MEDICINE HURST VENTRICULAR ELECTROCARDIOGRAPHY - Part 2 ppsx

29 Figure 4.1 Suppose the heart generates only three electrical forces. Having made this supposition, let the electrical forces be visualized as vectors (arrows), as shown in this figure. Note that arrow number one is directed to the right, inferiorly and anteriorly. Arrow number two is directed to the left, inferiorly, and parallel with the frontal plane. Arrow number three is directed to the left and posteriorly. There are five major factors that determine the characteristics of the vectors (arrows) that represent the electrical forces of the heart. They are: (a) the location of the heart in the thorax; (b) the transmission of the electrical forces of the heart to the body surface; (c) the exact location and anatomic features of the atria and ventricles; (d) the unique anatomy of the conduction system; and (e) the sensitivity of the measuring device (the electrocardiograph machine). The Location of the Heart in the Thorax Austin Flint, of auscultation fame, published a beautiful drawing of the heart (Fig. 4.2) in 1859. [1] Note that the heart is nearer to the anterior portion of the chest wall than it is to the lateral or posterior portions. 30 Figure 4.2 Austin Flint (1859) published this diagram and legend. A. The relations of the heart to the thoracic parietes. The letters a, b, c, etc., indicate the ribs. The numbers 1, 2, 3, etc., mark the intercostal spaces. The vertical line denotes the median line. The right triangle extending over a portion of the surface of the heart represents the "superficial cardiac region" as delineated on the chest with sufficient accuracy for practical purposes. The cross on the fourth rib shows the situation of the nipple. The relations of the ventricles, auricles, apex of the heart, aorta, and pulmonary artery to the ribs and intercostal spaces, the median line, and the nipple are accurately indicated. B. The relations of the heart to the pulmonary organs, liver, and stomach. The quadrangular space in which the heart is uncovered by lung is the "superficial cardiac region," represented more accurately than in Figure 4.2A. The relative situations of the left lobe of the liver the stomach and inferior border of the heart are correctly represented. (Reproduced with permission from Flint A: A Practical Treatise on the Diagnosis, Pathology, and Treatment of Diseases of the Heart. Philadelphia, Blanchard and Lea, 1859, p 15. Book reprinted by The Cardiac Classics of Cardiology Library, Birmingham, Alabama.) The Transmission of the Electrical Forces of the Heart to the Body Surface The electrical forces generated by the heart are transmitted through the tissues of the body to the skin. Whenever an electrocardiogram is recorded from the right wrist, the deflection has the same size and shape as when it is recorded from the right upper arm (Fig. 4.3A, left). Similarly, when the electrocardiogram is recorded from an electrode placed on the right ankle, the deflection has the same size and shape as when it is recorded from an electrode on the right knee (Fig. 4.3A, right). This suggests that the tissue of the legs and arms transmits electrical forces to the skin without great difficulty. It also indicates, as will be discussed later, that any portion of the legs or arms is "electrically" equidistant from the origin of the electrical forces generated by the heart. As Dr. Harvey Estes has pointed out in personal communication, the extremities are like wires attached to the trunk, and a connection made at any point along the wire will produce the same recording. The lower extremities represent an upside down, Y-shaped wire. 31 Figure 4.3 A. The electrocardiograms shown were recorded by placing an electrode on the right wrist (top left) and right upper arm (bottom left). The size and shape of the electrocardiographic deflections are the same. The other electrocardiograms were recorded by placing an electrode on the right ankle (top right) and right knee (bottom right). Again, the size and shape of the electrocardiographic deflections are the same. This simple experiment shows that electrically speaking, the ankles are no further away from the heart (the origin of electrical activity) than the knees, and the wrists are no further away than the upper arms. B. The electrocardiographic deflections were recorded from the front of the chest and the back of the thorax. The deflection recorded from the front of the chest is larger than that recorded from the back. This difference occurs because an electrode placed on the front of the chest is nearer the heart than one on the back or on the extremities. When an electrocardiogram is recorded from an electrode placed on the back of the thorax, the waves will be smaller than in an electrocardiogram recorded from an electrode on the front of the thorax (Fig. 4.3B). This occurs because the sampling electrode placed on the front is nearer the electrical field generated by the heart than when it is placed on the back. In addition to this, the lung tissue, which is sparse anteriorly as compared to posterolaterally, impedes the transmission of the electrical field to a greater degree posteriorly than anteriorly. The precordial deflection will also be larger than the deflections recorded from the extremities (Fig. 4.3A). The size of the electrical forces recorded from the body surface decreases considerably when the sampling electrode is moved from the anterior portion of the chest toward the extremities, but after about 10cm, the electrodes have to be moved greater and greater distances before there is a change in the magnitude of the recorded electrical forces (Fig. 4.4). The point, remarks Dr. Estes, is that recordings made beyond 10cm are made in a region where the isopotential lines have become so "thinned out" that distance is relatively unimportant; therefore, the surface points can be considered to be equidistant. 