Electrocardiografía

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.^[20][21][22] 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,^[23] 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 sports physical out of concern for sudden cardiac death.^[24]

Besides the standard electrocardiograph machine, there are other devices that can record 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). The development of the Holter monitor led to the creation of the implantable loop recorder, which performs the same function but is an implantable device with batteries that last for 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 (2018), Samsung Galaxy Watch 4 (2021) and newer devices.

Leads are broken down into three types: limb; augmented limb; and precordial or chest. A 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).^[35]

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.^[36]

The 12 standard ECG leads and electrodes are listed below.^[37] All leads are effectively bipolar, with one positive and one negative electrode; the term "unipolar" is not true, nor useful.^[33]

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/silver chloride conductor.^[38] 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.^[39]

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):

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

WCT is therefore a virtual electrode which sits slightly posteriorly to the heart. It is a useful point, from which the electrical potential of the precordial leads is measured.^[33]

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:^[33]

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

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

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

In a 12-lead ECG, all leads except the limb leads are assumed to be unipolar (aVR, aVL, aVF, V₁, V₂, V₃, V₄, V₅, and V₆). The measurement of a voltage requires two contacts and so, electrically, the unipolar leads are measured from the common lead (negative) and the unipolar lead (positive). This averaging for the common lead and the abstract unipolar lead concept makes for a more challenging understanding and is complicated by sloppy usage of "lead" and "electrode". In fact, instead of being a constant reference, V_W 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.^[40] 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."^[41]

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.^[42][43] The limb leads form the points of what is known as Einthoven's triangle.^[44]

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

${\displaystyle I=LA-RA}$

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

${\displaystyle II=LL-RA}$

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

${\displaystyle III=LL-LA}$

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: $${\displaystyle aVR=RA-{\frac {1}{2}}(LA+LL)={\frac {3}{2}}(RA-V_{W})}$$
• Lead augmented vector left (aVL) has the positive electrode on the left arm. The negative pole is a combination of the right arm electrode and the left leg electrode: $${\displaystyle aVL=LA-{\frac {1}{2}}(RA+LL)={\frac {3}{2}}(LA-V_{W})}$$
• Lead augmented vector foot (aVF) has the positive electrode on the left leg. The negative pole is a combination of the right arm electrode and the left arm electrode: $${\displaystyle aVF=LL-{\frac {1}{2}}(RA+LA)={\frac {3}{2}}(LL-V_{W})}$$

Together with leads I, II, and III, augmented limb leads aVR, aVL, and aVF form the basis of the hexaxial reference system, which is used to calculate the heart's electrical axis in the frontal plane.^[45]

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.^[46]

The precordial leads lie in the transverse (horizontal) plane, perpendicular to the other six leads. The six precordial electrodes act as the positive poles for the six corresponding precordial leads: (V₁, V₂, V₃, V₄, V₅, and V₆). Wilson's central terminal is used as the negative pole. Recently, unipolar precordial leads have been used to create bipolar precordial leads that explore the right to left axis in the horizontal plane.^[47]

En raras ocasiones, se pueden colocar electrodos adicionales para generar otras derivaciones con fines diagnósticos específicos. Las derivaciones precordiales derechas se pueden usar para estudiar mejor la patología del ventrículo derecho o la dextrocardia (y se denotan con una R (p. ej., V5R ₎ . Las derivaciones posteriores (V7 _a V9 ₎ se pueden usar para demostrar la presencia de un infarto de miocardio posterior. La derivación de Lewis o derivación S5 (que requiere un electrodo en el borde esternal derecho en el segundo espacio intercostal) se puede usar para detectar mejor la actividad auricular en relación con la de los ventrículos. ^{[ 48 ]}

Se puede insertar un electrodo esofágico en una parte del esófago donde la distancia a la pared posterior de la aurícula izquierda es de aproximadamente 5-6  mm (permaneciendo constante en personas de diferentes edades y pesos). ^{[ 49 ]} Un electrodo esofágico permite una diferenciación más precisa entre ciertas arritmias cardíacas, en particular el aleteo auricular , la taquicardia por reentrada del nodo AV y la taquicardia por reentrada auriculoventricular ortodrómica . ^{[ 50 ]} También puede evaluar el riesgo en personas con síndrome de Wolff-Parkinson-White , así como terminar la taquicardia supraventricular causada por reentrada . ^{[ 50 ]}

