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Display and analysis of electrocardiographic data
Display and Analysis of Electrocardiographic
Data*
HUBERT V. PIPBERGER, M.D. and HANNA A. PIPBERGER, Washington,
D
B.A.
D. C.
arrhythmias could be analyzed in detail, and electrocardiography became increasingly centered toward irregularities of the heart beat. This initial orientation was due to a large extent to the available type of display of electrocardiographic data. Thus, a wealth of new mformation was obtained. Since the diagnosis of arrhythmias requires essentially identification of P waves and QRS complexes, a more detailed analysis of the various waveforms appeared of lesser importance at this time. electrocardiographers were satisIn general, fied with available recording methods, and improvements in display of data were neglected. Analysis of other cardiac abnormalities by electrocardiogram developed on the basis of recognizable changes in shape of the P, QRS and T waves. Amplitude and duration were the main parameters for evaluation of the record. Descriptive terms such as peaked P waves or sagging S-T segments found widespread use and characterized not only the waves proper but also the whole era of electrocardiographic analysis. Relatively few attempts were made to quantitate data of the early days of electrocardiography. This trend became even stronger with the advent of unipolar leads, which were thought to represent mainly local processes in portions of the heart underlying the electrodes. Records were analyzed one by one, and for a long time no need was felt for a ‘unifying concept in interpretation of leads. In reports of electrocardiograms, terms such as “significant” and “diagnostic” Q waves could be found frequently without precise definition of the limits of diagnostic significance. In spite of such shortcomings, it remains a remarkable fact that a wealth of useful information was
of electrocardioISPLAY and analysis graphic data are mutually interdependent. Not every type of display lends itself to every Spatial information, for type of analysis. instance, cannot be conveniently derived from the scalar time-voltage records most commonly used in clinical practice. Displays of spatial vectorcardiograms on the other hand lack most of the time information of the electrocardiogram, such as P-R intervals and Q-T intervals. Any choice of analytic measurements requires necessarily the selection of a display of specific data which facilitates these measurements. It has to be kept in mind that practically all known forms of display of electrocardiographic data have some shortcomings which may make extraction of certain analytic parameters impractical or impossible. Compromises are therefore necessary, and some measurements may have to be given up in favor of others. Whenever a certain method of display is selected, it is most important to keep its shortcomings in mind. SCALAR LEAD DISPLAY Most commonly, voltage variations in time are displayed on XY coordinates where the horizontal X axis represents the time scale. Calibrated paper or film speed allows for easy time measurements along this horizontal X axis. The voltage level is indicated at any given moment by the height of the curve Calibrameasured along the vertical Y axis. tion signals allow for easy comparison with known voltage values. The original Einthoven limb leads were recorded in this fashion. The method proved Because most productive in diagnostic terms. of easy access to time information, cardiac
* From the Veterans Administration Research Center for Cardiovascular Data Processing, Mt. Alto Veterans Administration Hospital, and the Department of Medicine, Georgetown University School of Medicine, Washington, D. C. This study was supported in part by a Public Health Service Research Grant from the National Heart Institute (CD 00064-04).
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extracted fro1 I I conventional scalar clectrocardiogratlls. Although much of the anal)Gs remained empirical and descriptive, the rcmarkable diagnostic intuition of early elcctrocardiographers will always stand as a markstonc in the development of this diagnostic tool.
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The first significant step toward a unified concept of display of electrocardiographic leads was made by Einthoven and co-workers’ QRS axis.” It in describing the “manifest was derived from the three original limb leads. Voltage levels were plotted on the sides of an equilateral triangle, the Einthoven triangle. Perpendicular projections of these lead voltages result in the well known frontal plane QRS axis. Since no leads outside of this plane were used, the display was limited to two dimensions. It was obvious to Einthoven that variations of voltage in the limb leads could be described by vectors, although the term was not used at the time. QRS axis” The significance of “manifest goes even beyond the introduction of vector concepts in electrocardiography. It represents also a classic example of data reduction, which is becoming of increasing importance in the present Construcera of electronic data processing. tion of a frontal plane vector in the Einthoven triangle makes it very obvious that only two leads are necessary to obtain this vector. Once the QRS axis is obtained, it can be projected on an infinite number of leads in this plane. Additional leads cannot contribute any new Disinformation and are therefore redundant. play of data by more than two leads in the frontal plane could have become obsolete after Einthoven’s description of the “manifest QRS axis.” Einthoven’s earIy description proved important mainly as an attempt to arrive at a unifying concept for several electrocardioIt was demonstrated that their graphic leads. deflections could be considered as one entity. Although frontal plane QRS axis remained relatively popular in the succeeding years, the concept was not extended to a significant degree. As already mentioned, this was due largely to prevailing concepts of “local” leads which were presumed to record from limited portions of the myocardium. With such a concept in mind there was no great need for analyzing Another electrocardiographic leads together. factor that played a significant role was the
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“ventricular gradient” which is derived from time integrals of various leads. Wilson and co-workers,2 who introduced this entity, measured areas above and below the baseline of limb leads during QRS and T to obtain it. Without discussing here the theoretic implications of the procedure, it is important to realize that for the first time a whole electrocardiographic complex was described by a single term. QRS axes of the frontal plane are derived mostly from peak deflections of limb leads, which are rarely simultaneous. Since only a portion of each complex is used, the resulting axis or vector cannot be considered an adequate representation of theAentire complex. Time integrals of QRS (AQRS) and T (AT) encompass, however, all parts of these waveforms. Although the concept of the ventricular gradient has found wide interest, its practical applications have never become widespread because manual determinations of area are very tedious and time-consuming. In most studies, time integrals were determined in the frontal plane only. -4 third dimension could be added only when corrected orthogonal leads with equal lead strength became available. This is a necessary condition because vectorial additions are possible only on the basis of leads with equal ratios between recorded amplitudes and strength of If such ratios current of the cardiac generator. (lead stren.gth) are uaequal, the resulting vectors will be seriously distorted. Time integrals from three corrected orthogonal leads can be and a spatial ventricular added, however, gradient can be obtained. More recently, digital computers have been used for integration of area. Time integrals of QRS and T and their sum, the spatial ventricular gradient, were found valuable aids for separating normal from abnormal records. Differentiation between various pathologic entities was relatively poor, however. This should not be a surprise because time integrals represent more or less a summary of an electrocardiographic complex. Much of the detail of the electrocardiogram is given up in this process. Except for screening procedures in epidemiologic studies, the pracresented
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\.ECTOR ELECTROCARDIOGRAPHY
After the emergence of vector concepts in electrocardiography in the ‘1930’s and 1940’s frequent attempts were made to translate the conventional 12-lead electrocardiogram into a vector representation. The method proposed by Grant3 became best known. It popularized vector concepts in electrocardiography. Since the 12-lead electrocardiogram was available to practically all clinicians, it could be used with advantage for demonstrating the feasibility of vector translations. Precordial chest leads were used in the same manner as the limb leads for construction of vectors. Thus a large number of physicians became familiar with vectorial concepts. Since the 12 leads of the conventional electrocardiogram are commonly recorded one by one, some inaccuracies are inherent in vector constructions from those leads. The magnitude of the errors was studied in this laboratory in a series of normal and abnormal electrocardiograms by using corrected orthogonal leads for comparison. Discrepancies in the onset of the QRS complex in different leads were found to be as much as 0.025 second. Peak deflections varied in time from one lead to another by a maximum of 0.024 second. Since in vector electrocardiography onsets of QRS as well as peak deflections are assumed to be simultaneous, the accuracy of vector construction becomes fairly low. This became obvious when maximal vectors plotted from peak deflections in a 12-lead electrocardiogram were compared with maximal vectors obtained from vectorcardioErrors were most striking in the horigrams. zontal plane, where they may reach 90 degrees. A further assumption made in this method is the identity of maximal QRS vectors in the frontal and horizontal planes. When spatial angles between maximal vectors of these planes were compared, it was found that they can differ by as much as 106 degrees. Determinations of spatial angles between two vectors such as QRS-T angles are also not feasible by this method. It was shown previously4 that spatial angles of less than 45 degrees when projected onto a plane may appear as an angle of approximately 180 degrees. Spatial determinations of vectors require precise knowledge of respective magnitudes which cannot be obtained from conventional leads with widely differing VOLUME
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strengths. Determination of instantaneous vectors such as 0.04 second QRS vectors are also not feasible because of the discrepancies in time of onset of the QRS complex. It soon became obvious that vector electrocardiography cannot be used for quantitative studies. The method is crude and allows only an estimation of spatial relationships that does not lead to quantitative statements. VECTOR LOOP DISPLAY
