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When Is the Vectorcardiogram Superior to the Scalar Electrocardiogram? TE-CHUAN CHOU, MD, FACC Cincinnati

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When Is the Vectorcardiogram Superior to the Scalar Electrocardiogram? TE-CHUAN CHOU, MD, FACC Cincinnati lACC Vol. 8, No.4 791 October 1986:791-9 SEMINAR ON THE CHANGING ROLE OF ELECTROCARDIOGRAPHY IN CLINICAL PRACTICE-II Borys Surawicz, MD, FACC and Charles Fisch, MD, FACC Guest Editors When Is the Vectorcardiogram Superior to the Scalar Electrocardiogram? TE-CHUAN CHOU, MD, FACC Cincinnati. Ohio The clinical usefulness of the vectorcardiogram is.well display. However, some information, such as that On documented by the numerous reports published in the cardiac chamber size and myocardial damage, can also last 3 decades. It has been found more reliable than the be obtained by other noninvasive tests that are often electrocardiogram for the diagnosis of atrial enlarge­ performed on the same patients. With the increasing ment and right ventricular hypertrophy. It is more sen­ awareness of cost-effectiveness of various laboratory sitive than the electrocardiogram in the recognition of procedures in medicine, the vectorcardiogram should no myocardial infarction, especially if the infarction is in­ longer be considered a routine cardiac test and should ferior or if it occurs in the presence of left bundle branch be requested only for a specific clinical purpose. When block or left anterior hemiblock. It is helpful in the properly utilized, vectorcardiography should remain a diagnosis of ventricular pre-excitation and in the local­ valuable diagnostic as well as teaching tool. ization of the bypass tract. Some repolarization abnor­ (J Am Coli CardioI1986;8:791-9) malities are more clearly demonstrated by the vector A major difference between the conventional electrocardio­ are more clearly identified in the vectorcardiogram. The gram and the vectorcardiogram is the method of display. scalar electrocardiogram may be derived from the vector­ The conventional scalar electrocardiogram depicts the vari­ cardiogram with a reasonable degree of accuracy. The scalar ation in the magnitude of the electrical potential generated complexes in the limb leads may be obtained by projecting from the heart in relation to time in the 12 or more leads the frontal plane vector loops on the hexaxial reference routinely obtained. In vectorcardiography the electrical ac­ system based on Einthoven's equilateral triangle, and the tivity of the heart is represented by a single dipole. The complexes in the precordial leads by the projection of the strength and spatial orientation of the dipole at each moment transverse plane vectors on the precordial lead axes. Ad­ are depicted by a spatial vector. The changing direction and ditional right-sided or posterior chest leads may be derived magnitude of the instantaneous vectors during each cardiac as needed. On the other hand, the usual form of the vec­ cycle are displayed as loops formed by joining the terminals torcardiogram is a still picture of one cardiac cycle. The of the vectors. The spatial vector loops are viewed in three scalar electrocardiogram is needed to study cardiac ar­ mutually perpendicular planes: the transverse, sagittal and rhythmias. frontal planes. It is apparent that the phasic changes of the Another theoretic advantage of the vectorcardiogram is electrical forces from the heart, especially their directions, related to the lead system used for its recording. Because the heart is not located in the center of the human torso, From the Division of Cardiology, Department of Medicine, University the actual directions of the electrical axes of the conventional of Cincinnati College of Medicine, Cincinnati, Ohio. Part I of this Seminar electrocardiographic leads are not identical to those that are appeared in the September 1986 issue of the Journal (Vol. 8, No.3, 710­ commonly assumed from the anatomic location of the elec­ 24). Address for reprints: Te-Chuan Chou, MD, University of Cincinnati, trodes. The lead axes of the standard limb leads form a Division of Cardiology, Mail Location #542, Cincinnati, Ohio 45267. scalene (Burger) rather than an equilateral triangle (1). Fur- © 1986 by the American College of Cardiology 0735-1097/86/$3.50 792 CHOU JACC Vol. 8, NO.4 VECTORCARDIOGRAM VERSUS ELECTROCARDIOGRAM October 1986:791-9 thennore, electrical potentials of equal strength applied to most reliable (II). However, because the electrode for lead these leads do not result in deflections ofthe same magnitude VI is in close proximity to the right atrium, prominent elec­ (2). Lead III records the largest voltage and lead I records trical forces from the enlarged but posteriorly located left the smallest. These artifacts prompted the design of several atrium may be masked . A similar problem is less likely to corrected orthogonal lead systems to record the vectorcar­ occur with the corrected lead system used to record the diogram, with the Frank lead system being the most com­ vectorcardiogram. monly used (3). The electrical axes of the X, Y and Z leads are perpendicular to each other. The scalar voltages recorded by each lead are uniform when electrical force of the same Ventricular Hypertrophy