Editing Electrocardiography (section)

==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 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'', which is the average of numerous leads. All clinical ECGs use '''Wilson's central terminal''' ('''WCT''') as the virtual electrode from which the precordial leads are measured, whose potential is defined as the average potential measured by the three standard limb leads.<ref name=AHA_ECG_pt1>{{Cite journal |last1=Klingfield |first1=Paul |last2=Gettes |first2=Leonard S. |last3=Bailey |first3=James J. |last4=Childers |first4=Rory |last5=Deal |first5=Barbara J. |last6=Hancock |first6=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 |journal=[[Circulation (journal)|Circulation]] |volume=115 |issue=10 |date=13 March 2007 |department=AHA/ACC/HRS Scientific Statements |publisher=[[American Heart Association]] |pages=1306–1324 |doi=10.1161/CIRCULATIONAHA.106.180200|pmid=17322457 }}</ref>

Commonly, 10 electrodes attached to the body are used to form 12 ECG leads, with each lead measuring a specific electrical potential difference.<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>

=== 12-Lead ECG ===
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>

Electrodes should be placed in standard positions, with 'left' or 'right' referring to [[anatomical]] directions, being the patient's left or right. 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 and electrodes are listed below.<ref>{{Cite journal |last1=Drew |first1=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 |pmid=15505110 |issn=0009-7322}}</ref> All leads are effectively bipolar, with one positive and one negative electrode; the term "unipolar" is not true, nor useful.<ref name="AHA_ECG_pt1" />

{| class="wikitable collapsible"
|+ ECG Electrodes and positioning
|-
! Type
! Name ([[American Heart Association|AHA]])
! Color ([[American Heart Association|AHA]])
! Placement
! Name ([[International Electrotechnical Commission|IEC]])
! Color ([[International Electrotechnical Commission|IEC]])
|-
! scope=rowgroup rowspan=4 | Limb
| RA (Right Arm)
| White
| On the right arm, below the shoulders, avoiding thick [[muscle]].
| R (Right)
| Red
|-
| LA (Left Arm)
| Black
| Symmetrical to the placement of the RA.
| L (Left)
| Yellow
|-
| RL (Right Leg)
| Green
| On the right leg, below the hips.
| N (Neutral)
| Black
|-
| LL (Left Leg)
| Red
| Symmetrical to the placement of the RL.
| F (Foot)
| Green
|-
! scope=rowgroup rowspan=6 | Precordial
| V1
| Brown & Red
| Fourth [[intercostal space]] on the right [[sternal]] border.
| C1
| White & Red
|-
| V2
| Brown & Yellow
| Fourth intercostal space on the left sternal border (symmetrical to V1).
| C2
| White & Yellow
|-
| V3
| Brown & Green
| Halfway between electrodes V2 and V4 (in a straight line).
| C3
| White & Green
|-
| V4
| Brown & Blue
| Fifth intercostal space on the [[midclavicular line]].
| C4
| White & Brown
|-
| V5
| Brown & Orange
| Left [[anterior axillary line]] on the same horizontal plane as V4. If the anterior axillary line is ambiguous, place halfway between V4 and V6.
| C5
| White & Black
|-
| V6
| Brown & Purple
| Left [[midaxillary line]] on the same horizontal plane as V4.
| C6
| White & Purple.
|}

{| class="wikitable collapsible"
|+ ECG Leads and Views
|-
! Type
! Name
! Lead view
|-
! rowspan=3 | Limb
| I
| From the RA to the LA. Along the frontal and horizontal planes at 0° (directly to the left).
|-
| II
| From the RA to the LL. Along the frontal plane at 60° clockwise from I.
|-
| III
| From the LA to the LL. Along the frontal plane at 120° clockwise from I.
|-
! rowspan=3 | Augmented Limb
| aVL
| From WCT to the LA. Along the frontal plane at -30° (which is 330° clockwise from I).
|-
| aVR
| From WCT to the RA. Along the frontal plane at -150° (which is 210° from I).
|-
| aVF
| From WCT to the LL. Along the front plane at 90°.
|-
! 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>
| Along the same horizontal line as V<sub>4</sub>, in the left [[anterior axillary line]].
|-
| V<sub>6</sub>
| Along the same horizontal line as 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>

===Virtual Electrode===

The virtual electrode is used to obtain useful measurements from the precordial leads, and also allows the creation of the augmented limb leads.

The virtual electrode is known as '''Wilson's Central Terminal''' ('''WCT'''). For the precordial leads, WCT is formed by averaging the three standard limb leads (I, II, and III):
:<math>
V_W = \frac{1}{3}(RA+LA+LL)
</math>

WCT is therefore a virtual electrode which sits slightly [[posterior (anatomy)|posteriorly]] to the heart. It is a useful point, from which the electrical potential of the precordial leads is measured.<ref name="AHA_ECG_pt1" />

WCT used to be used as a reference for the virtual limb leads, however use in this way produced leads with very small amplitudes. Goldberger's modification is now used to produce each augmented limb lead, aVF, aVR, and aVL, which produces 50% larger amplitude leads than the standard WCT. Goldberger's WCT is formed according to the following:<ref name="AHA_ECG_pt1" />
:<math>
aVR = \frac{LA + LL}{2}
</math>
:<math>
aVL = \frac{RA + LL}{2}
</math>
:<math>
aVF = \frac{RA + LA}{2}
</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.