A more satisfactory method for displaying vectors is the display of vector loops. It was developed when cathode-ray oscilloscopes became available to medical researchers in the 1930’s. In contrast to the simple methods already described, this procedure leads to a continuous display of all vectorial forces generated during P, QRS and T. The technic is well known and does not need any detailed description here. Like any other method for displaying vectors, this method is based on the assumption that electromotive forces of the heart originate from a point-like fixed dipole source. This is not the place to discuss the merits or faults of the dipole theory. It should be kept in mind, however, that it was offered as a first approximation only. As such it has proved most helpful, and no other concept is available at present to replace it. The dipole hypothesis offered the electrocardiographer for the first time a rational basis for quantitation of the entire body surface electrocardiogram. Search for additional multipole components has not yet yielded any useful diagnostic information which could be applied in clinical practice. A complete description of vector variations in magnitude and direction can be given by a three-lead orthogonal lead system. Although these continuous vectorial changes during P, QRS and T occur in three-dimensional space, vector loops are displayed for convenience in two dimensions as planar projections. The feasibility of three-dimensional displays has been demonstrated, but fairly complex instruPlanar mentation is needed for this purpose. displays are most commonly recorded in the frontal, sagittal and horizontal planes, which are mutually perpendicular. For each such projection, two orthogonal leads are connected to the plates of an oscilloscope, and the cathoderay beam is deflected along their two axes resulting in the well-known display of vector loops. For timing purposes the beam is interrupted at
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regular intervals which allows time measurements when the beam is moving fast enough. Permanent records can be obtained by taking photographs from the oscilloscope screen. The leads for obtaining vector loops should mutually perpendicular. be, as mentioned, Furthermore, they should have the same strength, i.e., the ratio between recorded amplitudes and strength of current of the cardiac generator should be the same in all leads. In the original vectorcardiographic lead systems, bipolar and unipolar leads were used whose directions were assumed to be vertical, sagittal and horizontal. Biophysicists, later studying the performance of leads in more detail, found that this is rarely the case for such conventional leads. Problems related to lead performance have been discussed elsewhere.6 The variability in lead performance of the early vectorcardiographic systems led to large discrepancies of display in vector loops when different systems were applied to the same subject. Discrepancies in direction up to 180 degrees were found beof these older systems. The tween some more recently developed corrected orthogonal leads5 were found to be considerably more constant in lead strength and direction, and the various corrected systems became comparable in quantitative terms. Constancy in lead performance is of utmost importance because quantitative analysis of records can hardly be justified when recording procedures vary from one subject to another. Thus, the spread of findings in various diagnostic groups may become unduly large and prohibitive for efficient differentiation. Display of planar vector loop projections has become relatively popular as an adjunct to the conventional 12-lead electrocardiogram. As already mentioned, vector loop displays do not allow time measurements in between complexes, Furthersuch as P-R, S-T and T-P segments. Time more, heart rates cannot be determined. markings of small loops such as P and T are almost always fused with each other, which also prevents time measurements such as P The same holds true for the beginduration. ning and end of QRS loops which are frequently hidden by P and T loops. Furthermore, these portions of QRS may be perpendicular to a given plane and not be represented at all. Determinations of instantaneous vectors from QRS loops are, therefore, not feasible with any however, accuracy. They can be obtained, when simultaneously recorded scalar orthog-
and
Pipherqer onal lcads are available in addition to the loop display. In the analysis of vector loop projections the main emphasis has been put on loop configuraand rotation. Main QRS, tion, orientation T or P axes were derived from maximal vectors or half-area vectors, which could be used for quantitative analysis. The shape of loops was found characteristic for certain abnormalities but could hardly be translated into numerical terms. The vector loop display was found an excellent means for teaching purposes because a composite picture of the heart’s electromotive forces was presented. In order to recover part of the lost temporal information in vector loop displays, recordings were also made on running film. Since this procedure leads to serious distortions of direction of vectors it has never been widely accepted. POLAR VECTORS AND EIGENVECTORS A new type of electrocardiographic display of data based on corrected orthogonal leads was developed after the introduction of the polar This term vector as a diagnostic parameter.6J represents an extension of Einthoven’s QRS axis of the frontal plane into three-dimensional The spatial orientation of a vector loop space. is defined by an axis perpendicular to its largest projection. The magnitude of this axis (polar vector) is given by the area enclosed by the loop. Any shift or rotation of the loop will always lead to a concomitant shift of the polar The plane of the projection of the vector. largest loop or principal plane can be determined by loop rotations in space by using a lead Alternate procedures are based on resolver.* loop planimetry in three planes6 or fitting the least square of a plane in the loop.9 When two mutually perpendicular axes are added to the polar vector, a new orthogonal frame of reference is obtained which is independent of the position or rotation of the heart. This type of loop analysis in its own frame of reference leads In the leastto relatively uniform standards. square fitting procedure9 the three axes in the new frame of reference are designated as Eigenvectors, which have both magnitude and direcintion. * Ratios between these Eigenvectors * Eigenvectors obtained by the least-square fitting procedure may differ slightly in direction and magnitude from axes obtained by resolution of leads. The latter are always referred to point E of the vector loop. Eigenvectors are based on loop configuration only. The difference was not found significant for analytic purposes. THE
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dicate the planarity of the loop and its main configuration (length/width). It is also possible to record scalar leads along these axes which contain the time information necessary for recognition of arrhythmias. lo-i3 McFee and Abildskov and their associates”J3 proposed the electrocardiogram” for this term “normalized One of the 3 new orthogtype of recording. onal leads is an indicator of the planarity of the loop keeping deflections at a minimum; a and second is kept equally positive and negative; the third one represents a maximal deflection. Analysis of the vectorcardiogram in its own frame of reference was found an excellent means for separation of normal from abnormal records.12 A comparison of the diagnostic information in Eigenvectors with other procedures showed, however, that discrimination between pathologic groups was relatively poor.14 It did not exceed that of time integrals. It is conceivable, however, that analysis of detail in normalized scalar leads will reveal diagnostic features not given by the spatial magnitude and direction of Eigenvectors. The same type of normalization and analysis has been performed on T and P 10ops.9,~~ Whereas Eigenvectors of T did not add significantly to other types of analysis of T, the polar P vector in conjunction with the maximal P vector was found an excellent means for separation of the normal P from P pulmonale and P mitrale.14 CURVES OF SPATIAL MAGNITUDE, ORIENTATION AND VELOCITY Since it was recognized for a long time that vector loop displays lacked very essential time information, other methods of display had to be explored to overcome this shortcoming. McFeel6 and Sayers and Abildskov and Hellerstein and their co-workers1+18 introduced a recording method of spatial data on a linear time scale. Analog computers are used for The spatial orienthis most ingenious display. tation of vectors is given by two angular curves, indicating azimuth and elevation angles of a Thus, a continuous Cartesian reference frame. display of spatial data on a time scale is presented in much the same way as in a scalar electrocardiographic lead. At any given moment, spatial magnitude, orientation and velocity can be readily determined with precision. Thus, the combined information of the scalar electrocardiogram and spatial vectorcardiogram becomes available in one set of curves. VOLUME
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The practical usefulness of this type of displa) of data has been evaluated recently on a staSince a large variability in contistical basis.19 figuration of curves of various pathologic entities was found, a clear separation was not possible on In a different the basis of configuration only. approach, consecutive points of data on each It curve were compared with normal ranges. was found that most of the diagnostic information is contained in records of spatial orientation, spatial magnitude and velocity contributing considerably less. This point-by-point analysis leads, however, to a prohibitive number of false positive findings that may reach 25 per cent for a single curve. Since ranges of spatial direction, defined by azimuth and elevation angles, lose their true ellipsoid form and become rectangular, a certain number of false negative findings cannot be excluded.19 Although the statistical pitfalls of this type of analysis can be overcome by certain types of numerical analysis, the possibility of graphic display of data is lost in the handling procedures of strictly numerical data. Display of electrocardiograms in form of time-based curves of spatial magnitude, orientation and velocity is the most comprehensive representation of electrocardiographic data. However, diagnostic information can be extracted only with great difficulty. It is questionable, therefore, whether the relatively great cost of the required analog computers can be justified for this display. ANALYSIS BY DIGITAL COMPUTER Since analysis of electrocardiograms by digital computer is not necessarily linked with any type of display, a detailed description of comComputational technic is not necessary here. puter outputs can be printed out in almost any graphic form, but such types of display are not an essential part of the computation proper. It is important to realize that all methods of digital computer analysis of the electrocardiogram are based exclusively on numerical data. Thus, many complex mathematical operations can be performed that do not necessarily lend themselves to a graphic or optical representation. The lack of a visual display has to be weighed against the greater efficiency of complex numerical classification methods.gPr4 Since accuracy and reliability of diagnosis will always remain the basic goal in analysis of the electrocardiogram, precision of diagnostic decisions will have to be the de-
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Avcrag-e Rate of Recognition
( 51 ______~. 1. 818 instantanrous 2. 3. 4. 5. 6. 7.
