The following statements, which need reemphasis, should not be considered redundant: (1) Since the ECG deals with electrical forces, it follows that very strictly speaking, electrocardiography can be considered vectorial.149,150 (2) Orthodoxically, a scalar quantity only has magnitude, whereas a vector quantity has magnitude, direction, and sense. When analyzing the vectorcardiogram (VCG), one should consider the activation of each muscle cell as producing an electrical force that can be represented by a vector depicting the spatial orientation and magnitude of this force.!49
During the spread of the activation process, innumerable electrical forces are generated. These multiple forces vary in magnitude and differ in direction. At any given moment, the resultant of these electrical forces can be represented by a spatial vector possessing magnitude, direction, and sense. This vector is referred to as an instantaneous vector and represents the resultant of all the forces of the heart acting at that particular moment. Immediately afterward, the wave of accession spreads to different areas of the myocardium, and the new instantaneous vector representing all the forces of the heart now occupies a different spatial position and has a different magnitude. This continues throughout the cardiac cycle, with the succeeding instantaneous vector occupying different spatial positions. If all manifest spatial vectors are diagrammatically represented as having a common point of origin, and if the distal points of the vectors are joined, a single spatial loop is formed for ventricular depolarization (QRS), ventricular repolarization (ST-T), and the atrial complex (P). The VCG consists of four different loops. The electrical activity of the atria is recorded as a small loop designated the P loop, the depolarization of the ventricles is recorded as a large loop designated the QRS loop, while the repolarization of the ventricles is recorded as a smaller loop designated the ST-T loop. Finally, at high magnifications, even a small U loop also can be recorded.149-153
Space: The Final Frontier
The theory of the truly spatial VCG is theoretically attractive. Because the heart is a tridimensional structure (located in space), its electrical activity should best be recorded by a spatial method. Indeed, space, as conceived by physicists through objects and their motion, has three dimensions, and positions are characterized by three numbers. The instant of an event is the fourth number. Four definite numbers correspond to every event; a definite event corresponds to any four numbers. Therefore, the world of events really forms a four-dimensional continuum. Unfortunately, judging by what is being published in the literature, the quest for finding an optimal method of visualizing the spatial loop apparently has been abandoned. Nevertheless, the spatial VCG is still of importance in children with congenital and acquired heart disease because in this population the criteria for pressure and volume overloading have proven value.151 In our opinion, it is also of great value in categorizing the various types of intraventricular conduction defects.151-153 While this may be attributed to the spatial technique per se, it also can be due to the use of instruments having a higher fidelity than routinely employed electrocardiographs. The VCG also has been found useful in detecting MI and certain types of RV enlargement.151-153 In practice, it has not been proven that the VCG gives more information than the routine 12-lead ECG,151 although some computer programs may still use the Frank orthogonal leads X, Y, and Z.
These programs thus constitute a 15-lead system. In addition, the time required to obtain a VCG is longer than the time required to record a 12-lead ECG. These are the main reasons for the decrease in the use of spatial vectorcardiography during recent decades. Other reasons are nonreimbursement and the continuously increasing interest in other noninvasive methods of recording electrical activity (such as signal averaging, body surface mapping, and heart rate variability) or nonelectrical activity (such as echocardiography or magnetic resonance imaging, which looks at planes from different views). To obtain the spatial VCG, electrodes are placed on the body surface in a way to record three leads whose planes are at right angles to each other. The true spatial VCG requires three corrected orthogonal leads with the following features151-154: (1) Mutual perpendicularity, with each lead being parallel to one of the rectilinear coordinate axes of the human body. Such axes are the horizontal, A'(left-to-right and right-to-left) axis; the vertical, Y (inferosuperior or superoinferior) axis; and sagittal, Z (anteroposterior or posteroanterior) axis. (2) Equal amplitude from the vectorial viewpoint. (3) Retention of the same magnitude and direction for all points where cardiac electromotive forces are generated. For example, even if the leads forming Einthoven's frontal plane were to be spatially correct, Einthoven's theory itself would make any electrodes placed for the purpose of obtaining the horizontal and sagittal planes (such as the tetrahedral system) spatially incorrect. The most widely used, corrected spatial VCG method probably is the one introduced by Frank.!54 Since the spatial loop cannot be analyzed tridimensionally, it is customary to study its planar projections (BnBi Fig. 11-40). By proper attachment to the oscilloscope, the X and Y leads are used for the frontal plane, the X and Z leads for the horizontal plane, and the Z and Y leads for the sagittal plane (of which the right side has been the most popular).
Differences between Electrovectorcardiography and Spatial Vectorcardiography
Spatial vectorcardiography is distinctly different from the various vectorial methods of ECG interpretation, such as those of Sodi-Pallares et al.7 and Grant.56,57 In clinical practice and in teaching, both seem to be considered equal, but this is so only for pragmatic and didactic reasons. Although the spatial VCG and the ECG should each be studied as distinct methods, most electrocardiographers either memorize loop patterns or attempt to derive the leads with which they are familiar from the corresponding QRS loops. Thus bipolar standard and unipolar extremity leads are derived from the frontal plane more or less as when, in clinical ECG, they are derived from the electrical axis. To do this in spatial vector loops, the electrical axis is equated with the maximal QRS vector that extends from the point of origin of the loop to its farthest point. The unipolar precordial leads are derived from the horizontal plane loops. Leads thus derived are different from the usual precordial ECG leads. The latter, as mentioned previously, record electrical forces moving toward or away from them, including local potentials that can be of different duration in different precordial leads.13,35,133 In the 12-lead ECG (especially when the precordial electrodes are misplaced), however, these forces can move spatially not only in a left-to-right and anteroposterior direction but also in an inferosuperior direction as in leads V5 and V6 in patients with a very superior and leftward deviation of the EA. On the other hand, the theory of spatial vectorcardiography states that the horizontal plane and unipolar leads derived from them just record left-to-right and anteroposterior forces and that they do not record local potentials so that any difference in the duration of intervals is merely an illusion133451 (0-»-0- Fig. 11-41). In spatial vectorcardiography, electrical forces oriented superiorly or inferiorly cannot be reflected in the horizontal plane but only in the frontal and sagittal planes. Most of the information contained in the sagittal plane is present in the frontal and horizontal planes. In practice, the sagittal plane is useful to act as a "judge" in cases of apparent discrepancy between the other two planes. For example, it serves to determine if a localized delay present in one of the two planes is "real" or is due to perpendicularity of vectors. It also serves for a better evaluation of the upward or downward direction of the initial 0.01- and 0.02-s vectors than the frontal plane. Projections of normal spatial QRS and ST-T loops in the corresponding planes are depicted in BhB; Fig. 11-42.
PREVIOUS | NEXT
Page: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27
Copyright ©2001-2002 The McGraw-Hill Companies. All rights reserved.
For further information about this site contact tech [email protected].
Last modified: August 10, 2002 .
Was this article helpful?