osborn wave'' at the J-point is [[pathognomonic]] of [[hypothermia]] or [[hypercalcemia]].<ref>{{Cite journal |vauthors=Otero J, Lenihan DJ |year=2000 |title=The "normothermic" Osborn wave induced by severe hypercalcemia |journal=Tex Heart Inst J |volume=27 |issue=3 |pages=316–317 |pmc=101092 |pmid=11093425}}</ref>
|
|-
|[[ST segment]]
|The ST segment connects the QRS complex and the T wave; it represents the period when the ventricles are depolarized.
|It is usually isoelectric, but may be depressed or elevated with [[myocardial infarction]] or ischemia. [[ST depression]] can also be caused by [[LVH]] or [[digoxin]]. [[ST elevation]] can also be caused by [[pericarditis]], [[Brugada syndrome]], or can be a normal variant (J-point elevation).
|
|-
|[[T wave]]
|The T wave represents the repolarization of the ventricles. It is generally upright in all leads except aVR and lead V1.
|Inverted T waves can be a sign of myocardial ischemia, [[LVH|left ventricular hypertrophy]], high [[intracranial pressure]], or metabolic abnormalities. Peaked T waves can be a sign of [[hyperkalemia]] or very early [[myocardial infarction]].
|160 ms
|-
|[[Corrected QT interval]] (QTc)
|The QT interval is measured from the beginning of the QRS complex to the end of the T wave. Acceptable ranges vary with heart rate, so it must be ''corrected'' to the QTc by dividing by the square root of the RR interval.
|A prolonged QTc interval is a risk factor for ventricular tachyarrhythmias and sudden death. Long QT can arise as a [[long QT syndrome|genetic syndrome]], or as a side effect of certain medications. An unusually short QTc can be seen in severe hypercalcemia.
|<440 ms
|-
|[[U wave]]
|The U wave is hypothesized to be caused by the repolarization of the interventricular septum. It normally has a low amplitude, and even more often is completely absent.
|A very prominent U wave can be a sign of hypokalemia, hypercalcemia or hyperthyroidism.<ref>{{Cite book |url=https://books.google.com/books?id=8s4TQ6yYHRkC |title=Making Sense of the ECG, Third Edition |last1=Houghton, Andrew R |last2=Gray, David |date=2012 |publisher=Hodder Education |isbn=978-1-4441-6654-5 |page=214}}</ref>
|
|-
|}

===Time-Frequency Analysis in ECG Signal Processing===
In electrocardiogram (ECG) signal processing, Time-Frequency Analysis (TFA) is an important technique used to reveal how the frequency characteristics of ECG signals change over time, especially in non-stationary signals such as arrhythmias or transient cardiac events.

'''Common Methods for Time-Frequency Analysis'''
{| class="wikitable"
|-
!Method
!Advantage
!Disadvantage
!Example
|-
|[[Short-time Fourier transform]]
|Simple to implement, suitable for analyzing steady or near-steady heart rhythms and easy to perform using Fast Fourier Transform (FFT).
|Time and frequency resolution are affected by window length, making it difficult to efficiently capture both short-term and long-term variations simultaneously.
|Monitoring short-term heart rate variability.
|-
|[[Wavelet transform]]
|Provides multi-resolution analysis, making it suitable for processing non-stationary signals.
|Computationally intensive.
|Local feature extraction of P-wave or T-wave and frequency analysis of atrial fibrillation signals.
|-
|[[Hilbert–Huang transform]]
|Suitable for fully non-stationary and nonlinear signals.
Provides instantaneous frequency distribution.
|Susceptible to mode mixing issues.
|Detection of transient heart rate variability.
|-
|-
|}

'''Steps for Time-Frequency Analysis'''

Step1: Preprocessing
*	Signal Denoising: Use wavelet denoising, band-pass filtering (0.5–50 Hz), or Principal Component Analysis (PCA) to remove electromyographic (EMG) noise.
*	Signal Segmentation: Segment the signal based on heartbeat cycles (e.g., R-wave detection).
Step2: Select an Appropriate TFA Method
*	Choose methods such as STFT, WT, or HHT based on the application requirements.
Step3: Compute the Time-Frequency Spectrum
*	Calculate the time-frequency distribution using the selected method to generate a time-frequency representation.
Step4: Feature Extraction
*	Extract power features from specific frequency bands, such as low-frequency (LF: 0.04–0.15 Hz) and high-frequency (HF: 0.15–0.4 Hz) components.
Step5: Pattern Recognition or Diagnosis
*	Apply machine learning or deep learning models to detect or classify cardiac events based on the time-frequency features.