vectors 5 initial and 5 terminal instantaneous vcctors (at 0.01 sec. intervals) Q amplitude. Q duration and R amplitude of 3 scalar orthogonal leads Q duration. Q amplitude, K amplitude and S amplitude of 3 scalar orthogonal leads QRS Eigenyectors SAQRS (time integral of QRS) Spatial maximal QRS vectors
96 94 57 63 51 50 46
The listed li,gurcs indicate the percentage of records correctly classifird in diagnostic groups. This procedure has been reported in more detail previously.r4 High rates of recognition indicate a high content of diagnostic information. Multiple instantaneous vectors yielded most of this information (groups 1 and 2). Scalar measurements were decidedly inferior to spatial data (groups 3 and 4). Eigenvectors, time integrals and maximal vectors were least informative (groups 5, 6 and 7). The 818 instantaneous vectors were obtained by dividing QRS in time into 8 equal parts.
factor for the selection of analytic methods. A typical demonstration of the essential difference between visual analyses and computer methods has been given for differential diagnosis.‘* On the basis of multiple instantaneous vectors, computations were performed in 24dimensional space. This procedure was found more efficient than any other analytic method. Although multidimensional operations do not represent a limitation mathematically, the electrocardiographer is hardly able to extend three-dimensional space conceptually. Analysis in multidimensional space is limited, therefore, to numerical expressions without direct visual representation. A side-product of digital computer analysis of electrocardiograms has been the determination of diagnostic information in various types of display or electrocardiographic measurements. A comparative listing of QRS parameters is given in Table I. Multiple instantaneous vectors yielded most of the diagnostic information. This should not be surprising because a considerable amount of detail is provided by a multitude of points. The information in measurements from scalar cisive
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Pipberger lcads was markedly less (Q amplitude and duration, K amplitude). When determination of the S amplitude was added, the diagnostic information increased slightly. This finding was of considerable interest because advantages and disadvantages of scalar vs. spatial analysis of the electrocardiogram have been argued for many years. Determination of the diagnostic information available in the two procedures provided for the first time a quantitative answer to this problem. A rather surprising degree of superiority of spatial data was found. This linding, however, does not imply selection of a given form of spatial electrocardiographic display. It only proves that time relations between leads, which are largely neglected in measurements of single leads, are essential for efficient diagnostic classification. The importance of simultaneous recording of scalar orthogonal leads is stressed by this finding. The information content of time integrals, Eigenvectors and maximal QRS vectors was considerably less than that of multiple instantaneous vectors. As already mentioned, these measurements represent mainly a summary of electrocardiographic complexes. DIFFERENTIAL ELECTROCARDIOGRAPHY A very simple electrocardiographic display method based on results of digital computations is being developed at this laboratory at present. Figure 1 shows the basic problem of diagnostic classification in electrocardiography. Ranges of electrocardiographic measurements are projected on two arbitrarily selected leads. Almost as a rule such projections overlap. A definite diagnostic decision cannot be made for any record falling in the range of overlap. When sufficiently large samples of records from various diagnostic entities are available, however, axes can be computed which result in optimal separations of pathologic groups. Leads taken along these axes will discriminate best between pathologic groups. Directions of such leads have been determined for the majority of diagnostic entities. It appears at present that a set of 5 leads may suffice for practical purposes. A typical example of such a lead separates for instance “left ventricular hypertrophy” from “anteroseptal infarct.” An attempt is being made to base the differentiation on very simple measurements such as presence or absence of Q waves, Q/R and R/S ratios, etc. Thus, reading and interpretation remain as convenient as possible. Since scalar THE
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leads are used in this display, no time information is lost and analysis of arrhythmias can be performed with the same tracings. The described tracings are obtained in the following manner. An ordinary set of electrodes as used for recording corrected orthogonal leads is applied. Instead of the regularly used resistor networks which yield orthogonal lead directions, new sets of resistors were computed. These combinations of resistors represent basically fixed settings of a lead resolver. Through lead resolution an infinite number of directions of leads can be obtained. Since only a limited number of differential leads is required, a continuous resolver can be replaced by a few fixed resolver settings. Each of these fixed positions requires a new set of resistors. A cathode-follower is necessary because most commercial direct-writing electrocardiographs have only a relatively low input impedance. Recording of differential electrocardiographic leads proved very rewarding because record analysis is reduced to a very few simple steps. Most of the parameters can be evaluated by simple inspection. A small box containing a lead-selector switch with the necessary resistors and cathode-follower can easily be attached to any portable electrocardiograph at a low cost.
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FIG. 1. Two typical ranges of vectors are shown in the middle of the diagram. They represent two diagnostic entities. Typically, such vector ranges have the form of ellipsoids and are three-dimensional. Projections of the ranges on two arbitrarily chosen electrocardiographic leads X and Y, result in overlapping distribution curves of findings. In the range of overlap, definite diagnostic decisions cannot be made. A lead axis can be computed, however, which results in an optimal separation of the two diagnostic entities (ax + /3Y). Such leads can bc determined for differentiation between various pathologic groups. (Reprinted from Progress in Cardiovascular Diseases9 by permission of Grune and Stratton, Inc.)
ANALYSIS OF HIGH FREQUENCY COMPONENTS High frequency components of the electrocardiogram consist of small fast notches, slurs and segments of high velocity. They cannot be recorded with slow-moving direct-writing electrocardiographs. In recent years the importance of such components has been pointed out repeatedly.20,21 High frequency content of the electrocardiogram was found increased after myocardial infarction. The question to be answered is whether or not this finding represents independent information. If in certain cases diagnoses of infarction can be made exclusively on the basis of high frequency components without any other electrocardiographic evidence for infarction, research in this field becomes extremely important. The presently available direct-writing electrocardiographs would not be adequate for the display of components of high velocity. DISCUSSION AND CONCLUSIONS This review of methods of electrocardiographic display and analysis can by no means be considered complete. First, an attempt was made VOLUME
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to consider certain advantages and disadvantages of methods in general use. Second, some of the more recent developments in this field were discussed. Those who have not followed electrocardiographic literature closely may consider the present situation rather bewildering because too many new concepts have been introduced. A closer look shows, however, that a certain consistency can be found in present trends which facilitates their evaluation. In the last two decades electrocardiography has, like many other medical tools, been subjected to a very thorough analysis by biophysicists, mathematicians and biostatisticians. It was to be expected that methods of display and analysis were profoundly influenced by this development. One of the major concepts to change display and analysis of the electrocardiogram was the introduction of the dipole hypothesis. This theory states that all electromotive forces generated by the heart can be considered as originating from a fixed-point dipole source.
