Hypertrophies and Intraventricular Conduction Defects Hypertrophies and Intraventricular Conduction Defects Causes, Presentation, and Significance Linda Josephson, MS, RN, CCRN-CMC There is an increasing need for nurses to interpret a 12-lead electrocardiogram, both in critical care units and in other areas. This can be a challenging task, especially in the presence of hypertrophies, bundle-branch blocks, and fascicular blocks. This article reviews the pathophysiology of intraventricular blocks and hypertrophy, characteristics found in the 12-lead electrocardiogram, and discusses what the significance of these findings may be. Keywords: Arrhythmias, Hypertrophies, Intraventricular conduction defects, Ischemic changes [DIMENS CRIT CARE NURS. 2010;29(6):259/275] There is an increasing need for nurses to be able to cardiodynamic conditions or changes. Hypertrophy of the accurately interpret a 12-lead electrocardiogram (ECG). different heart chambers place patients at increased risk The accurate detection of ischemic changes and the con- for cardiovascular events and may be a contributing cause firmation of arrhythmic changes all require the accurate of intraventricular conduction defects themselves.3,4 Epi- interpretation of a 12-lead ECG.1 However, there has demiological studies on the prevalence of BBBs and hy- traditionally been less emphasis placed on nurses being pertrophies demonstrate that the prevalence of both these able to interpret a 12-lead ECG and a greater expectation conditions increases with age.3,5 As the population of the that nurses would be adept at arrhythmia detection using United States continues to age, there will be an increased a 1- or 2-lead monitoring system. Because of that, many prevalence for cardiac disturbances. The presence of both nurses who are currently in practice may not have re- these conditions indicates that the individual has a higher ceived instruction in nursing school on how to read a 12- risk of other cardiac comorbidities.6 lead ECG. Some nurses may not get all the education they This article reviews the pathophysiology of intraven- need in their critical care orientation program. As a result, tricular blocks and hypertrophy, characteristics found in nurses may not feel confident or comfortable in the in- the 12-lead ECG, and discusses what the significance of terpretation of a 12-lead ECG. Even for nurses who have these findings may be. taken a formal ECG course, interpreting an ECG may not always be done accurately.2 This may be especially chal- ANATOMY OF THE HEART’S ELECTRICAL lenging for some arrhythmias such as bundle-branch CONDUCTION SYSTEM blocks (BBBs), fascicular blocks, and hypertrophies. The sinoatrial node (SA node) is the pacemaker of the These conduction defects and hypertrophies can have heart. It is located high in the left atrium near the superior considerable clinical significance as precursors to other vena cava. The SA node is connected to the right atrium DOI: 10.1097/DCC.0b013e3181f0be8d November/December 2010 259 Copyright @ 2010 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. Hypertrophies and Intraventricular Conduction Defects via Bachman bundle and to the atrioventricular (AV) node TABLE 1 Blood Supply to the Cardiac via internodal pathways that travel down the left atrium. Conduction System The AV node is located in the right atrium also, but low in the atrial wall just above the tricuspid valve. It serves to Coronary slow the impulse SA node, to allow the atria to contract Structure Blood Supply Artery and push the last of the blood from the atria into the SA node RCA RCA ventricles just before the ventricular systole (Figure 1). AV node AV nodal artery from RCA RCA Conduction of the impulse continues down through Bundle of His Left anterior descending via septal LCA the bundle of His, which is a continuation of fibers from perforator arteries the AV node. The bundle of His continues for a short distance, and then it bifurcates, forming the right and left Right bundle branch Left anterior descending artery LCA bundle branches (LBBs). The right side of the bundle Left anterior fascicle Left anterior descending artery LCA becomes the right bundle branch (RBB), which is long and Left posterior fascicle Left anterior descending and LCA, RCA slender. It travels down the right side of the intraventricu- posterior descending artery lar septum to the base. At the base of the septum, the RBB diverges into the Purkinje fibers, which serves to stimulate Abbreviations: AV, atrioventricular; LCA, left coronary artery; RCA, right coronary the right ventricle to contract. artery; SA, sinoatrial. On the left side, the left common bundle continues for a short distance before it also bifurcates into the left anterior Normally, the bundle of His receives a dual blood fascicle (LAF) and the left posterior fascicle (LPF). The supply; the bundle receives blood from the left anterior LAF, like the RBB, is long and thin. It travels down the left descending (LAD) artery and the posterior descending ar- side of the intraventricular septal wall and runs toward the tery (PDA) (Table 1). anterior-superior papillary muscle. The RBB conducts the The RBB is supplied by the LAD artery. Because the impulse from the bundle of His to the anterior and lateral left RBB is a long and slender structure, it is vulnerable to ventricular (LV) wall. The posterior fascicle travels down the damage. However, this structure has many bifurcations posterior wall of the intraventricular septum in the direction that may be less turbulent. The LBB is much more robust of the posterior-inferior papillary muscle. It is responsible for than the RBB. The anterior and medial fascicles receive stimulating the posterior ventricular wall to contract.7,8 their blood supply from the LAD artery via the septal perforating branches of the LAD artery. The posterior fas- BLOOD SUPPLY TO THE CONDUCTION SYSTEM cicle, like the bundle of His, has a dual blood supply from 9,10 The right coronary artery (RCA) provides the blood sup- both the LAD artery and PDA (Figure 2). ply to the SA node in 55% of the population, and the left circumflex artery provides the blood supply to the SA VECTOR DEVELOPMENT OF THE QRS COMPLEX node in the remaining 45% of the population. The blood During depolarization of the heart, many vectors develop supply to the AV node is commonly supplied by the RCA as the myocardium contracts. However, to simplify the in the vast majority of the population, 90% or so. In the concept, the depolarization of the ventricles can be divided 10% of the population that does not have this arrange- into a 2-phase event.7,11 The first vector is developed when ment, the AV node is supplied by the left circumflex artery. the septal wall of the ventricles depolarizes. The septum Figure 2. Blood supply to the conduction system. Courtesy of Figure 1. Conduction system. Abbreviation: AV, atrioventricular. ecgpedia, Rob Kreuger, medical illustrator; C. Carl Jaffe, MD, car- Courtesy of Wikimediacommons, drawing by Madhero88. Licensed diologist. Licensed under creative commons. http://creativecommons. under creative commons: http://creativecommons.org/licenses/by/2.5/. org/licenses/by/2.5/. 260 Dimensions of Critical Care Nursing Vol. 29 / No. 6 Copyright @ 2010 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. Hypertrophies and Intraventricular Conduction Defects depolarizes from the left side to the right. This creates a outflow tract of the left ventricle, exposing it to additional vector that travels in a rightward and slightly anterior stress. It is not uncommon to see a bifascicular block of the direction. On the surface ECG for leads that are on the RBB and the LAF. Both the RBB and LAF usually have a right side, such as aVR, and V1-V2, the first part of the single blood supply, usually from the LAD artery. The QRS complex is positive, and a small r wave is inscribed. single blood supply also makes them vulnerable to damage. For leads that are on the left side, such as I, aVL, V5-V6, A block of the LPF is relatively rare. This is due to the because the septal vector moves away from the positive LPF being stronger and more robust fascicle, than either the electrode of the leads, the first part of the QRS complex is RBB or the LAF. Additionally, the LPF also has a dual blood negative and inscribes a small q wave. This is sometimes supply from both the right and left coronary artery system.8,9 called a septal q wave.9-11 The second phase of the ventricular depolarization is Right Bundle-Branch Block the activation of the free wall of the right and left ventricle. When a right BBB (RBBB) occurs, the septum continues to The free walls of both ventricles contract simultaneously. depolarize normally. The impulses from the SA node travel However, because the left ventricle has the greatest muscle to the AV node and then continue through the bundle of mass, it generates the greatest electrical force and domi- His and down the LBB as usual. The impulse depolarizes nates the morphology of the QRS complex. The contrac- the septum in the normal fashion, from left to right, and tion of the free wall of the ventricles produces the main then the left ventricle depolarizes.7 Then the impulses vector of the heart, which travels from right to left and travel from the left ventricle to the right ventricle, across somewhat downward.9-11 the septal wall and through the myocardium to stimulate The result of this is that in leads that are right-sided, aVR, the right ventricle and cause depolarization (Table 2). V1 and V2, the deflection is seen primarily as a negative one On the surface ECG, the first portion of the QRS com- after the small initial r wave.