Spatial vectorcardiography

"The intrinsic effects are those signalling events which occur in the immediate neighborhood of the contacts; the extrinsic effects at some distance from them. In all direct leads from the heart, whatsoever the method employed, these two sets of effects are intermixed and are often very difficult to disentangle. Until complete explanation is possible and until the extrinsic effects are more fully understood than they are at present, we cannot be too circumspect in interpreting curves taken by leads direct from the heart muscle." (Lewis, Meakins, and White, 1914.)³⁵. The basic tenet of spatial vectorcardiography which has been stressed is that all electrocardiograms recorded in man-by the very nature of the applicable technics employed-are derivatives of the spatial cardiac vector. When several areas which are generating electricity coexist within one electrolytic conductor, the effective electromotive field force recorded is the balance of all of the forces active at any one time. It is difficult to conceive how a record of electrical forces generated by a certain area can be obtained separately, and how it can be, uninfluenced by the electrical forces generated some distance away. The presence of several different areas generating electricity within a conducting medium does not permit such analysis to be made, particularly when semi-direct leads are employed. It is otherwise difficult to explain the similarity between the records obtained by direct and semi-direct leads when the exploring electrode is moved along a line perpendicular to the cardiac surface. If one assumes that any lead merely "taps" the cardiac vector, an exploring electrode moving along the same axis of derivation will record electrocardiograms of essentially the same configuration since the projections of the cardiac vector are not changed. The voltage would vary according to the distance from the electrode to the dipole center. When the distance becomes very small, as with chest leads, a somewhat disproportionate representation of the positive or negative pole may theoretically occur. It is possible but not probable that this disproportionate representation in the instance of direct leads might provide increased information on "localized" abnormalities. It is believed that nearly all of the significant information pertaining to the balance of forces throughout the periods of depolarization and repolarization is contained in the cardiac vector. However, it is conceivable that in respect to the recording of the current of injury a decrease in the distance between the exploring electrode and the source of the current of injury might prove advantageous. Until more experience is gained and instrumentation is improved, final judgment should be reserved in this matter. The use of high amplitude recording and continuously moving film should provide supplementary information in connection with this problem. Although arrhythmias have not been discussed, they can also be recorded and analyzed by similar technics. The use of moving film and high amplitude recording would permit timing and analysis of atrial activity. While it is possible to derive the configuration of the various electrocardiographic leads from the spatial vectorcardiogram, it is not possible to derive the vector loop from a set of routinely recorded leads. Special leads recorded simultaneously and preferably at increased speed are necessary for this procedure. The error which can arise from even a minute error in phase can be very great, as was pointed out as early as 1920 by Fahr.⁷ Since simple vector recording devices are now available, such laborious methods of synthesis would seem to be unnecessary. Attempts have recently been made to reconstruct the cardiac vector in the horizontal plane from routinely recorded unipolar thoracic leads.³⁶ A comparison of the loops obtained in this manner with those actually recorded reveals a great many discrepancies. The basic error in attempting to reconstruct the cardiac vector from routine precordial leads derives from the fact that nearly identical unipolar precordial leads may be obtained when the spatial vectors differ markedly and represent distinctly different clinical and physiologic entities. By the technic of spatial vectorcardiography the total resultant electromotive forces of the heart are recorded. The routine unipolar and bipolar leads can be derived from the appropriate projection of the spatial vectorcardiogram. From the horizontal projection one can derive the configuration of the multiple thoracic leads; from the sagittal projection, the multiple esophageal leads; and from the frontal projection, leads I, II, III, and VR, VL and VF. By means of spatial vectorcardiography it is possible to distinguish left ventricular hypertrophy from left bundle branch block, and right ventricular hypertrophy from right bundle branch block. Such distinction is not possible at times with routine electrocardiographic leads. Furthermore, infarction of the posterior surface of the heart may be diagnosed by means of spatial vectorcardiography alone. The technic employed by the authors, which is based on a cube system of electrode placement, was found to represent the spatial position of the cardiac vector most adequately. Although this system does not utilize the principles of the equilateral triangle of Einthoven, the results can readily be linked up with the accumulated information obtained by presently available methods. The choice and preference of technic depend largely upon what one hopes to obtain from spatial vectorcardiography. If spatial vectorcardiography is to be utilized mainly to explain the electrocardiographic findings in the frontal plane, adherence to a system utilizing the Einthoven triangle might seem desirable. It is believed, however, that the inherent value of spatial vectorcardiography lies beyond this goal. Since all of the electrocardiograms obtained in man are believed to be derivatives of the spatial cardiac vector, the spatial vectorcardiogram should include all of the information which any number and combination of leads can supply. One of the advantages of the cube technic of spatial vectorcardiography lies in the ease of correlating the present routine leads with each plane. The greater yield of clinical information which can be obtained from spatial vectorcardiography as to hypertrophy, conduction disturbance and infarction is another distinct advantage.