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The \.alidit>. of this first approximation has been discussed elsewhere in this symposium. The assumption of “local” effects on certain leads contradicts the dipole theory. Accordin,q to present knowledge, precordial leads may contain a minor contribution from nondipolar sources. An arbitrarily selected R wave of a precordial lead, for instance, may consist of 85 per cent dipolar and 15 per cent nondipolar content. Depending upon the polarity of the nondipolar content, the R amplitude may be Visual increased or decreased by a fraction. analysis would in both cases not reveal which part of the R wave is of dipolar or nondipolar origin. Until leads which will differentiate between dipolar and nondipolar sources become available and practical, the latter need not be considered. With presently available display methods dipolar contributions predominate to such an extent that any search for multipole components in ordinary electrocardiographic records remains futile. The present inability to recognize or define any portion of the electrocardiogram as nondipolar leaves us with a very powerful basic tool, the dipole concept. Dipole variations in time, in magnitude and direction can be completely described by three orthogonal leads. Records of dipole variations can be obtained in the form of scalar leads, vector loops, or any Selection other type of display of spatial data. of the type of display has become now strictly a problem of practicality in terms of accessibility of the information and ease of analysis. Although it has become obvious that precordial leads contain predominantly dipolar information, these leads were not found an efficient means for the display of dipole variations. Some of the reasons have been discussed in conjunction with vector electrocardiography. Since extraction of “local” information proved impractical also, the question arises whether or not recording of precordial leads should be To justify their further use has continued. become extremely difficult. Tradition alone can hardly be considered sufficient reason to obtain a series of records from each patient without rational basis. They can be used only for empirical and descriptive analysis without contributing any new information not already contained in simpler three-lead orthogonal lead systems. Practically all recent advances in electrocardiographic display methods were based on the dipole concept. The great variety of
and Pipberger new display procedures can be easily understood when their common source of information is remembered. Whether we consider displays of vector loops, curves of spatial magnitude and orientation or any other type of spatial data presentation, we can always refer to the same origin, the dipole, whose variations in time are expressed in terms of spatial magnitude and orientation. This unifying concept has greatly facilitated teaching and understanding of electrocardiography. The approach to analysis of electrocardiograms has changed considerably in the last decades. Whereas empirical analysis and intuition provided a wealth of valid information in the past, more recently a general trend toward quantitation of electrocardiographic data developed. As a first step, normal ranges were determined for single measurements such as Q duration or Q amplitude. This led to a separation of normal from abnormal findings without precise differentiation of pathologic entities. With the advent of automatic data processing technic, more complex statistical tools became available. One analytic procedure could easily be compared with another, and the results could be expressed numerically in terms of diagnostic recognition rates.14 Thus, a quantitative evaluation of diagnostic and analytic procedures became possible. The probability of being in a certain diagnostic category could be determined for each record at the same time. Results of the evaluation of various analytic procedures have been discussed. It was found that the diagnostic information content of spatial measurements exceeded largely that of scalar records when they were taken one by one. Since all spatial information can also be obtained from simultaneously recorded scalar orthogonal leads, it became obvious that the simultaneity of recording represented the main factor. From there it follows that recording of scalar leads in sequence will always be accompanied by a loss of diagnostic information. A further finding was the limited information available by all methods which use a few representative terms for a whole electrocardiographic record. Time integrals such as the ventricular gradient, polar vectors, Eigenvectors and similar items were of only limited value in diagnostic differentiation. They represent powerful tools, however, when normal records need to be separated from abnormal ones without specific diagnosis. It became obvious from these studies that a THE
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certain minimum of detailed information needs to be provided to achieve reasonable efficiency in diagnostic classification. At present it appears that eight points during QRS and eight points during S-T and T may be sufficient for this purpose. Instantaneous vectors need to be determined for each of these points. Frequency-density-distribution functions were found an efficient means for diagnostic classification of individual records. Although such procedures offer great advantages for efficient analysis of electrocardiographic data, their complexity requires use of digital computers. The display of such data as well as the graphic representation of their analysis in multidimensional space are impractical. The question arises, therefore, whether electrocardiographic display will remain an essential part of electrocardiography in the It has become a restraint rather than future. an advantage. It cannot be expected that facilities for data processing will become available to all practicing physicians in the near future. Other methods need to be developed, therefore, to bridge the obvious gap. Differential electrocardiography, as described, may be a step in this direction. Results of analyses of large record samples by computers represent the basis of this method. Some of the advantages obtained in more complex computational procedures could thus be incorporated into daily practice, using singlechannel electrocardiographic recorders which are commonly available now. One of the main drawbacks of conventional analysis of the electrocardiogram has always been the inconsistency in interpretation.22 It is not very realistic to use the skills of trained electrocardiographers as a general standard. The increasing replacement of subjective evaluations by quantitative statements may lead to another risk, however. The persuasiveness of numbers is great and should not be underestimated. As long as the limitations of the electrocardiogram as a diagnostic tool are kept in mind, the advantages of quantitative analysis The will always outweigh the disadvantages. electrocardiogram should always remain one building stone only in the over-all evaluation of the patient. SUMMARY
A review of the most common types of electrocardiographic display and analysis, including those developed more recently, was given. VOLUME
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Advantages and disadvantages of scalar lead recordings, vector loop displays, curves of spatial magnitude, orientation and velocity, polar vectors and Eigenvectors were evaluated. Analysis of data from these displays was compared with analysis by digital computer, which is based almost exclusively on numerical terms. Since graphic display of numerical operations proved impractical, the question was raised, whether or not display of electrocardiographic data represents an unnecessary restraint for efficient analysis. A new method of differential electrocardiography is described. It is based on computed entities. Leads ranges of various diagnostic which discriminate best between diagnostic groups can be obtained by resolution of orthogonal leads. Some of the results of large scale computations can thus be incorporated in routine electrocardiography. Quantitative comparison of various analytical procedures showed a marked superiority of spatial parameters over scalar leads recorded in sequence. Rates of diagnostic recognition were best when a series of instantaneous vectors was available for QRS, S-T and T. The ventricular gradient, polar vectors, Eigenvectors and curves of spatial magnitude, orientation and velocity were found less efficient for diagnostic classification. REFERENCES 1. EINTHOVEN,W., FAHR, G. and DEWAART, A. ober die Richtung und die manifeste G&se der Potentialschwankungen in men&lichen Herzen und iiber den Einfluss der Herzlage auf die Form des Elektrokardiogramms. Arch. ges. Physiol., 150 : 275, 1913. 2. WILSON, F. N., MCLEOD, A. G., BARKER, P. S. and JOHNSTON,F. D. The determination and the significance of the areas of ventricular deflections of the electrocardiogram. Am. Heart J., 10: 46, 1934. 3. GRANT, R. P. Clinical Electrocardiography. The Spatial Vector Approach. New York, 1957. McGraw-Hill. 4. BALL, M. F. and PIPBERGER, H. V. The normal spatial QRS-T angle of the orthogonal vectorcardiogram. Am. Heart J., 56: 611, 1958. 5. PIPBERGER, H. V. Current status and persistent problems of electrode placement and lead systems for vectorcardiography and electrocardiography. Prog. Cardiovas. Dir., 2: 248, 1959. 6. BURGER, H. C. and VAANE, J. P. A criterion characterizing the orientation of a vectorcardiogram in space. Am. Heart J., 56: 29, 1958. Quantitative Vector Electrocardi7. BRI~BERG, L oaraohv. Baltimore. Md. 1960. Waverlv Press. 8. SC&IT~, ‘0. H. Cathode-ray presentation of three dimensional data. J. Appl. Physics, 18: 819, 1947.
