Graphical Abstract
Key words: anatomy, cCabrera sequence/format, electrocardiography
The human electrocardiogram (ECG) was first recorded with a mercury capillary electrometer by Augustus Waller in May 1887. Willem Einthoven improved ECG recordings with a string galvanometer he invented and published first recording of the human ECG with his new instrument in 1902.1 The mechanism underlying the P-QRS-T–wave complex he recorded was further elucidated in his 1908 publication2 referring to anatomical and physiological insights, advocated by Sunao Tawara (Tahara) in 1906.3 Modern 12-lead electrocardiography was subsequently established by Frank N. Wilson in 1934.4 Currently, it serves as one of the fundamental functional diagnostic tests in clinical cardiology. Twelve-lead ECG reflects precise physiological features of the heart, namely electrical propagation. Therefore, understanding of the three-dimensional (3D) local propagation of excitation,5 as well as the summation and cancellation of electrical forces taking place simultaneously in multiple locations within the heart, is essential.6
Take-Home Messages
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A comprehensive understanding of the 3D relationships between the cardiac structures and each electrode is fundamental to reading 12-lead ECGs.
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Applying the concept of anatomical nomenclature is essential for logical interpretation of the 12-lead ECG.
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The Cabrera sequence/format applied is useful for intuitive, anatomical comprehension of the findings in the limb leads.
These physiologic features must be interpreted in the context of the cardiac and thoracic anatomy of a given individual, which can be variable. The major anatomical factors necessary for precise interpretation of the 12-lead ECG include the location of the heart, shape of the heart, anatomy of the conduction system, and tissues/organs/fluid/air around the heart, including the lungs and especially pericardial effusion.7,8 Thus, appreciation of individual 3D anatomical relationships between the heart and electrodes is fundamental. However, resources illustrating appropriate 3D relationships between nondistorted real hearts and each electrode have been limited.9,10 In addition, mixed use of anatomical and Valentine nomenclatures in the field of cardiology has contributed to confusion in the understanding of the 12-lead ECG.11, 12, 13 Therefore, we herein present a comprehensive anatomical review of the fundamental anatomical aspects essential for understanding the 12-lead ECG, based on pressure-perfused, fixed hearts from the UCLA Wallace A. McAlpine14 and Amara-Yad Project Collections (Figures 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12).
Figure 1.
Relationship Between the Limb Leads and the Heart
The standard augmented unipolar (aVR, aVL, aVF) and bipolar (I, II, III) limb electrodes are overlaid on the intact (A) and sectioned (B) heart viewed from the anterior direction. The limb leads are all about frontal plane anatomy, providing vertical (superior or inferior) and horizontal (right or left) spatial information. They cannot provide spatial information about the sagittal (anterior or posterior) direction. Thus, to interpret the limb leads, structures of interest should always be viewed from the anterior direction and need to be projected and described on these frontal plane images. For example, the noncoronary aortic sinus (N) can be described as the structure located right and inferior to the left coronary aortic sinus (L). Then, each P-wave and QRS-wave morphology of the structure of interest should be considered in spatial association with each electrode within the context of each atrial or ventricular mass (Figure 9), respectively.
Figure 2.
Correlative Changes in the Limb Leads
Changes observed in each limb lead should not be interpreted individually without a correlative point of view. All changes found in the standard 6 limb leads should follow a common correlative rule reflecting structural anatomy. For example, when the focus of a premature ventricular contraction shifts from the posterolateral free wall (yellow star) to the medial free wall (red star) of the right ventricular outflow tract (A), an increase in Q-wave amplitude in I is observed. This is combined with a decrease in R-wave amplitude in II, an increase in R-wave amplitude in III, a decrease in Q-wave amplitude in aVR, and an increase in Q-wave amplitude in aVL (B and C), reflecting the right-to-left move of the arrhythmogenic focus and resultant change in propagation vector (yellow and red dotted arrows). In general, therefore, a leftward shift is combined with a tendency of decrease in II/III R-wave ratio (increase in II/III Q-wave ratio), decrease in aVR/aVL Q-wave ratio (increase in aVR/aVL R-wave ratio), and decrease in I and aVL R-wave amplitude (increase in I and aVL Q-wave amplitude), and vice versa. Correlative changes in the limb leads by the spatial shift/difference of interest could become more anatomically intuitive when using Cabrera sequence/format (Figures 10 and 11).
Figure 3.