'''Application Scenarios'''

Heart Rate Variability Analysis (HRV):
*	Time-frequency analysis helps to separate sympathetic and parasympathetic nervous system activity.

Atrial Fibrillation Detection:
*	Analyze the time-frequency characteristics of atrial activity.

Ventricular Fibrillation Analysis:
*	Detect time-frequency changes in high-frequency abnormal components.


=== Limb leads and electrical conduction through the heart <span id="How the Electrical Conduction Through the Heart Creates the ECG"></span> ===
[[File:Animation of ECG Limb Leads.gif|thumb|530x530px|Formation of limb waveforms during a pulse]]
The animation shown to the right illustrates how the path of electrical conduction gives rise to the ECG waves in the limb leads. 
What is green zone ?
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.<ref>{{Cite web |url=https://simple-cardio.blogspot.com/2012/12/ecg-ekg-paper.html |title=ECG (EKG) Paper. |last=Cardio-online |date=2012-12-12 |website=Simple Cardiology |access-date=2019-10-20}}</ref><ref>{{Cite web |url=https://www.cvphysiology.com/Arrhythmias/A014 |title=Volume Conductor Principles and ECG Rules of Interpretation |website=CV Physiology |access-date=2019-10-22}}</ref> 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.<ref>{{Citation |last1=Sattar |first1=Yasar |title=Electrocardiogram |date=2022 |url=http://www.ncbi.nlm.nih.gov/books/NBK549803/ |work=StatPearls |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=31747210 |access-date=2022-10-28 |last2=Chhabra |first2=Lovely}}</ref>

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.<ref>{{Citation |last1=Noble |first1=R. Joe |title=Electrocardiography |date=1990 |url=https://www.ncbi.nlm.nih.gov/books/NBK354/ |work=Clinical Methods: The History, Physical, and Laboratory Examinations |editor-last=Walker |editor-first=H. Kenneth |edition=3rd |publisher=Butterworths |isbn=9780409900774 |pmid=21250195 |access-date=2019-10-22 |last2=Hillis |first2=J. Stanley |last3=Rothbaum |first3=Donald A. |editor2-last=Hall |editor2-first=W. Dallas |editor3-last=Hurst |editor3-first=J. Willis}}</ref>

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&nbsp;m/s. 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.<ref>{{cite journal |last1=Scher |first1=Allen M. |last2=Young |first2=Allan C. |last3=Malmgren |first3=Arthur L. |last4=Erickson |first4=Robert V. |title=Activation of the Interventricular Septum |journal=Circulation Research |date=January 1955 |volume=3 |issue=1 |pages=56–64 |doi=10.1161/01.RES.3.1.56 |pmid=13231277 |doi-access=free }}</ref><ref>{{Cite web |url=https://www.cvphysiology.com/Arrhythmias/A016 |title=Ventricular Depolarization and the Mean Electrical Axis |website=CV Physiology |access-date=2019-10-22}}</ref>

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]]. 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.<ref>{{Citation |last1=Kashou |first1=Anthony H. |title=ST Segment |date=2022 |url=http://www.ncbi.nlm.nih.gov/books/NBK459364/ |work=StatPearls |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=29083566 |access-date=2022-10-28 |last2=Basit |first2=Hajira |last3=Malik |first3=Ahmad}}</ref>

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.<ref>{{Cite journal |last=Lukas |first=Anton |date=2016-06-29 |title=Electrophysiology of Myocardial Cells in the Epicardial, Midmyocardial, and Endocardial Layers of the Ventricle |journal=Journal of Cardiovascular Pharmacology and Therapeutics |language=en |doi=10.1177/107424849700200108 |pmid=10684443 |volume=2 |issue=1 |pages=61–72|s2cid=44968291 }}</ref>

===Ischemia and infarction===
{{Main|Electrocardiography in myocardial infarction}}
Ischemia or [[NSTEMI|non-ST elevation myocardial infarctions]] (non-STEMIs) may manifest as [[ST depression]] or inversion of [[T wave]]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]] in ischemic myocardium. This then progresses over a period of minutes to elevations of the [[ST segment]] by at least 1&nbsp;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.<ref name="Alpert-2000">{{Cite journal |vauthors=Alpert JS, Thygesen K, Antman E, Bassand JP |year=2000 |title=Myocardial infarction redefined – a consensus document of The Joint European Society of Cardiology/American College of Cardiology Committee for the redefinition of myocardial infarction |journal=J Am Coll Cardiol |volume=36 |issue=3 |pages=959–969 |doi=10.1016/S0735-1097(00)00804-4 |pmid=10987628|doi-access=free }}</ref>