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Pipberger H. V., STALLMANN, I’. LV., YAN(J, K. and DRAPER, H. W. Digital computer analysis of the normal and abnormal electrocardiogram. Prog. Cardiouas. Dis.. 5: 378, 1963. RIJLANT, P. L’i?lectrogendse globale du coeur chez l’homme. Electrocardiographic vectorielle et vectorcardiographie. Acta cardiol., 13: 349, 1958. MCFEE, R., WILKINSON, R. S. and ABILDSKOV,J. A. On the normalization of the electrical orientation of the heart and the representation of electrical axis by means of an axis map. Am. Heart J., 62: 391, 1961. PIPBERGER,H. V. and CARTER,T. N. Analysis of the normal and abnormal vectorcardiogram in its own reference frame. Circulation,25 : 827, 1962. ABILDSKOV,J. A., MCFEE, R. and SCHECTER,G. L. Further observations with the normalized electrocardiogram and axis map. Am. Heart J., 65: 220, 1963. PIPBERGER, H. V. and STALLMANN, F. W. Computation of differential diagnosis in electrocardiography. Ann. New York Acad. SC., in press. MCFEE, R. A trigonometric computer with electrocardiographic application. Rev. Scientifi Z&r., 21: 420, 1950. SAYERS, B. McA., SILBERBERG,F. G. and DURIE,
9. PIPBERGER,
10.
11.
12.
13.
14.
15.
16.
and Piptxrger I). F. The electrocardiographic spatial maqnitude curve in man. .4m. Heart J., 49: 323, 1955. 17. ARILDSKOV, .J. A., INGERSON,W. B. and HISEY, B. L. A linear time scale for spatial vectorcardiographic data. Circulation, 14: 556, 1956. 18. HELLERSTEIN, H. K. and HAMLIN, R. QRS component of the spatial vectorcardiogram and of the spatial magnitude and velocity electrocardiograms of the normal dog. Am. J. Cordial., 6: 1049, 1960. 19. YANO, K. and PIPBERGER, H. V. Spatial magnitude, orientation and velocity of the normal and abnormal QRS complex. Circulation, 29: 107, 1964. 20. LANGNER, P. H., JR., GESELOWITZ, D. V. and MANSURE, F. T. High frequency components in the electrocardiograms of normal subjects and of patients with coronary heart disease. Am. Heart J., 62: 746, 1961. 21. FRANKE, E. K., BRAUNSTEIN, I. F. and ZILLNER, D. C. Study of high frequency components in electrocardiogram by power spectrum analysis. Circulation Res., 10: 870, 1962. 22. SEGALL, H. N. The electrocardiogram and its interpretation: A study of reports by 20 physicians on a set of 100 electrocardiograms. Canad. M. A. J., 82: 2, 1960.
THE AMERICAN JOURNAL OF CARDIOLOGY
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HUBERT V. PIPBERGER, M.D. and HANNA A. PIPBERGER, Washington,
D
B.A.
D. C.
arrhythmias could be analyzed in detail, and electrocardiography became increasingly centered toward irregularities of the heart beat. This initial orientation was due to a large extent to the available type of display of electrocardiographic data. Thus, a wealth of new mformation was obtained. Since the diagnosis of arrhythmias requires essentially identification of P waves and QRS complexes, a more detailed analysis of the various waveforms appeared of lesser importance at this time. electrocardiographers were satisIn general, fied with available recording methods, and improvements in display of data were neglected. Analysis of other cardiac abnormalities by electrocardiogram developed on the basis of recognizable changes in shape of the P, QRS and T waves. Amplitude and duration were the main parameters for evaluation of the record. Descriptive terms such as peaked P waves or sagging S-T segments found widespread use and characterized not only the waves proper but also the whole era of electrocardiographic analysis. Relatively few attempts were made to quantitate data of the early days of electrocardiography. This trend became even stronger with the advent of unipolar leads, which were thought to represent mainly local processes in portions of the heart underlying the electrodes. Records were analyzed one by one, and for a long time no need was felt for a ‘unifying concept in interpretation of leads. In reports of electrocardiograms, terms such as “significant” and “diagnostic” Q waves could be found frequently without precise definition of the limits of diagnostic significance. In spite of such shortcomings, it remains a remarkable fact that a wealth of useful information was
of electrocardioISPLAY and analysis graphic data are mutually interdependent. Not every type of display lends itself to every Spatial information, for type of analysis. instance, cannot be conveniently derived from the scalar time-voltage records most commonly used in clinical practice. Displays of spatial vectorcardiograms on the other hand lack most of the time information of the electrocardiogram, such as P-R intervals and Q-T intervals. Any choice of analytic measurements requires necessarily the selection of a display of specific data which facilitates these measurements. It has to be kept in mind that practically all known forms of display of electrocardiographic data have some shortcomings which may make extraction of certain analytic parameters impractical or impossible. Compromises are therefore necessary, and some measurements may have to be given up in favor of others. Whenever a certain method of display is selected, it is most important to keep its shortcomings in mind. SCALAR LEAD DISPLAY Most commonly, voltage variations in time are displayed on XY coordinates where the horizontal X axis represents the time scale. Calibrated paper or film speed allows for easy time measurements along this horizontal X axis. The voltage level is indicated at any given moment by the height of the curve Calibrameasured along the vertical Y axis. tion signals allow for easy comparison with known voltage values. The original Einthoven limb leads were recorded in this fashion. The method proved Because most productive in diagnostic terms. of easy access to time information, cardiac
* From the Veterans Administration Research Center for Cardiovascular Data Processing, Mt. Alto Veterans Administration Hospital, and the Department of Medicine, Georgetown University School of Medicine, Washington, D. C. This study was supported in part by a Public Health Service Research Grant from the National Heart Institute (CD 00064-04).
VOLUME14, SEPTEMBER1964
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extracted fro1 I I conventional scalar clectrocardiogratlls. Although much of the anal)Gs remained empirical and descriptive, the rcmarkable diagnostic intuition of early elcctrocardiographers will always stand as a markstonc in the development of this diagnostic tool.
