Vectorcardiographic Lead Systems
Fig. 16.1. The basic principle of vectorcardiography is illustrated based on ideal uniform lead fields which are mutually orthogonal being set up by parallel electrodes on opposite sides of the torso (bipolar configuration).
There are both uncorrected and corrected VCG lead systems. The uncorrected VCG systems do not consider the distortions caused by the boundary and internal inhomogeneities of the body. The uncorrected lead systems assume that the direction of the spatial line connecting an electrode pair yields the orientation of the corresponding lead vector. Currently it is known that this assumption is inaccurate, as is discussed later. In any event, these uncorrected lead systems are no longer in clinical use.
The goal of the corrected lead system is to perform an orthonormal measurement of the electric heart vector. In an orthonormal measurement both of the following requirements are fulfilled:
Fig. 16.2 The mirror vectorcardiograph constructed by Hubert Mann was the first instrument to produce a vectorcardiogram. It has three coils symmetrically placed at 120° intervals around a mirror. Thus it produces a vectorcardiogram in the frontal plane from the three limb leads of Einthoven. (Mann, 1938a).
In 1956 Ernest Frank (Frank, 1956) published a vectorcardiographic lead system that was based on his previously published data of image surface (Frank, 1954). Because the image surface was measured for a finite, homogeneous thorax model, the volume conductor model for the Frank VCG-lead system was also the same. In the following, we first discuss the design principles of the Frank lead system. Then we discuss the construction of each orthogonal component of the measurement system. Though we refer here to the original publication of Frank, we use the consistent coordinate system described in the Appendix.
Fig. 16.9 Determination of the foot-to-head component (z-component) in the Frank lead system. The image space shown on the left corresponds to the actual sagittal plane on the right.
Fig. 16.10 Determination of the back-to-front component (x-component) in the Frank lead system. The image space shown on the left corresponds to the actual transverse plane on the right.
Fig. 16.11 The lead matrix of the Frank VCG-system. The electrodes are marked I, E, C, A, M, F, and H, and their anatomical positions are shown. The resistor matrix results in the establishment of normalized x-, y-, and z-component lead vectors, as described in the text.
McFee and Parungao (1961) published a simple VCG lead system called the axial system, based on a lead field theoretic approach. In addition, the heart was modeled with a volume source and the thorax was assumed to be homogeneous.
The three uniform lead fields were designed according to the principle discussed in Section 11.6.10. To detect the three orthogonal components of the electric heart vector, three pairs of (single or multiple) electrodes must be used on each coordinate axis, one on either side of the heart. McFee and Parungao recognized that the closer to the heart the electrodes are placed the more electrodes must be used to achieve a homogeneous lead field within the heart's area.
Otto H. Schmitt and Ernst Simonson developed many versions of vectorcardiographic lead systems, calling them stereovectorelectrocardiography (SVEC). The third version, SVEC III, was published in 1955 (Schmitt and Simonson, 1955). It requires a total of 14 electrodes and creates a lead field in the thorax which is very symmetric in relation to the sagittal plane. The lead system is described in Figure 16.13.
In the SVEC III lead system, the electrodes are located on the thorax in the following way: The torso is divided angularly into 30° symmetric sectors about a central vertical axis so that, starting with 1 at the front, Arabic numerals up to 12 divide the torso vertically. Roman numerals refer to interspaces at the sternum and are carried around horizontally on a flat panel so that a grid is established on which a location such as V 7 would mean a location at the vertical level of the fifth interspace and at the middle of the back.
E. J. Fischmann, M. R. Barber, and G. H. Weiss (1971) constructed a VCG lead system that measures the equivalent electric dipole according to the Gabor-Nelson theorem.
Their equipment consisted of a matrix of 7 × 8 electrodes on the back of the patient and 11 × 12 on the chest. The latter were fixed on rods that could move along their axes. Similar electrode matrices with 7 × 7 electrodes were also placed on the sides of the patient. When the moving-rod electrodes are pressed against the surface of the thorax, their movement gives information about the thorax shape. This information is needed in the solution of the Gabor-Nelson equation.
This lead system was not intended for clinical use but rather for the demonstration of the Gabor-Nelson theory in the measurement of the vectorcardiogram.
In 1971 Clifford V. Nelson and his collaborators published a lead system suitable for clinical use based on the Gabor-Nelson theorem (Nelson et al., 1971). The lead system includes electrodes placed on three levels of the thorax with eight on each level, one electrode on the head, and one on the left leg. The electrode rows are designated A, B, and C, as shown in Figure 16.14. The levels are determined by measuring the distance H' between the suprasternal notch and umbilicus. This distance is divided by 8, and the rows are placed at 1/8 H', 4/8 H', and 7/8 H' from either notch or umbilicus.
As shown in Figure 16.14, electrodes 1 and 5 are placed at center-back and midsternal line, respectively. Electrodes 2, 3, and 4 are equally spaced on the right side, and electrodes 6, 7, and 8 are equally spaced on the left side. If the arms intervene on level C, electrodes 3 and 7 are placed on the right arm and left arm, respectively. The angle θ is the angle between the surface of the thorax and the frontal plane.
Resistors of 500 kΩ (R) are connected to the electrodes on rows A, B, and C (see Figure 16.15). From these resistors, on each three levels four (Rx and Ry) are variable and are adjusted according to the shape of the thorax of the patient to obey the Gabor-Nelson theory. The adjustment is made so that
|Rx /R = sin θ||(16.1)|
|Ry /R = cos θ|
where θ = the angle between the surface normal and the sagittal plane
Nelson and co-workers claim that on the basis of their measurements this VCG lead system is much more accurate than the McFee or Frank lead systems. Furthermore, this system should be very insensitive to electrode misplacement.
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