32 Figure 4.4 The influence of distance on the size of the electrocardiographic deflections. When electrodes are placed near the heart (central circle on the front of the chest), the size of the deflections is influenced considerably by their nearness to the origin of electrical activity. For electrodes placed in the middle circle, the size of the deflections is influenced less by their proximity to the heart than when they are placed in the central circle. The size of the deflections recorded by electrodes placed on the body in the outer circle will be influenced very little by the distance from the heart. When electrodes are placed within the outer circle, they are considered to be electrically equidistant from the origin of electrical activity. The Precise Location and Anatomic Features of the Atria and Ventricles The names of the four chambers of the heart the right atrium, left atrium, right ventricle, and left ventricle prevent us from perceiving the precise location of these structures in the thorax. The right atrium is in reality located to the right and slightly anterior to the left atrium. The left atrium is a posterior structure and is actually located in a central position within the chest. The right ventricle is located to the right and is predominantly an anterior structure, while the left ventricle rests on the left leaf of the diaphragm in a left lateral and slightly anterior position. The anatomic position of the cardiac structures is shown in Figures 4.5, 4.6, and 4.7. The reader should recall that the heart is located more vertically in tall, thin individuals and more horizontally in broad-chested, obese individuals. 33 Figure 4.5 The gross anatomy of the heart (frontal view). In order to understand electrocardiography it is necessary to know cardiac anatomy. The technique of magnetic resonance imaging (MRI) can be used to show the frontal view, transverse view (see Fig. 4.6), and left lateral view (see Fig. 4.7) of the heart. They are the same views that must be kept in mind as one analyzes the electrical forces of the heart. (Image of Dr. Mark Lowell; provided by Dr. Roderic I. Pettigrew and the Radiology Department of Emory University Hospital). Figure 4.6 The gross anatomy of the heart (transverse view). A. Magnetic resonance image (transverse view) showing the left and right ventricles. (Image of Dr. Mark Lowell; provided by Dr. Roderic I. Pettigrew and the Radiology Department of the Emory University Hospital.) B. Magnetic resonance image (transverse view) showing the left atrium, right atrium, right ventricle and left ventricle. (Image of Dr. Mark Lowell; provided by Dr. Roderic I. Pettigrew and the Radiology Department of the Emory University Hospital.) 34 Figure 4.7 The gross anatomy of the heart (magnetic resonance image of left lateral view). (Image of Dr. Mark Lowell; provided by Dr. Roderic I. Pettigrew and the Radiology Department of Emory University Hospital.) In addition to their location, the size, thickness, and integrity of the walls of the four cardiac chambers are major determinants of the electrical field created by the heart. The heart of a normal newborn exhibits a right and left ventricle of equal wall thickness, whereas the left ventricle of a 1-year-old child and an adult has a thicker wall than the right ventricle. These normal anatomic conditions influence the characteristics of the heart's electrical field. A large right or left atrium may be associated with large, deformed P waves in the electrocardiogram. A hypertrophied right ventricle may produce large rightward and anteriorly directed electrical forces, whereas a hypertrophied left ventricle may produce large leftward and slightly posteriorly directed electrical forces. Damage to the left ventricle, as with myocardial infarction, may also alter the electrical field. All of these conditions will be discussed later. The objective of the current discussion is to emphasize that the location of the chambers of the heart and the anatomical status of the muscle influence the characteristics of the heart's electrical field and its distribution. The Cardiac Conduction System The cardiac impulse is a self-perpetuating process that begins in the sinoatrial node (SA node). The SA node is normally "beating" a certain number of times each minute, and periodically leaks electrical potential, 35 causing the neighboring cells to depolarize. This node is located at the junction of the superior vena cava, the right atrium, and right atrial appendage. Anton Becker, one of the modern authorities on the conduction system of the heart, [2-5] does not believe that there is any specialized conduction tissue within the atria. However, he maintains that there are preferential electrical pathways within the atria, pointing out that the right atrium is a "bag of holes." [5] There are five such holes, created by the openings of the superior and inferior vena cava, the opening of the coronary sinus, the fossa ovalis, and the opening of tricuspid valve. [5] There