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Pipberger general availability of’ one-channel recorders, which by itself does not express the need for unifying concepts in electrocardiograph)-. VENTRICULAR A
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The first significant step toward a unified concept of display of electrocardiographic leads was made by Einthoven and co-workers’ QRS axis.” It in describing the “manifest was derived from the three original limb leads. Voltage levels were plotted on the sides of an equilateral triangle, the Einthoven triangle. Perpendicular projections of these lead voltages result in the well known frontal plane QRS axis. Since no leads outside of this plane were used, the display was limited to two dimensions. It was obvious to Einthoven that variations of voltage in the limb leads could be described by vectors, although the term was not used at the time. QRS axis” The significance of “manifest goes even beyond the introduction of vector concepts in electrocardiography. It represents also a classic example of data reduction, which is becoming of increasing importance in the present Construcera of electronic data processing. tion of a frontal plane vector in the Einthoven triangle makes it very obvious that only two leads are necessary to obtain this vector. Once the QRS axis is obtained, it can be projected on an infinite number of leads in this plane. Additional leads cannot contribute any new Disinformation and are therefore redundant. play of data by more than two leads in the frontal plane could have become obsolete after Einthoven’s description of the “manifest QRS axis.” Einthoven’s earIy description proved important mainly as an attempt to arrive at a unifying concept for several electrocardioIt was demonstrated that their graphic leads. deflections could be considered as one entity. Although frontal plane QRS axis remained relatively popular in the succeeding years, the concept was not extended to a significant degree. As already mentioned, this was due largely to prevailing concepts of “local” leads which were presumed to record from limited portions of the myocardium. With such a concept in mind there was no great need for analyzing Another electrocardiographic leads together. factor that played a significant role was the
in
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“ventricular gradient” which is derived from time integrals of various leads. Wilson and co-workers,2 who introduced this entity, measured areas above and below the baseline of limb leads during QRS and T to obtain it. Without discussing here the theoretic implications of the procedure, it is important to realize that for the first time a whole electrocardiographic complex was described by a single term. QRS axes of the frontal plane are derived mostly from peak deflections of limb leads, which are rarely simultaneous. Since only a portion of each complex is used, the resulting axis or vector cannot be considered an adequate representation of theAentire complex. Time integrals of QRS (AQRS) and T (AT) encompass, however, all parts of these waveforms. Although the concept of the ventricular gradient has found wide interest, its practical applications have never become widespread because manual determinations of area are very tedious and time-consuming. In most studies, time integrals were determined in the frontal plane only. -4 third dimension could be added only when corrected orthogonal leads with equal lead strength became available. This is a necessary condition because vectorial additions are possible only on the basis of leads with equal ratios between recorded amplitudes and strength of If such ratios current of the cardiac generator. (lead stren.gth) are uaequal, the resulting vectors will be seriously distorted. Time integrals from three corrected orthogonal leads can be and a spatial ventricular added, however, gradient can be obtained. More recently, digital computers have been used for integration of area. Time integrals of QRS and T and their sum, the spatial ventricular gradient, were found valuable aids for separating normal from abnormal records. Differentiation between various pathologic entities was relatively poor, however. This should not be a surprise because time integrals represent more or less a summary of an electrocardiographic complex. Much of the detail of the electrocardiogram is given up in this process. Except for screening procedures in epidemiologic studies, the pracresented
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\.ECTOR ELECTROCARDIOGRAPHY
After the emergence of vector concepts in electrocardiography in the ‘1930’s and 1940’s frequent attempts were made to translate the conventional 12-lead electrocardiogram into a vector representation. The method proposed by Grant3 became best known. It popularized vector concepts in electrocardiography. Since the 12-lead electrocardiogram was available to practically all clinicians, it could be used with advantage for demonstrating the feasibility of vector translations. Precordial chest leads were used in the same manner as the limb leads for construction of vectors. Thus a large number of physicians became familiar with vectorial concepts. Since the 12 leads of the conventional electrocardiogram are commonly recorded one by one, some inaccuracies are inherent in vector constructions from those leads. The magnitude of the errors was studied in this laboratory in a series of normal and abnormal electrocardiograms by using corrected orthogonal leads for comparison. Discrepancies in the onset of the QRS complex in different leads were found to be as much as 0.025 second. Peak deflections varied in time from one lead to another by a maximum of 0.024 second. Since in vector electrocardiography onsets of QRS as well as peak deflections are assumed to be simultaneous, the accuracy of vector construction becomes fairly low. This became obvious when maximal vectors plotted from peak deflections in a 12-lead electrocardiogram were compared with maximal vectors obtained from vectorcardioErrors were most striking in the horigrams. zontal plane, where they may reach 90 degrees. A further assumption made in this method is the identity of maximal QRS vectors in the frontal and horizontal planes. When spatial angles between maximal vectors of these planes were compared, it was found that they can differ by as much as 106 degrees. Determinations of spatial angles between two vectors such as QRS-T angles are also not feasible by this method. It was shown previously4 that spatial angles of less than 45 degrees when projected onto a plane may appear as an angle of approximately 180 degrees. Spatial determinations of vectors require precise knowledge of respective magnitudes which cannot be obtained from conventional leads with widely differing VOLUME
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strengths. Determination of instantaneous vectors such as 0.04 second QRS vectors are also not feasible because of the discrepancies in time of onset of the QRS complex. It soon became obvious that vector electrocardiography cannot be used for quantitative studies. The method is crude and allows only an estimation of spatial relationships that does not lead to quantitative statements. VECTOR LOOP DISPLAY
A more satisfactory method for displaying vectors is the display of vector loops. It was developed when cathode-ray oscilloscopes became available to medical researchers in the 1930’s. In contrast to the simple methods already described, this procedure leads to a continuous display of all vectorial forces generated during P, QRS and T. The technic is well known and does not need any detailed description here. Like any other method for displaying vectors, this method is based on the assumption that electromotive forces of the heart originate from a point-like fixed dipole source. This is not the place to discuss the merits or faults of the dipole theory. It should be kept in mind, however, that it was offered as a first approximation only. As such it has proved most helpful, and no other concept is available at present to replace it. The dipole hypothesis offered the electrocardiographer for the first time a rational basis for quantitation of the entire body surface electrocardiogram. Search for additional multipole components has not yet yielded any useful diagnostic information which could be applied in clinical practice. A complete description of vector variations in magnitude and direction can be given by a three-lead orthogonal lead system. Although these continuous vectorial changes during P, QRS and T occur in three-dimensional space, vector loops are displayed for convenience in two dimensions as planar projections. The feasibility of three-dimensional displays has been demonstrated, but fairly complex instruPlanar mentation is needed for this purpose. displays are most commonly recorded in the frontal, sagittal and horizontal planes, which are mutually perpendicular. For each such projection, two orthogonal leads are connected to the plates of an oscilloscope, and the cathoderay beam is deflected along their two axes resulting in the well-known display of vector loops. For timing purposes the beam is interrupted at
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regular intervals which allows time measurements when the beam is moving fast enough. Permanent records can be obtained by taking photographs from the oscilloscope screen. The leads for obtaining vector loops should mutually perpendicular. be, as mentioned, Furthermore, they should have the same strength, i.e., the ratio between recorded amplitudes and strength of current of the cardiac generator should be the same in all leads. In the original vectorcardiographic lead systems, bipolar and unipolar leads were used whose directions were assumed to be vertical, sagittal and horizontal. Biophysicists, later studying the performance of leads in more detail, found that this is rarely the case for such conventional leads. Problems related to lead performance have been discussed elsewhere.6 The variability in lead performance of the early vectorcardiographic systems led to large discrepancies of display in vector loops when different systems were applied to the same subject. Discrepancies in direction up to 180 degrees were found beof these older systems. The tween some more recently developed corrected orthogonal leads5 were found to be considerably more constant in lead strength and direction, and the various corrected systems became comparable in quantitative terms. Constancy in lead performance is of utmost importance because quantitative analysis of records can hardly be justified when recording procedures vary from one subject to another. Thus, the spread of findings in various diagnostic groups may become unduly large and prohibitive for efficient differentiation. Display of planar vector loop projections has become relatively popular as an adjunct to the conventional 12-lead electrocardiogram. As already mentioned, vector loop displays do not allow time measurements in between complexes, Furthersuch as P-R, S-T and T-P segments. Time more, heart rates cannot be determined. markings of small loops such as P and T are almost always fused with each other, which also prevents time measurements such as P The same holds true for the beginduration. ning and end of QRS loops which are frequently hidden by P and T loops. Furthermore, these portions of QRS may be perpendicular to a given plane and not be represented at all. Determinations of instantaneous vectors from QRS loops are, therefore, not feasible with any however, accuracy. They can be obtained, when simultaneously recorded scalar orthog-
and