Un electrograma intracardíaco (ICEG) es esencialmente un ECG con algunas derivaciones intracardíacas adicionales (es decir, dentro del corazón). Las derivaciones estándar del ECG (derivaciones externas) son I, II, III, aVL, V1 _y V6 _. Se añaden de dos a cuatro derivaciones intracardíacas mediante cateterismo cardíaco. El término "electrograma" (EGM), sin mayor especificación, generalmente se refiere a un electrograma intracardíaco. ^{[ 51 ]}

Un informe estándar de ECG de 12 derivaciones (un electrocardiograma) muestra un trazado de 2,5 segundos de cada una de las doce derivaciones. Los trazados suelen estar dispuestos en una cuadrícula de cuatro columnas y tres filas. La primera columna corresponde a las derivaciones de las extremidades (I, II y III), la segunda a las derivaciones aumentadas de las extremidades (aVR, aVL y aVF) y las dos últimas a las derivaciones precordiales (V1 _a V6 ₎ . Además, puede incluirse una tira de ritmo como cuarta o quinta fila. ^{[ 45 ]}

La sincronización a lo largo de la página es continua y registra los trazados de los 12 conductores durante el mismo período de tiempo. En otras palabras, si la salida se registrara con agujas sobre papel, cada fila cambiaría de conductor a medida que el papel se desliza bajo la aguja. Por ejemplo, la fila superior registraría primero el conductor I, luego el conductor aVR, luego V1 _y finalmente V4 _, por lo que ninguno de estos cuatro trazados de los conductores corresponde al mismo período de tiempo, ya que se registran secuencialmente a lo largo del tiempo. ^{[ 52 ]}

Cada una de las 12 derivaciones del ECG registra la actividad eléctrica del corazón desde un ángulo diferente y, por lo tanto, se alinea con diferentes áreas anatómicas del corazón. Se dice que dos derivaciones que observan áreas anatómicas vecinas son contiguas . ^{[ 45 ]}

Además, dos derivaciones precordiales adyacentes se consideran contiguas. Por ejemplo, aunque V4 _es una derivación anterior y V5 _es una derivación lateral, son contiguas porque están una al lado de la otra.

La interpretación del ECG es, en última instancia, un proceso de reconocimiento de patrones. Para comprender los patrones encontrados, es útil entender la teoría de lo que representan los ECG. Esta teoría se basa en el electromagnetismo y se resume en los cuatro puntos siguientes: ^{[ 56 ]}

• La despolarización del corazón hacia el electrodo positivo produce una deflexión positiva.
• La despolarización del corazón en dirección opuesta al electrodo positivo produce una deflexión negativa.
• La repolarización del corazón hacia el electrodo positivo produce una deflexión negativa.
• La repolarización del corazón en dirección opuesta al electrodo positivo produce una deflexión positiva.

Así, la dirección general de la despolarización y repolarización produce una deflexión positiva o negativa en el trazado de cada electrodo. Por ejemplo, la despolarización de derecha a izquierda produciría una deflexión positiva en el electrodo I, ya que los dos vectores apuntan en la misma dirección. En cambio, esa misma despolarización produciría una deflexión mínima en V1 _y V2 _, dado que los vectores son perpendiculares; este fenómeno se denomina isoeléctrico.

El ritmo normal produce cuatro entidades: una onda P , un complejo QRS , una onda T y una onda U , cada una de las cuales tiene un patrón bastante único.

• La onda P representa la despolarización auricular.
• El complejo QRS representa la despolarización ventricular.
• La onda T representa la repolarización ventricular.
• La onda U representa la repolarización del músculo papilar.

Los cambios en la estructura del corazón y su entorno (incluida la composición sanguínea) modifican los patrones de estas cuatro entidades.

La onda U no suele observarse y su ausencia generalmente se ignora. La repolarización auricular suele estar oculta en el complejo QRS, mucho más prominente, y normalmente no se puede visualizar sin electrodos adicionales especializados.

Los ECG normalmente se imprimen en una cuadrícula. El eje horizontal representa el tiempo y el eje vertical el voltaje. Los valores estándar en esta cuadrícula se muestran en la imagen adyacente a 25 mm/seg (o 40 ms por mm): ^{[ 57 ]}

• Un recuadro pequeño mide 1  mm × 1  mm y representa 0,1 mV × 0,04 segundos.
• Una caja grande mide 5  mm × 5  mm y representa 0,5 mV × 0,20 segundos.