Relationship Between the Precordial Leads and the Heart
The standard precordial electrodes are overlaid on the heart viewed from the anterior (A) and superior (B and C) directions. Generally, V1 and V2 are located anterior to the right atrial appendage and right ventricular outflow tract, respectively. V3 is located in the middle part of the anterior interventricular groove. V4 is located left anteriorly to the cardiac apex.15 Compared with V1 to V4, the distance from V5 and V6 to the heart is greater because of the intervening left lung (C). Primarily, the precordial leads inform horizontal (axial) plane anatomy, mainly providing horizontal (right or left) and sagittal (anterior or posterior) spatial information (B and C). Thus, to interpret the precordial leads, structures of interest should be viewed from the superior direction and need to be projected and described on these horizontal plane images. For example, the right coronary aortic sinus (R) can be described as the structure located right anterior to the left coronary aortic sinus (L) and left anterior to the noncoronary aortic sinus (N). Then, each P-wave and QRS-wave morphology of the structure of interest should be considered in spatial association with each electrode within the context of each atrial or ventricular mass (Figure 9), respectively.
Figure 4.
Height Difference of Each Precordial Lead Demonstrated With Correlative Anatomy
Although horizontal, two-dimensional images are frequently used in the literature to describe the precordial leads (Figure 3), V1 through V6 are not located on a single flat horizontal plane. The horizontal planes involving the V1/V2 (A), V3, V4-V6 (B) electrodes are different. Each plane is located at the superior, middle, and inferior levels of the ventricular mass, respectively. Acknowledgment of this height difference is required to interpret the precordial leads. In the superior plane (A), the right ventricular outflow tract is located anterior to the left ventricular outflow tract and left anterior to the aortic root, posterior to V2. The superolateral papillary muscle (orange star) and crest of the ventricular septum (green star) supporting the right coronary aortic sinus (R) are located on the superior plane (A). The inferomedial papillary muscle (blue star) and inferoseptal process of the left ventricle (white star) between the left ventricle and the right atrium are located on the inferior plane (B). N = noncoronary aortic sinus.
Figure 5.
Importance of the Height Difference of Each Precordial Lead
Although not as intuitive as the limb inferior leads, the precordial leads can provide spatial information about vertical (superior or inferior) direction to some extent because of their height difference. For example, virtual progressive dissection demonstrates the isolated left ventricular myocardium (B) by removing other structures (including the aortic root, right ventricle, atria, coronary vessels, and thorax) from (A) and viewing it from the superior direction (B). This kind of traditional two-dimensional, horizontal plane image cannot differentiate the height of each precordial lead from the height of each structure of interest. However, the septal and aortic margins of the left ventricular summit (green star) adjacent to the right coronary aortic sinus (R) are actually located on the superior plane at the level of V1 and V2, and the inferoseptal process of the left ventricle (white star) is located on the inferior plane at the level of V4 through V6 (Figure 4).16 Thus, in addition to the inferior axis, ventricular arrhythmias originating from the septal and aortic margins of the left ventricular summit17 generally demonstrate S-wave predominance in V1 and V2 electrodes (C), also referred to as the abrupt V3 transition, as the focus gets closer to the V1 and V2 in the plane. On the other hand, ventricular arrhythmias originating from the inferoseptal process18 generally demonstrate R-wave concordance throughout V1-V6 (D) with the superior axis, as the region is distant from all the V1-V6 leads. Thus, ventricular arrhythmias originating from the left ventricular base, namely the left ventricular ostium,19 should not necessarily show R-wave concordance in the precordial leads. L = left coronary aortic sinus; N = noncoronary aortic sinus.
Figure 6.
Variable Relationship Between the Fixed Precordial Leads and the Heart
There are significant individual variations in the size and orientation of the heart and shape of the thoracic cage, all of which affect the three-dimensional relationships between the electrodes and cardiac structures of interest and the extent of extracardiac tissues/organs/fluid/air, such as the lungs. One of the major limitations of the precordial leads is this variation, as the electrodes are always placed in a fixed location by referring to the 4th and 5th intercostal spaces. As a result, the relative location of the precordial electrodes shown with (upper panels) or without (bottom panels) ribs can be inferior (A), level (B), or superior (C) to the ventricular mass.20 Note that the number of each side rib attaching to the sternum can also be variable; generally, it is 7 (A and B) and rarely, it is 6 or 8 (C). This spatial variation of the precordial leads against the heart can affect the electrocardiographic findings in the precordial leads. For example, when the same monofocal, premature ventricular contraction (asterisks) is recorded from precordial leads placed in different locations (2 intercostal spaces above ∼ 1 intercostal space below), R-wave progression with transition zone shift from V6 toward V4 can be observed as the leads move downward, rendering each QRS-wave appearance different (D). The spatial variation is also relevant particularly in women, where the precordial leads could be misplaced because of the breast.21 Legend for circles: red = V1; yellow = V2; green = V3; brown = V4; black = V5; purple = V6.