Pipherqer onal lcads are available in addition to the loop display. In the analysis of vector loop projections the main emphasis has been put on loop configuraand rotation. Main QRS, tion, orientation T or P axes were derived from maximal vectors or half-area vectors, which could be used for quantitative analysis. The shape of loops was found characteristic for certain abnormalities but could hardly be translated into numerical terms. The vector loop display was found an excellent means for teaching purposes because a composite picture of the heart’s electromotive forces was presented. In order to recover part of the lost temporal information in vector loop displays, recordings were also made on running film. Since this procedure leads to serious distortions of direction of vectors it has never been widely accepted. POLAR VECTORS AND EIGENVECTORS A new type of electrocardiographic display of data based on corrected orthogonal leads was developed after the introduction of the polar This term vector as a diagnostic parameter.6J represents an extension of Einthoven’s QRS axis of the frontal plane into three-dimensional The spatial orientation of a vector loop space. is defined by an axis perpendicular to its largest projection. The magnitude of this axis (polar vector) is given by the area enclosed by the loop. Any shift or rotation of the loop will always lead to a concomitant shift of the polar The plane of the projection of the vector. largest loop or principal plane can be determined by loop rotations in space by using a lead Alternate procedures are based on resolver.* loop planimetry in three planes6 or fitting the least square of a plane in the loop.9 When two mutually perpendicular axes are added to the polar vector, a new orthogonal frame of reference is obtained which is independent of the position or rotation of the heart. This type of loop analysis in its own frame of reference leads In the leastto relatively uniform standards. square fitting procedure9 the three axes in the new frame of reference are designated as Eigenvectors, which have both magnitude and direcintion. * Ratios between these Eigenvectors * Eigenvectors obtained by the least-square fitting procedure may differ slightly in direction and magnitude from axes obtained by resolution of leads. The latter are always referred to point E of the vector loop. Eigenvectors are based on loop configuration only. The difference was not found significant for analytic purposes. THE
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dicate the planarity of the loop and its main configuration (length/width). It is also possible to record scalar leads along these axes which contain the time information necessary for recognition of arrhythmias. lo-i3 McFee and Abildskov and their associates”J3 proposed the electrocardiogram” for this term “normalized One of the 3 new orthogtype of recording. onal leads is an indicator of the planarity of the loop keeping deflections at a minimum; a and second is kept equally positive and negative; the third one represents a maximal deflection. Analysis of the vectorcardiogram in its own frame of reference was found an excellent means for separation of normal from abnormal records.12 A comparison of the diagnostic information in Eigenvectors with other procedures showed, however, that discrimination between pathologic groups was relatively poor.14 It did not exceed that of time integrals. It is conceivable, however, that analysis of detail in normalized scalar leads will reveal diagnostic features not given by the spatial magnitude and direction of Eigenvectors. The same type of normalization and analysis has been performed on T and P 10ops.9,~~ Whereas Eigenvectors of T did not add significantly to other types of analysis of T, the polar P vector in conjunction with the maximal P vector was found an excellent means for separation of the normal P from P pulmonale and P mitrale.14 CURVES OF SPATIAL MAGNITUDE, ORIENTATION AND VELOCITY Since it was recognized for a long time that vector loop displays lacked very essential time information, other methods of display had to be explored to overcome this shortcoming. McFeel6 and Sayers and Abildskov and Hellerstein and their co-workers1+18 introduced a recording method of spatial data on a linear time scale. Analog computers are used for The spatial orienthis most ingenious display. tation of vectors is given by two angular curves, indicating azimuth and elevation angles of a Thus, a continuous Cartesian reference frame. display of spatial data on a time scale is presented in much the same way as in a scalar electrocardiographic lead. At any given moment, spatial magnitude, orientation and velocity can be readily determined with precision. Thus, the combined information of the scalar electrocardiogram and spatial vectorcardiogram becomes available in one set of curves. VOLUME
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The practical usefulness of this type of displa) of data has been evaluated recently on a staSince a large variability in contistical basis.19 figuration of curves of various pathologic entities was found, a clear separation was not possible on In a different the basis of configuration only. approach, consecutive points of data on each It curve were compared with normal ranges. was found that most of the diagnostic information is contained in records of spatial orientation, spatial magnitude and velocity contributing considerably less. This point-by-point analysis leads, however, to a prohibitive number of false positive findings that may reach 25 per cent for a single curve. Since ranges of spatial direction, defined by azimuth and elevation angles, lose their true ellipsoid form and become rectangular, a certain number of false negative findings cannot be excluded.19 Although the statistical pitfalls of this type of analysis can be overcome by certain types of numerical analysis, the possibility of graphic display of data is lost in the handling procedures of strictly numerical data. Display of electrocardiograms in form of time-based curves of spatial magnitude, orientation and velocity is the most comprehensive representation of electrocardiographic data. However, diagnostic information can be extracted only with great difficulty. It is questionable, therefore, whether the relatively great cost of the required analog computers can be justified for this display. ANALYSIS BY DIGITAL COMPUTER Since analysis of electrocardiograms by digital computer is not necessarily linked with any type of display, a detailed description of comComputational technic is not necessary here. puter outputs can be printed out in almost any graphic form, but such types of display are not an essential part of the computation proper. It is important to realize that all methods of digital computer analysis of the electrocardiogram are based exclusively on numerical data. Thus, many complex mathematical operations can be performed that do not necessarily lend themselves to a graphic or optical representation. The lack of a visual display has to be weighed against the greater efficiency of complex numerical classification methods.gPr4 Since accuracy and reliability of diagnosis will always remain the basic goal in analysis of the electrocardiogram, precision of diagnostic decisions will have to be the de-
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Avcrag-e Rate of Recognition
( 51 ______~. 1. 818 instantanrous 2. 3. 4. 5. 6. 7.
vectors 5 initial and 5 terminal instantaneous vcctors (at 0.01 sec. intervals) Q amplitude. Q duration and R amplitude of 3 scalar orthogonal leads Q duration. Q amplitude, K amplitude and S amplitude of 3 scalar orthogonal leads QRS Eigenyectors SAQRS (time integral of QRS) Spatial maximal QRS vectors
96 94 57 63 51 50 46
The listed li,gurcs indicate the percentage of records correctly classifird in diagnostic groups. This procedure has been reported in more detail previously.r4 High rates of recognition indicate a high content of diagnostic information. Multiple instantaneous vectors yielded most of this information (groups 1 and 2). Scalar measurements were decidedly inferior to spatial data (groups 3 and 4). Eigenvectors, time integrals and maximal vectors were least informative (groups 5, 6 and 7). The 818 instantaneous vectors were obtained by dividing QRS in time into 8 equal parts.
factor for the selection of analytic methods. A typical demonstration of the essential difference between visual analyses and computer methods has been given for differential diagnosis.‘* On the basis of multiple instantaneous vectors, computations were performed in 24dimensional space. This procedure was found more efficient than any other analytic method. Although multidimensional operations do not represent a limitation mathematically, the electrocardiographer is hardly able to extend three-dimensional space conceptually. Analysis in multidimensional space is limited, therefore, to numerical expressions without direct visual representation. A side-product of digital computer analysis of electrocardiograms has been the determination of diagnostic information in various types of display or electrocardiographic measurements. A comparative listing of QRS parameters is given in Table I. Multiple instantaneous vectors yielded most of the diagnostic information. This should not be surprising because a considerable amount of detail is provided by a multitude of points. The information in measurements from scalar cisive
and
Pipberger lcads was markedly less (Q amplitude and duration, K amplitude). When determination of the S amplitude was added, the diagnostic information increased slightly. This finding was of considerable interest because advantages and disadvantages of scalar vs. spatial analysis of the electrocardiogram have been argued for many years. Determination of the diagnostic information available in the two procedures provided for the first time a quantitative answer to this problem. A rather surprising degree of superiority of spatial data was found. This linding, however, does not imply selection of a given form of spatial electrocardiographic display. It only proves that time relations between leads, which are largely neglected in measurements of single leads, are essential for efficient diagnostic classification. The importance of simultaneous recording of scalar orthogonal leads is stressed by this finding. The information content of time integrals, Eigenvectors and maximal QRS vectors was considerably less than that of multiple instantaneous vectors. As already mentioned, these measurements represent mainly a summary of electrocardiographic complexes. DIFFERENTIAL ELECTROCARDIOGRAPHY A very simple electrocardiographic display method based on results of digital computations is being developed at this laboratory at present. Figure 1 shows the basic problem of diagnostic classification in electrocardiography. Ranges of electrocardiographic measurements are projected on two arbitrarily selected leads. Almost as a rule such projections overlap. A definite diagnostic decision cannot be made for any record falling in the range of overlap. When sufficiently large samples of records from various diagnostic entities are available, however, axes can be computed which result in optimal separations of pathologic groups. Leads taken along these axes will discriminate best between pathologic groups. Directions of such leads have been determined for the majority of diagnostic entities. It appears at present that a set of 5 leads may suffice for practical purposes. A typical example of such a lead separates for instance “left ventricular hypertrophy” from “anteroseptal infarct.” An attempt is being made to base the differentiation on very simple measurements such as presence or absence of Q waves, Q/R and R/S ratios, etc. Thus, reading and interpretation remain as convenient as possible. Since scalar THE
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leads are used in this display, no time information is lost and analysis of arrhythmias can be performed with the same tracings. The described tracings are obtained in the following manner. An ordinary set of electrodes as used for recording corrected orthogonal leads is applied. Instead of the regularly used resistor networks which yield orthogonal lead directions, new sets of resistors were computed. These combinations of resistors represent basically fixed settings of a lead resolver. Through lead resolution an infinite number of directions of leads can be obtained. Since only a limited number of differential leads is required, a continuous resolver can be replaced by a few fixed resolver settings. Each of these fixed positions requires a new set of resistors. A cathode-follower is necessary because most commercial direct-writing electrocardiographs have only a relatively low input impedance. Recording of differential electrocardiographic leads proved very rewarding because record analysis is reduced to a very few simple steps. Most of the parameters can be evaluated by simple inspection. A small box containing a lead-selector switch with the necessary resistors and cathode-follower can easily be attached to any portable electrocardiograph at a low cost.