El recuadro "grande" está representado por un grosor de línea mayor que los recuadros pequeños.

La velocidad de impresión estándar en Estados Unidos es de 25  mm por segundo (5 cuadros grandes por segundo), pero en otros países puede llegar a ser de 50  mm por segundo. Se utilizan velocidades más rápidas, como 100 y 200  mm por segundo, durante los estudios de electrofisiología.

No todos los aspectos de un ECG dependen de registros precisos o de una escala conocida de amplitud o tiempo. Por ejemplo, determinar si el trazado corresponde a un ritmo sinusal solo requiere el reconocimiento y la comparación de características, y no la medición de amplitudes o tiempos (es decir, la escala de las cuadrículas es irrelevante). En cambio, los requerimientos de voltaje de la hipertrofia ventricular izquierda requieren conocer la escala de la cuadrícula.

En un corazón normal, la frecuencia cardíaca es la velocidad a la que se despolariza el nódulo sinoauricular, ya que es la fuente de despolarización del corazón. La frecuencia cardíaca, al igual que otros signos vitales como la presión arterial y la frecuencia respiratoria, cambia con la edad. En adultos, una frecuencia cardíaca normal se sitúa entre 60 y 100 lpm (normocardiocardio), mientras que en niños es mayor. ^{[ 58 ]} Una frecuencia cardíaca inferior a la normal se denomina bradicardia (<60 en adultos) y superior a la normal se denomina taquicardia (>100 en adultos). Una complicación de esto se produce cuando las aurículas y los ventrículos no están sincronizados y la frecuencia cardíaca debe especificarse como auricular o ventricular (por ejemplo, la frecuencia ventricular en la fibrilación ventricular es de 300 a 600 lpm, mientras que la frecuencia auricular puede ser normal [60-100] o más rápida [100-150]).

En corazones en reposo normales, el ritmo fisiológico del corazón es el ritmo sinusal normal (RSN). El ritmo sinusal normal produce el patrón prototípico de onda P, complejo QRS y onda T. Generalmente, cualquier desviación del ritmo sinusal normal se considera una arritmia cardíaca . Por lo tanto, la primera pregunta al interpretar un ECG es si existe o no ritmo sinusal. Un criterio para el ritmo sinusal es que las ondas P y los complejos QRS aparezcan en una relación 1:1, lo que implica que la onda P causa el complejo QRS. ^{[ 52 ]}

Una vez establecido o no el ritmo sinusal, la segunda cuestión es la frecuencia cardíaca. En el caso del ritmo sinusal, esta se refiere a la frecuencia de las ondas P o de los complejos QRS, ya que su relación es 1:1. Si la frecuencia es demasiado rápida, se trata de taquicardia sinusal , y si es demasiado lenta, de bradicardia sinusal .

Si no se trata de un ritmo sinusal, entonces es necesario determinar el ritmo antes de proceder con una interpretación más detallada. Algunas arritmias con hallazgos característicos:

• La ausencia de ondas P con complejos QRS "irregularmente irregulares" es el sello distintivo de la fibrilación auricular .
• Un patrón en forma de "diente de sierra" con complejos QRS es el sello distintivo del aleteo auricular .
• Un patrón de onda sinusoidal es el sello distintivo del aleteo ventricular .
• La ausencia de ondas P, los complejos QRS anchos y la frecuencia cardíaca rápida constituyen una taquicardia ventricular .

La determinación de la velocidad y el ritmo es necesaria para dar sentido a la interpretación posterior.

El corazón tiene varios ejes, pero el más común, con diferencia, es el eje del complejo QRS (las referencias a "el eje" implican el eje QRS). Cada eje puede determinarse computacionalmente para obtener un número que represente grados de desviación respecto a cero, o puede clasificarse en varios tipos. ^{[ 59 ]}

El eje QRS es la dirección general del frente de onda de despolarización ventricular (o vector eléctrico medio) en el plano frontal. A menudo, basta con clasificar el eje en uno de tres tipos: normal, desviado a la izquierda o desviado a la derecha. Los datos poblacionales muestran que un eje QRS normal se encuentra entre −30° y 105°, donde 0° corresponde a la derivación I y los valores positivos son inferiores y los negativos superiores (lo que se comprende mejor gráficamente como el sistema de referencia hexaaxial ). ^{[ 60 ]} Más allá de +105° se encuentra la desviación del eje a la derecha y más allá de −30° la desviación del eje a la izquierda (el tercer cuadrante, de −90° a −180°, es muy raro y constituye un eje indeterminado). Un método abreviado para determinar si el eje QRS es normal es observar si el complejo QRS es mayoritariamente positivo en las derivaciones I y II (o en la derivación I y aVF si +90° es el límite superior de la normalidad). ^{[ 61 ]}