Figure 7.
Importance of Using Anatomical Nomenclature to Understand Electrocardiograms
In anatomical nomenclature (attitudinal nomenclature14), the directions cranial, caudal, right, left, ventral, and dorsal are referred to as superior, inferior, right, left, anterior, and posterior, respectively (A). In the context of the entire heart, each anterior and inferior surface corresponds to the sternocostal (with the right ventricular free wall adjacent) and diaphragmatic surfaces of the heart, respectively (A and C). As the concept of 12-lead electrocardiography is based on frontal, horizontal, and sagittal planes (Figures 1, 2, 3, and 4), understanding of the structural anatomy based on this anatomical nomenclature is fundamental.11, 12, 13 However, in the field of cardiology, Valentine nomenclature (nonattitudinal nomenclature14) has been widely used (B). This is based on the point of view of pathologists, who hold the recovered hearts in their hands. The heart is shown as if it stands on its apex (B). The diaphragmatic surface facing the hands is referred to as the “posterior” aspect, the superior surface as the “anterior” aspect, and “right” and “left” as the right and left cardiac chambers, respectively (A and B). Although this terminology is somewhat convenient and common, we should apply anatomical nomenclature for logical interpretation of the 12-lead electrocardiogram. For example, the right ventricle and left atrium should be recognized as the anterior and posterior chambers, respectively. Confusingly, in echocardiography, both “inferior” (anatomical nomenclature) and “anterior” (Valentine nomenclature) are used in a mixed fashion to describe left ventricular segments facing each other (the insertion represents the sectional plane along the dotted line in C).22 To understand 12-lead electrocardiography, this “anterior” segment of the left ventricle should be recognized as being superior to the inferior segment (C).
Figure 8.
Difficulties Caused by Using the Valentine Nomenclature
When the traditional terms “anterior/posterior fascicle” of the left bundle branch, based on Valentine nomenclature, are used, it is difficult to explain superior-axis deviation in the setting of “anterior fascicular block.” However, understanding them as the superior/inferior fascicle, based on anatomical nomenclature (A), the superior fascicular block (B) is intuitively understood as superior-axis deviation, as represented by the limb leads. Another example is that when the left ventricular papillary muscles are described as anterolateral and posteromedial, based on Valentine nomenclature, it is difficult to describe the QRS morphology of a ventricular arrhythmia originating from each papillary muscle. However, when the papillary muscles are referred to as superolateral (orange star) and inferomedial, (blue star) based on anatomical nomenclature (C), each inferomedial and superolateral axis of each ventricular arrhythmia can be intuitively understood, as represented in the limb leads. A ventricular arrhythmia originating from the inferomedial papillary muscle also exhibits a Q-wave in V3-V6, as it is located on the inferior plane (Figure 4), compared with an arrhythmia originating from the superolateral papillary muscle located on the superior plane, exhibiting an RS-wave in V3-V6. Similarly, the right and left bundle branches, based on Valentine nomenclature, actually demonstrate anterior and posterior alignment in anatomical nomenclature (D). Therefore, a block of each bundle should show anterior-axis and posterior-axis deviation, respectively, as represented by the anterior chest lead (V1). N = noncoronary aortic sinus; R = right coronary aortic sinus.
Figure 9.