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 (V<sub>1</sub> and V<sub>2</sub>). 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 V<sub>6</sub>). 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).<ref>{{Citation |last1=Warner |first1=Matthew J. |title=Inferior Myocardial Infarction |date=2022 |url=http://www.ncbi.nlm.nih.gov/books/NBK470572/ |work=StatPearls |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=29262146 |access-date=2022-10-28 |last2=Tivakaran |first2=Vijai S.}}</ref>

===Artifacts===
An ECG tracing is affected by patient motion. Some rhythmic motions (such as shivering or [[tremor]]s) can create the illusion of cardiac arrhythmia.<ref>{{Cite journal |last1=Segura-Sampedro |first1=Juan José |last2=Parra-López |first2=Loreto |last3=Sampedro-Abascal |first3=Consuelo |last4=Muñoz-Rodríguez |first4=Juan Carlos |year=2015 |title=Atrial Flutter EKG can be useful with the proper Electrophysiological Basis |journal=International Journal of Cardiology |volume=179 |pages=68–69 |doi=10.1016/j.ijcard.2014.10.076 |pmid=25464416}}</ref> Artifacts are distorted signals caused by a secondary internal or external sources, such as muscle movement or interference from an electrical device.<ref name="Takla">{{Cite journal |last1=Takla |first1=George |last2=Petre |first2=John H. |last3=Doyle |first3=D John |last4=Horibe |first4=Mayumi |last5=Gopakumaran |first5=Bala |s2cid=10614183 |year=2006 |title=The Problem of Artifacts in Patient Monitor Data During Surgery: A Clinical and Methodological Review |journal=Anesthesia & Analgesia |volume=103 |issue=5 |pages=1196–1204 |doi=10.1213/01.ane.0000247964.47706.5d |pmid=17056954|doi-access=free }}</ref><ref>{{Cite journal |last1=Kligfield |first1=Paul |last2=Gettes |first2=Leonard S. |last3=Bailey |first3=James J. |last4=Childers |first4=Rory |last5=Deal |first5=Barbara J. |last6=Hancock |first6=E. William |last7=van Herpen |first7=Gerard |last8=Kors |first8=Jan A. |last9=Macfarlane |first9=Peter |date=2007-03-13 |title=Recommendations for the standardization and interpretation of the electrocardiogram: part I: The electrocardiogram and its technology: a scientific statement from the American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society: endorsed by the International Society for Computerized Electrocardiology |journal=Circulation |volume=115 |issue=10 |pages=1306–1324 |doi=10.1161/CIRCULATIONAHA.106.180200 |pmid=17322457|doi-access=free }}</ref>

Distortion poses significant challenges to healthcare providers,<ref name="Takla" /> who employ various techniques<ref>{{Cite web |url=https://www.physio-control.com/WorkArea/DownloadAsset.aspx?id=2147489452 |title=Minimizing ECG Artifact |date=2015 |website=Physio-Control |publisher=Physio-Control, Inc., Redmond WA |format=PDF |access-date=21 October 2017}}</ref> and strategies to safely recognize<ref name="Fahim H Jafary">{{Cite journal |last=Jafary |first=Fahim H |year=2007 |title=The "incidental" episode of ventricular fibrillation: A case report |journal=Journal of Medical Case Reports |volume=1 |issue=1 |pages=72 |doi=10.1186/1752-1947-1-72 |pmc=2000884 |pmid=17760955 |doi-access=free }}</ref> these false signals.{{medical citation needed|date=August 2015}} Accurately separating the ECG artifact from the true ECG signal can have a significant impact on patient outcomes and [[Legal liability|legal liabilities]].<ref>{{Cite journal |last1=Mangalmurti |first1=Sandeep |last2=Seabury |first2=Seth A. |last3=Chandra |first3=Amitabh |last4=Lakdawalla |first4=Darius |last5=Oetgen |first5=William J. |last6=Jena |first6=Anupam B. |year=2014 |title=Medical professional liability risk among US cardiologists |journal=American Heart Journal |volume=167 |issue=5 |pages=690–696 |doi=10.1016/j.ahj.2014.02.007 |pmc=4153384 |pmid=24766979}}</ref>{{UNMED|date=August 2015}}