strength is applied to each lead. The clinical value of the vectorcardiogram depends on Left Ventricular Hypertrophy the amount of information it may supply that is not readily The electrocardiographic and vectorcardiographic rec­ available from the conventional scalar electrocardiogram. ognition of left ventricular hypertrophy are based mainly on The purpose of this paper is to review and compare the the increased magnitude of the QRS forces directed leftward diagnostic reliability of the electrocardiogram and vector­ and posteriorly. Secondary repolarization abnormalities, if cardiogram in some of the major pathologic entities. present, give additional support to the diagnosis. Romhilt et al. (12) were able to identify by the vectorcardiogram 61% of 70 hearts with anatomic left ventricular hypertrophy Atrial Abnormalities in an autopsy series. No false positive diagnoses were made Left and right atrial enlargement. The electrocardio­ among 23 hearts without left ventricular hypertrophy. In a graphic diagnosis of left atrial enlargement is generally based larger series reported by Dower and Horn (13), the vector­ on the findings of an increase in the duration of the P waves, cardiogram recognized 41.7% of autopsy proved cases of that are often notched in some of the limb leads (the P left ventricular hypertrophy with 11.4% false positive di­ mitrale pattern), leftward shift of the frontal plane P axis agnoses. In each series the overall accuracy of the vector­ and a prominent negative component of the P wave in lead cardiographic diagnosis was similar to that of the electro­ V I (4). Although the equivalent changes may be observed cardiogram. In a study of 100 autopsy-proved cases of left in the vectorcardiogram, the vectorcardiographic diagnosis ventricular hypertrophy reported by Abbott-Smith and Chou of left atrial enlargement emphasizes the magnitude of the (14), the vectorcardiogram was less sensitive than the elec­ maximal posterior P vectors in the transverse or sagittal trocardiogram when the voltage criteria alone were used. A planes (5,6) . In 21 consecutive patients with mitral valve correct diagnosis of left ventricular hypertrophy was made disease and angiographically documented left atrial enlarge­ in 33% with 11.7% false positive diagnoses. The electro­ ment reported by Benchimol et al. (7), the electrocardiogram cardiogram was able to recognize 41.9% of the hearts with was diagnostic or suggestive of left atrial enlargement in left ventricular hypertrophy, with a false positive diagnosis 43% and the vectorcardiogram in 86%. In a series (6) of in 8.8%. When other QRS changes and the ST vector and 20 patients with the electrocardiographic findings of the P T loop abnormalities were also utilized, however, the vec­ pulmonale pattern, 10 were found to have vectorcardio­ torcardiogram recognized 50% of the cases without an in­ graphic signs of left atrial enlargement. All 10 patients had crease in false positive diagnoses. clinical evidence of left-sided heart disease only. In the same From the available data it appears that the vectorcardi­ study only 49 of 100 consecutive patients with the P pul­ ogram is less sensitive than the electrocardiogram in the monale pattern had a disease in which right atrial enlarge­ diagnosis of left ventricular hypertrophy. The attenuation ment might be expected. The results suggested that criteria of the voltage obtained from the C electrode (which is lo­ depending on the P wave voltage and axis in the frontal cated near the cardiac apex) in the recording of the X lead plane alone have low specificity. This is also true when the of the Frank lead system is probably responsible for some increased duration and notching of the P wave are used for of the false negative diagnoses related to the voltage criteria. the diagnosis of left atrial enlargement (8). These changes On the other hand, the morphologic changes in the QRS may be present in patients with an intra-atrial conduction loop may be useful in improving the specificity of the di­ defect without atrial enlargement due to a condition such agnosis , especially in younger patients in whom the voltage as chronic pericarditis, myocarditis and atrial ischemia, in­ criteria alone are not reliable . In the transverse plane, there farction or fibrosis (9,10) . is a leftward displacement of the initial and terminal de­ The increased sensitivity of the vectorcardiogram in the flections of the loop and its distal area is larger than the diagnosis of left atrial enlargement is probably related to proximal area. These are some of the typical vectorcardio­ the lead system used. Among the electrocardiographic cri­ graphic changes seen in left ventricular hypertrophy that are teria for the diagnosis of left atrial enlargement, a prominent not easily identifiable in the scalar electrocardiogram (14). negative component ofthe P wave in lead VI has been found Gaum et a1. (15) analyzed the vectorcardiogram and elec- JACC Vol. 8, No.4 CHOU 793 October 1986:791-9 VECTORCARDIOGRAM VERSUS ELECTROCARDIOGRAM trocardiogram in patients with supravalvular aortic stenosis tional electrocardiographic criteria for right ventricular hy­ and coarctation of the aorta, They observed a characteristic pertrophy.
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