are also five ''holes'' in the left atrium. They are the openings of four pulmonary veins and the opening of the mitral valve. By comparison, each ventricle has only two ''holes." Becker also points out that some of the tissue surrounding some of the holes in the atria is fibrous tissue and not muscle. [5] The remaining atrial tissue, made up of atrial cells crowded together, forms the preferential electrical pathways. These preferential electrical pathways are called internodal tracts. They are labeled as the anterior, middle, and posterior tracts. The anterior tract was first described by physiologist Jean Bachmann [6,7] of the Emory University School of Medicine. It travels anteriorly from the sinus node to reach the atrioventricular node, and simultaneously travels into the left atrium. [7] The middle tract travels from the sinus node and passes posteriorly around the superior vena cava and down the atrial septum to reach the atrioventricular node. The posterior tract travels posteriorly through the crista terminalis and down the posterior portion of the atrial septum to the atrio-ventricular node. [7] James has depicted the internodal tracts as shown in Figure 4.8. The right atrium is depolarized initially. [8] This produces electrical forces that are directed to the left, inferiorly, parallel with the frontal plane or slightly anteriorly (Fig. 4.9). This vector is referred to as P1. The impulse reaches the atrioventricular node at about the time it reaches the left atrium. The atrioventricular node delays the impulse while the left atrium undergoes depolarization. The left atrium produces electrical forces that are directed to the left, inferiorly, and slightly posteriorly. This vector is referred to as P2. The mean P vector is the summation of the vectors representing the depolarization of the right and left atria. The mean P vector is directed to the left and inferiorly, and is commonly parallel with the frontal plane. This vector is referred to as Pm. The wave of depolarization spreads rapidly through the atria. It does not, as it does in the ventricles, spread from the endocardium to the epicardium; instead, it spreads in a ripple-like fashion through the atrial myocardium. Having passed through the atria, the electrical stimuli arrive at the atrioventricular node, which is located in the lower portion of the right atrium. The electrical impulse then passes down the common bundle (the bundle of His) [9] and the left and right bundle branches until it reaches the Purkinje fibers. The right and left bundle branches are endocardial structures. The left bundle branch fans out as shown in Tawara's classic diagram (Fig. 4.10). [10] A diagrammatic illustration of the left and right ventricular conduction system is shown in Figure 4.11. 36 Figure 4.8 Diagram showing the three internodal pathways: Anterior (A), middle (M), and posterior (P). Bachmann's bundle (BB) contains the major interatrial pathway and the first portion of the anterior internodal pathway. RV = right ventricle, LV = left ventricle; Ao = aorta; SN = sinus node; AVN=AV node. (Modified with permission from James TN: The connecting pathways between the sinus node and the A-V node and between the right and the left atria in the human heart. Am Heart J 1963; 66:489.) 37 Figure 4.9 Depolarization of the atria. A. Mean vector representing depolarization of the right atrium. This vector is referred to as P1. B. Mean vector representing depolarization of the left atrium. This vector is referred to as P2. C. Mean vector representing depolarization of both atria. This vector is referred to as Pm. Figure 4.10 Tawara's view of the left bundle branch. This diagram is taken from the monograph by Tawara (1906), [10] which established and elucidated the significance of the atrioventricular conduction axis. It shows the fanlike arrangement of the left bundle branch. The clinical value of the so-called concept of hemiblocks should not be extended to presume that the left bundle branch is arranged anatomically in bifascicular fashion. As shown here, it is arranged as a fan, and if it divides at all, it forms three rather than two divisions. (From Tawara S: Das reizleitungssystem des saugetierherzens. Jena, Gustav Fischer, 1906.) 38 Figure 4.11 The left bundle branch provides an early twig to the left upper portion of the interventricular septum. The left bundle divides to form two branches, although Tawara called it tripartite. The major divisions are the anterior-superior and the posterior-inferior divisions. The arrows indicate the direction of depolarization of the myocytes that results from the electrical stimulus transmitted by the conduction system. The electrical impulses created by the sinoatrial node itself, the atrioventricular node, the common bundle of His, the left and right bundle branches, and the Purkinje fibers are not recorded by the electrocardiograph machine when the sampling electrodes are placed on the skin surface. The atrioventricular node actually slows the transmission of electrical impulses. The speed of electrical propagation in the atrioventricular node is 200mm per second. The speed of electrical propagation in the bundle branches and Purkinje fibers is 4000mm per second, and the speed of propagation in the ventricular muscle is 1000mm per second. [11] The myocardial cells slow the transmission of electrical impulses as compared to the speed of impulse transmission in the bundle branches and Purkinje fibers. The atrial and ventricular electrocardiogram recorded from the skin surface is produced by the depolarization and repolarization of the atrial and ventricular muscle cells (myocytes). Whereas the wave of excitation (depolarization) in the ventricles progresses, for the most part, from endocardium to epicardium in an orderly manner, some of the Purkinje fibers undoubtedly transmit the electrical stimuli into the midportion of the ventricular muscle wall where depolarization of the myocytes occurs at the same time as it occurs in the endocardium. In fact, part of the ventricular muscle may be depolarized toward the endocardium. Still, the overall wave of depolarization of the ventricles spreads from endocardium to epicardium. [...]