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,<ref>{{Cite journal |last1=Batchvarov |first1=Velislav N. |last2=Malik |first2=Marek |last3=Camm |first3=A. John |date=November 2007 |title=Incorrect electrode cable connection during electrocardiographic recording |journal=Europace |volume=9 |issue=11 |pages=1081–1090 |doi=10.1093/europace/eum198 |pmid=17932025 |doi-access=free }}</ref> and has resulted in improper diagnosis and treatment including unnecessary use of [[thrombolytic]] therapy.<ref>{{Cite journal |last=Chanarin N., Caplin J., Peacock A. |year=1990 |title="Pseudo reinfarction": a consequence of electrocardiogram lead transposition following myocardial infarction |journal=Clinical Cardiology |volume=13 |issue=9 |pages=668–669 |doi=10.1002/clc.4960130916 |pmid=2208827|doi-access=free }}</ref><ref>{{Cite journal |last=Guijarro-Morales A., Gil-Extremera B., Maldonado-Martín A. |year=1991 |title=ECG diagnostic errors due to improper connection of the right arm and leg cables |journal=International Journal of Cardiology |volume=30 |issue=2 |pages=233–235 |doi=10.1016/0167-5273(91)90103-v |pmid=2010249}}</ref>

=== A Method for Interpretation ===
Whitbread, consultant nurse and paramedic, suggests ten rules of the normal ECG, deviation from which is likely to indicate pathology.<ref>{{cite journal |last1=Whitbread |first1=Mark |title=Reading a normal ECG |journal=British Journal of Cardiac Nursing |date=January 2006 |volume=1 |issue=1 |pages=32–33 |doi=10.12968/bjca.2006.1.1.20382 }}</ref> These have been added to, creating the 15 rules for 12-lead (and 15- or 18-lead) interpretation.<ref>{{cite journal |last1=Mallinson |first1=Tom |title=Additional rules for reading an electrocardiogram |journal=Journal of Paramedic Practice |date=2 March 2023 |volume=15 |issue=3 |pages=95–97 |doi=10.12968/jpar.2023.15.3.95 }}</ref>

Rule 1: All waves in aVR are negative.

Rule 2: The ST segment (J point) starts on the isoelectric line (except in V1 & V2 where it may be elevated by not greater than 1&nbsp;mm).

Rule 3: The PR interval should be 0.12–0.2 seconds long.

Rule 4: The QRS complex should not exceed 0.11–0.12 seconds.

Rule 5: The QRS and T waves tend to have the same general direction in the limb leads.

Rule 6: The R wave in the precordial (chest) leads grows from V1 to at least V4 where it may or may not decline again.

Rule 7: The QRS is mainly upright in I and II.

Rule 8: The P wave is upright in I II and V2 to V6.

Rule 9: There is no Q wave or only a small q (<0.04 seconds in width) in I, II and V2 to V6.

Rule 10: The T wave is upright in I II and V2 to V6. The end of the T wave should not drop below the isoelectric baseline.

Rule 11: Does the deepest S wave in V1 plus the tallest R wave in V5 or V6 equal >35&nbsp;mm?

Rule 12: Is there an [[Epsilon wave]]?

Rule 13: Is there an J wave?

Rule 14: Is there a [[Wolff–Parkinson–White syndrome|Delta wave]]?

Rule 15: Are there any patterns representing an [[Myocardial infarction|occlusive myocardial infarction]] (OMI)?

==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]]; 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.<ref>{{cite journal |last1=Montague |first1=Brian T. |last2=Ouellette |first2=Jason R. |last3=Buller |first3=Gregory K. |title=Retrospective Review of the Frequency of ECG Changes in Hyperkalemia |journal=Clinical Journal of the American Society of Nephrology |date=March 2008 |volume=3 |issue=2 |pages=324–330 |doi=10.2215/CJN.04611007 |pmid=18235147 |pmc=2390954 }}</ref>