Importance of Considering Each Structure Within the Context of Each Atrial or Ventricular Mass
Even if the structures of interest are described based on anatomical nomenclature, it is important to think of each structure either in the atria or ventricles in the context of each atrial or ventricular mass, respectively. This is because relative location of the structure within the atrial or ventricular mass matters to interpret P-wave (arrowheads in C) or QRS-wave morphology, respectively. For example, the P-wave of pacemaps of the right inferior pulmonary vein (RIPV; red star) and left inferior pulmonary vein (LIPV; blue star) can show inferior axis (C). This is because the pulmonary vein component extends at the roof/dome of the left atrium, located superior within the context of the atrial mass.23 P-wave amplitudes of the pacemaps of the inferior pulmonary veins in II (red and blue stars) are lower than those of the right superior pulmonary vein (RSPV; yellow star) and left superior pulmonary vein (LSPV; pink star), respectively, as superior pulmonary veins are superior to inferior pulmonary veins (A). Additionally, P-wave duration is wider in the left pulmonary veins, as the right pulmonary veins are adjacent to the atrial septum, thus achieving atrial synchrony. Furthermore, the positive P-wave in V1 exhibits lower amplitude in the RSPV compared with the one in the RIPV, reflecting the posteroinferior location of the latter relative to the former (B). These observations exhibit the excellent spatial resolution of the 12-lead electrocardiogram, as well as the importance of a detailed understanding of the three-dimensional structural anatomy of the living heart in the context of anatomical nomenclature. Pacemaps of the left atrial appendage (LAA; green star) and right atrial appendage (RAA; white star) are also demonstrated, showing typical negative deflection of the P-wave in aVL and V1, respectively, reflecting its spatial relationship with these leads relative to the atrial mass (A and B).
Figure 10.
Concept of the Cabrera Sequence/Format
Compared with precordial leads, it is peculiar that limb leads are not aligned in an anatomically consecutive fashion (A). By creating a –aVR lead using a vertically flipped image of aVR, it is feasible to align the limb leads in an anatomically consecutive fashion (B). This is referred to as the Cabrera sequence/format,24, 25, 26, 27 also referred to as the panoramic display. Even though the use of the Cabrera sequence was recommended as an alternative presentation standard of limb leads,25 it has not been routinely used in worldwide clinical practice except in Sweden, where it has been adopted as national standard since 1979.26 For example, when the QRS-wave morphology of a premature ventricular contraction (Figures 2 and 11) originating from the medial free wall of the right ventricular outflow tract (red star) is visualized in this Cabrera sequence/format, alongside anatomical images viewed from the anterior direction, the propagation vector (red dotted arrows) becomes immediately apparent. R = right coronary aortic sinus.
Figure 11.
Feasibility and Utility of the Cabrera Sequence/Format
As shown in Figure 10, the Cabrera sequence/format with the heart viewed from the anterior direction (A to D) can be applied to understand the propagation vector (yellow and red dotted arrows) of ventricular arrhythmias (A and B), propagation vector of the normal sinus P-wave (C), and extent of myocardial damage (white dotted curve) and culprit coronary artery region as revealed by the distribution of ST-segment elevation (asterisks) (D). The concept of transition zone (R/S-wave ratio becomes 1.0) can be applied similarly to the precordial leads, indicating perpendicular direction (white dotted lines) to the main propagation vector (A and B) in the context of ventricular arrhythmia. Compared with the detailed descriptions in Figure 2, the limb lead interpretation is visually organized and anatomically intuitive. L = left coronary aortic sinus; N = noncoronary aortic sinus; R = right coronary aortic sinus.
Figure 12.
Resources to Facilitate 3-Dimensional Understanding
The 3-dimensional relationship between the precordial leads and the heart is illustrated by using a digital stereolithography (STL) polygon model (Supplemental STL File) obtained from the clinical cardiac computed tomography datasets and demonstrated with three-dimensional prints (thermoplastic polyurethane, 30% scale). The thoracic cage and each lung are detachable. The STL file of this model can be opened and printed using commercially available software and printers, allowing for interactive exploration and enhanced understanding of the complex but fundamental three-dimensional electrocardiographic anatomy.
Funding Support and Author Disclosures
This work was made possible by support from NIH grant P01 HL164311 (to K.S.) and the UCLA Amara-Yad Project (https://www.uclahealth.org/medical-services/heart/arrhythmia/about-us/amara-yad-project). The authors have reported that they have no relationships relevant to the contents of this paper to disclose.
Acknowledgments
The authors are thankful to the individuals who donated their bodies for the advancement of education and research. They are grateful to the OneLegacy Foundation and the National Institutes of Health (P01 HL164311 to Dr Shivkumar), which formed the basis for obtaining donor hearts for research and for funding this effort, respectively, and they also thank the UCLA Amara-Yad Project for supporting this work. The authors are thankful to Dr Olujimi A. Ajijola for establishing and maintaining an organ procurement pipeline for research. They deeply appreciate Research Operations Manager Ms Hunter N. Strause for her dedication and support for these projects.
Footnotes
The authors attest they are in compliance with human studies committees and animal welfare regulations of the authors’ institutions and Food and Drug Administration guidelines, including patient consent where appropriate. For more information, visit the Author Center.
Appendix
For a supplemental STL file, please see the online version of this paper.
Supplementary Material
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