... leads were born and called V1, V2, V3, V4, V5, V6, and V3R (Fig 4 .21 A) The precordial V lead axes are shown in Figures 4 .21 B and 4 .21 C The magnitude of the deflections from each precordial lead was adequate for analysis because the electrode positions were near the heart 49 Figure 4 .21 Unipolar chest lead axes A When unipolar chest leads are used, they are identified as V1, V2, V3, V4, V5, V6, and V3R... electrode positions are influenced by the electrical forces from all parts of the heart Electrode positions 1, 2, and 3 are influenced by the right ventricle more than positions 4, 5, and 6, but the electrocardiographic recordings made from positions 1, 2, and 3 reveal left ventricular electrical forces to a greater extent than they do right ventricular electrical forces Electrode positions 5 and 6 are influenced... recorded from a specific electrode site using a Wilson unipolar extremity lead could be augmented if the extremity-to-central terminal electrode was disconnected when the exploring electrode was placed on that extremity.[17] As a result, leads aVR, aVL, and aVF were born (Fig 4 .24 )  52 Figure 4 .24 Goldberger's lead system A Lead aVR is created by placing an electrode on the left arm (L) and leg (F), and... (Represented as Vectors) Onto the Lead Axes The projection of electrical forces (represented as vectors) onto the lead axes used in electrocardiography must be carefully considered.[1 9 -2 1]This concept can be illustrated by the use of a screen, pencil, and light source (Fig 4 .27 ). [20 ] Suppose a pencil (which represents an electrical force) is placed in front of a screen (which represents a lead axis), and... Figure 4 .29 A Imagine that this force is directed posteriorly because the zero potential plane is oriented in such a way that the transitional pathway passes between electrode positions V3 and V4 Figure 4 .29 B shows the hexaxial lead system superimposed on the spatially oriented vector shown in Figure 4 .29 A Figure 4 .29 C shows how the extremity lead axes would be influenced by the vector Figure 4 .29 The... sampling sites at a time This machine was replaced by the portable electronic, direct-writing machine, which also recorded from one or two sampling sites at a time The latter machine was replaced by the modern portable, electronic, direct-writing machine, which records from 12 sampling sites simultaneously (Fig 4.13) The direct-writing machine does not record with the precision of the photographic machine,... recorded by V leads with those recorded by the CR, CL, and CF leads, in an effort to determine which system yielded the best clinical information (Fig 4 .22 ) It was eventually concluded that the V lead system was superior to the others 50 Figure 4 .22 The precordial deflections at all six precordial electrode positions using CR, CL, CF, and V leads The deflections recorded on leads I, II, and III are... paper speed is preset to a uniform 25 mm per second Figure 4.13 A modern direct-writing electrocardiograph machine The machine records all 12 leads simultaneously The recordings of three leads are displayed one above the other so that leads I, II, and III can be viewed on the first segment of the paper These are followed by the recordings of aVR, aVL, and aVF, V1, V2, and V3; and V4, V5, and 41 V6 The... bucket of a double-bucket apparatus The subjects' extremities were immersed in saline-soaked cotton held within the inner bucket (which was actually a porous pot), and each wire was attached to the outer bucket, which contained zinc sulfate Sir Thomas Lewis published the photograph shown in Figure 4.15 in the fourth edition of his book Clinical Electrocardiography, published in 1 928 .[13] Improvements... forces actually generated by the heart The three electrical forces do not occur simultaneously; Force 1 occurs a brief moment before Force 2, and Force 2 occurs a brief moment before Force 3 Force 1 is produced by depolarization of the interventricular septum; Force 2 is due to depolarization of the endocardial layers of the right and left ventricles, and Force 3 is due to the depolarization of the thicker, . of the interventricular septum. The left bundle divides to form two branches, although Tawara called it tripartite. The major divisions are the anterior-superior and the posterior-inferior divisions occurs a brief moment before Force 2, and Force 2 occurs a brief moment before Force 3. Force 1 is produced by depolarization of the interventricular septum; Force 2 is due to depolarization of the. precordial V leads were born and called V 1 , V 2 , V 3 , V 4 , V 5 , V 6 , and V 3R (Fig. 4 .21 A). The precordial V lead axes are shown in Figures 4 .21 B and 4 .21 C. The magnitude of the deflections from