Rhythm disturbances or arrhythmias:<ref>{{Cite web |date=2018-02-19 |title=Arrhythmia |url=https://www.nhs.uk/conditions/arrhythmia/ |access-date=2022-10-28 |website=nhs.uk |language=en}}</ref>
* [[Atrial fibrillation]] and [[atrial flutter]] 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]]
** [[Atrioventricular reentrant tachycardia]]
** [[Junctional ectopic tachycardia]]
** [[Atrial tachycardia]]
*** [[Ectopic atrial tachycardia]] (unicentric)
*** [[Multifocal atrial tachycardia]]
*** [[Paroxysmal atrial tachycardia]]
** Sinoatrial nodal reentrant tachycardia
* [[Wide complex tachycardia]]
** [[Ventricular flutter]]
** [[Ventricular fibrillation]]
** [[Ventricular tachycardia]] (monomorphic ventricular tachycardia)
** [[Torsades de pointes]] (polymorphic ventricular tachycardia)
* [[Pre-excitation syndrome]]
** [[Lown–Ganong–Levine syndrome]]
** [[Wolff–Parkinson–White syndrome]]
* [[J wave]] (Osborn wave)

[[Heart block]] and conduction problems:
* [[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
** [[Right bundle branch block#Incomplete right bundle branch block|Incomplete right bundle branch block]] (IRBBB)
** Complete [[right bundle branch block]] (RBBB)
* Left bundle
** Incomplete left bundle branch block (ILBBB)
** Complete [[left bundle branch block]] (LBBB)
** [[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 syndrome]]s, genetic and drug-induced
* Right and left atrial abnormality

Electrolytes disturbances and intoxication:
* [[Digoxin poisoning|Digitalis intoxication]]
* Calcium: [[hypocalcemia]] and [[hypercalcemia]]
* Potassium: [[hypokalemia]] and [[hyperkalemia]]
* [[Serotonin Syndrome|Serotonin toxicity]]

Ischemia and infarction:
* [[Wellens' syndrome]] (LAD occlusion)
* de Winter T waves (LAD occlusion) <ref>{{Cite journal |last=de Winter |first=Robert |s2cid=205040240 |date=6 Nov 2008 |title=A New ECG Sign of Proximal LAD Occlusion |journal=NEJM |volume=359 |issue=19 |pages=2071–2073 |doi=10.1056/NEJMc0804737 |pmid=18987380|doi-access=free }}</ref>
* [[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]]
* [[Right ventricular strain]] or S1Q3T3 (can be seen in [[pulmonary embolism]])

Other phenomena:
* [[Cardiac aberrancy]]
** [[Ashman phenomenon]]
* [[Concealed conduction]]
*[[Electrical alternans]]

==History==
[[File:Willem Einthoven ECG.jpg|thumb|An early commercial ECG device (1911)]]
[[File:BASA-532K-1-2-15-Ran Bosilek.jpg|thumb|ECG from 1957]]
* 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.<ref>{{Cite ODNB |doi=10.1093/ref:odnb/37794 |title=Muirhead, Alexander (1848–1920), electrical engineer |last=Birse |first=Ronald M }}</ref>
* 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.<ref>{{Cite journal |last=Rogers |first=Mark C. |date=1969 |title=Historical Annotation: Sir John Scott Burdon-Sanderson (1828-1905) A Pioneer in Electrophysiology |journal=Circulation |volume=40 |issue=1 |pages=1–2 |doi=10.1161/01.CIR.40.1.1 |pmid=4893441 |doi-access=free }}</ref>
* In 1887, [[Augustus Desiré Waller|Augustus Waller]]<ref name="Waller_1887">{{Cite journal |last=Waller AD |year=1887 |title=A demonstration on man of electromotive changes accompanying the heart's beat |journal=J Physiol |volume=8 |issue=5 |pages=229–34 |doi=10.1113/jphysiol.1887.sp000257 |pmc=1485094 |pmid=16991463}}</ref> 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.<ref name=naming/> Einthoven probably chose the initial letter P to follow the example set by [[Descartes]] in [[geometry]].<ref name=naming/> 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,<ref name="naming">{{Cite journal |last=Hurst JW |date=3 November 1998 |title=Naming of the Waves in the ECG, With a Brief Account of Their Genesis |journal=Circulation |volume=98 |issue=18 |pages=1937–42 |doi=10.1161/01.CIR.98.18.1937 |pmid=9799216|doi-access=free }}</ref> 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]].<ref>{{Cite journal |last=Interwoven W |year=1901 |title=Un nouveau galvanometre |journal=Arch Neerl Sc Ex Nat |volume=6 |page=625}}</ref>
* In 1901, Einthoven, working in [[Leiden]], the [[Netherlands]], used the [[string galvanometer]]: the first practical ECG.<ref>{{Cite journal |vauthors=Rivera-Ruiz M, Cajavilca C, Varon J |date=29 September 1927 |title=Einthoven's String Galvanometer: The First Electrocardiograph |journal=Texas Heart Institute Journal  |volume=35 |issue=2 |pages=174–78 |pmc=2435435 |pmid=18612490}}</ref> 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.<ref name="Cooper_1986">{{Cite journal |last=Cooper JK |year=1986 |title=Electrocardiography 100 years ago. Origins, pioneers, and contributors |journal=N Engl J Med |volume=315 |issue=7 |pages=461–64 |doi=10.1056/NEJM198608143150721 |pmid=3526152}}</ref>
* 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.<ref>{{Cite journal |last=Blackford, John M., MD |date=1 May 1927 |title=Electrocardiography: A Short Talk Before the Staff of the Hospital |journal=Clinics of the Virginia Mason Hospital |volume=6 |issue=1 |pages=28–34}}</ref>
* In 1937, [[Taro Takemi]] invented a new portable electrocardiograph machine.<ref>{{Cite news |url=https://www.hsph.harvard.edu/takemi/about-the-program/dr-taro-takemi/ |title=Dr. Taro Takemi |date=2012-08-27 |work=Takemi Program in International Health |access-date=2017-10-21 |language=en-US}}</ref>
* 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.<ref name="nicehist">{{Cite web |year=2009 |title=A (not so) brief history of electrocardiography. |url=https://ecglibrary.com/ecghist.html}}</ref>
* In the late 1940s, [[Rune Elmqvist]] invented an inkjet printer involving 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.<ref>{{cite web
  | title = A (not so) brief history of electrocardiography
  | publisher = ECG Library
  | date = 2006-01-03
  | url = https://www.ecglibrary.com/ecghist.html
  | doi =
  | accessdate = 2021-01-11
  | archive-date = 2012-02-02
  | archive-url = https://web.archive.org/web/20120202003013/http://www.ecglibrary.com/ecghist.html
  | url-status = live
  }}</ref>