El eje QRS normal generalmente apunta hacia abajo y a la izquierda , siguiendo la orientación anatómica del corazón dentro del tórax. Un eje anormal sugiere un cambio en la forma y orientación física del corazón o un defecto en su sistema de conducción que provoca que los ventrículos se despolaricen de forma anómala. ^{[ 52 ]}

La extensión de un eje normal puede ser de +90° o 105° dependiendo de la fuente.

Todas las ondas en un trazado de ECG y los intervalos entre ellas tienen una duración predecible, un rango de amplitudes aceptables ( voltajes ) y una morfología típica. Cualquier desviación del trazado normal es potencialmente patológica y, por lo tanto, de importancia clínica. ^{[ 62 ]}

Para facilitar la medición de las amplitudes y los intervalos, un ECG se imprime en papel cuadriculado a una escala estándar: cada 1  mm (un pequeño recuadro en el papel de ECG estándar de 25 mm/s) representa 40 milisegundos de tiempo en el eje x y 0,1 milivoltios en el eje y. ^{[ 63 ]}

En el procesamiento de señales de electrocardiograma (ECG), el análisis tiempo-frecuencia (TFA) es una técnica importante que se utiliza para revelar cómo cambian las características de frecuencia de las señales de ECG con el tiempo, especialmente en señales no estacionarias como arritmias o eventos cardíacos transitorios.

Métodos comunes

Steps

Step 1: 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).

Step 2: Select an Appropriate TFA Method

• Choose methods such as STFT, WT, or HHT based on the application requirements.

Step 3: Compute the Time-Frequency Spectrum

• Calculate the time-frequency distribution using the selected method to generate a time-frequency representation.

Step 4: 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.

Step 5: 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.

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 the 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.^[66][67] 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.^[68]

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.^[69]

Frame 13 depicts an interesting phenomenon in an over-simplified fashion. It depicts the depolarization as it starts to travel down the interventricular septum, through the bundle of His and bundle branches. After the Bundle of His, the conduction system splits into the left bundle branch and the right bundle branch. Both branches conduct action potentials at about 1 m/s. 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.^[70][71]

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.^[72]

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.^[73]

Ischemia or non-ST elevation myocardial infarctions (non-STEMIs) may manifest as ST depression or inversion of T waves. It may also affect the high frequency band of the QRS.

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 mm. Over a period of hours, a pathologic 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.^[74]

The coronary artery that has been occluded can be identified in an STEMI based on the location of ST elevation. The left anterior descending (LAD) artery supplies the anterior wall of the heart, and therefore causes ST elevations in anterior leads (V₁ and V₂). The LCx supplies the lateral aspect of the heart and therefore causes ST elevations in lateral leads (I, aVL and V₆). The right coronary artery (RCA) usually supplies the inferior aspect of the heart, and therefore causes ST elevations in inferior leads (II, III and aVF).^[75]

An ECG tracing is affected by patient motion. Some rhythmic motions (such as shivering or tremors) can create the illusion of cardiac arrhythmia.^[76] Artifacts are distorted signals caused by a secondary internal or external sources, such as muscle movement or interference from an electrical device.^[77][78]

Distortion poses significant challenges to healthcare providers,^[77] who employ various techniques^[79] and strategies to safely recognize^[80] these false signals. Accurately separating the ECG artifact from the true ECG signal can have a significant impact on patient outcomes and legal liabilities.^[81]

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,^[82] and has resulted in improper diagnosis and treatment including unnecessary use of thrombolytic therapy.^[83][84]

Whitbread, consultant nurse and paramedic, suggests ten rules of the normal ECG, deviation from which is likely to indicate pathology.^[85] These have been added to, creating the 15 rules for 12-lead (and 15- or 18-lead) interpretation.^[86]

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 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 mm?

Rule 12: Is there an Epsilon wave?

Rule 13: Is there an J wave?

Rule 14: Is there a Delta wave?

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

La palabra deriva del griego electro , que significa relacionado con la actividad eléctrica; kardia , que significa corazón; y graph , que significa "escribir". ^{[ 101 ]}