=== Etymology ===
The word is derived from the [[Greek language|Greek]] ''electro'', meaning related to electrical activity; ''kardia'', meaning heart; and ''graph'', meaning "to write".<ref>{{Cite web |last= |first= |title=The Interesting History of EKGs |url=https://info.nhanow.com/blog/the-interesting-history-of-ekgs |access-date=2024-01-21 |website=info.nhanow.com |language=en-us}}</ref>

==See also==
* [[Signal-averaged electrocardiogram]]
* [[Electrical conduction system of the heart]]
* [[Electroencephalography]]
* [[Electrogastrogram]]
* [[Electropalatography]]
* [[Electroretinography]]
* [[Emergency medicine]]
* [[Forward problem of electrocardiology]]
* [[Heart rate]]
* [[Heart rate monitor]]
* [[Wireless ambulatory ECG]]

==Notes==
{{notelist}}

==References==
{{Reflist|30em}}

==External links==
{{Commons category|ECG}}
* [https://www.ecgpedia.org/A4/ECGpedia_on_1_A4En.pdf The whole ECG course on 1 A4 paper] from [https://en.ecgpedia.org/wiki/Main_Page ECGpedia], a wiki encyclopedia for [https://en.ecgpedia.org/wiki/ECG_course a course on interpretation of ECG]
* [https://ecg.bidmc.harvard.edu/maven/mavenmain.asp Wave Maven – a large database of practice ECG questions] provided by [[Beth Israel Deaconess Medical Center]]
* [https://www.physionet.org/physiobank/database/#ecg PysioBank – a free scientific database with physiologic signals (here ecg) ]
* [https://ekg.academy EKG Academy – free EKG lectures, drills and quizzes]
* [https://ecg.utah.edu/lesson/1 ECG Learning Center] created by Eccles Health Sciences Library at [[University of Utah]]

{{Emergency medicine}}
{{Cardiac procedures}}
{{Electrodiagnosis}}
{{Cardiovascular physiology}}

{{Authority control}}

[[Category:Cardiac electrophysiology]]
[[Category:Diagnostic cardiology]]
[[Category:Electrodiagnosis]]
[[Category:Electrophysiology]]
[[Category:Mathematics in medicine]]
[[Category:Medical tests]]
[[Category:Dutch inventions]]
[[Category:Science and technology in the Netherlands]]
[[Category:Biology in the Netherlands]]