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FIG. 1. Two typical ranges of vectors are shown in the middle of the diagram. They represent two diagnostic entities. Typically, such vector ranges have the form of ellipsoids and are three-dimensional. Projections of the ranges on two arbitrarily chosen electrocardiographic leads X and Y, result in overlapping distribution curves of findings. In the range of overlap, definite diagnostic decisions cannot be made. A lead axis can be computed, however, which results in an optimal separation of the two diagnostic entities (ax + /3Y). Such leads can bc determined for differentiation between various pathologic groups. (Reprinted from Progress in Cardiovascular Diseases9 by permission of Grune and Stratton, Inc.)
ANALYSIS OF HIGH FREQUENCY COMPONENTS High frequency components of the electrocardiogram consist of small fast notches, slurs and segments of high velocity. They cannot be recorded with slow-moving direct-writing electrocardiographs. In recent years the importance of such components has been pointed out repeatedly.20,21 High frequency content of the electrocardiogram was found increased after myocardial infarction. The question to be answered is whether or not this finding represents independent information. If in certain cases diagnoses of infarction can be made exclusively on the basis of high frequency components without any other electrocardiographic evidence for infarction, research in this field becomes extremely important. The presently available direct-writing electrocardiographs would not be adequate for the display of components of high velocity. DISCUSSION AND CONCLUSIONS This review of methods of electrocardiographic display and analysis can by no means be considered complete. First, an attempt was made VOLUME
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to consider certain advantages and disadvantages of methods in general use. Second, some of the more recent developments in this field were discussed. Those who have not followed electrocardiographic literature closely may consider the present situation rather bewildering because too many new concepts have been introduced. A closer look shows, however, that a certain consistency can be found in present trends which facilitates their evaluation. In the last two decades electrocardiography has, like many other medical tools, been subjected to a very thorough analysis by biophysicists, mathematicians and biostatisticians. It was to be expected that methods of display and analysis were profoundly influenced by this development. One of the major concepts to change display and analysis of the electrocardiogram was the introduction of the dipole hypothesis. This theory states that all electromotive forces generated by the heart can be considered as originating from a fixed-point dipole source.
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Piphergcr
The \.alidit>. of this first approximation has been discussed elsewhere in this symposium. The assumption of “local” effects on certain leads contradicts the dipole theory. Accordin,q to present knowledge, precordial leads may contain a minor contribution from nondipolar sources. An arbitrarily selected R wave of a precordial lead, for instance, may consist of 85 per cent dipolar and 15 per cent nondipolar content. Depending upon the polarity of the nondipolar content, the R amplitude may be Visual increased or decreased by a fraction. analysis would in both cases not reveal which part of the R wave is of dipolar or nondipolar origin. Until leads which will differentiate between dipolar and nondipolar sources become available and practical, the latter need not be considered. With presently available display methods dipolar contributions predominate to such an extent that any search for multipole components in ordinary electrocardiographic records remains futile. The present inability to recognize or define any portion of the electrocardiogram as nondipolar leaves us with a very powerful basic tool, the dipole concept. Dipole variations in time, in magnitude and direction can be completely described by three orthogonal leads. Records of dipole variations can be obtained in the form of scalar leads, vector loops, or any Selection other type of display of spatial data. of the type of display has become now strictly a problem of practicality in terms of accessibility of the information and ease of analysis. Although it has become obvious that precordial leads contain predominantly dipolar information, these leads were not found an efficient means for the display of dipole variations. Some of the reasons have been discussed in conjunction with vector electrocardiography. Since extraction of “local” information proved impractical also, the question arises whether or not recording of precordial leads should be To justify their further use has continued. become extremely difficult. Tradition alone can hardly be considered sufficient reason to obtain a series of records from each patient without rational basis. They can be used only for empirical and descriptive analysis without contributing any new information not already contained in simpler three-lead orthogonal lead systems. Practically all recent advances in electrocardiographic display methods were based on the dipole concept. The great variety of
and Pipberger new display procedures can be easily understood when their common source of information is remembered. Whether we consider displays of vector loops, curves of spatial magnitude and orientation or any other type of spatial data presentation, we can always refer to the same origin, the dipole, whose variations in time are expressed in terms of spatial magnitude and orientation. This unifying concept has greatly facilitated teaching and understanding of electrocardiography. The approach to analysis of electrocardiograms has changed considerably in the last decades. Whereas empirical analysis and intuition provided a wealth of valid information in the past, more recently a general trend toward quantitation of electrocardiographic data developed. As a first step, normal ranges were determined for single measurements such as Q duration or Q amplitude. This led to a separation of normal from abnormal findings without precise differentiation of pathologic entities. With the advent of automatic data processing technic, more complex statistical tools became available. One analytic procedure could easily be compared with another, and the results could be expressed numerically in terms of diagnostic recognition rates.14 Thus, a quantitative evaluation of diagnostic and analytic procedures became possible. The probability of being in a certain diagnostic category could be determined for each record at the same time. Results of the evaluation of various analytic procedures have been discussed. It was found that the diagnostic information content of spatial measurements exceeded largely that of scalar records when they were taken one by one. Since all spatial information can also be obtained from simultaneously recorded scalar orthogonal leads, it became obvious that the simultaneity of recording represented the main factor. From there it follows that recording of scalar leads in sequence will always be accompanied by a loss of diagnostic information. A further finding was the limited information available by all methods which use a few representative terms for a whole electrocardiographic record. Time integrals such as the ventricular gradient, polar vectors, Eigenvectors and similar items were of only limited value in diagnostic differentiation. They represent powerful tools, however, when normal records need to be separated from abnormal ones without specific diagnosis. It became obvious from these studies that a THE
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certain minimum of detailed information needs to be provided to achieve reasonable efficiency in diagnostic classification. At present it appears that eight points during QRS and eight points during S-T and T may be sufficient for this purpose. Instantaneous vectors need to be determined for each of these points. Frequency-density-distribution functions were found an efficient means for diagnostic classification of individual records. Although such procedures offer great advantages for efficient analysis of electrocardiographic data, their complexity requires use of digital computers. The display of such data as well as the graphic representation of their analysis in multidimensional space are impractical. The question arises, therefore, whether electrocardiographic display will remain an essential part of electrocardiography in the It has become a restraint rather than future. an advantage. It cannot be expected that facilities for data processing will become available to all practicing physicians in the near future. Other methods need to be developed, therefore, to bridge the obvious gap. Differential electrocardiography, as described, may be a step in this direction. Results of analyses of large record samples by computers represent the basis of this method. Some of the advantages obtained in more complex computational procedures could thus be incorporated into daily practice, using singlechannel electrocardiographic recorders which are commonly available now. One of the main drawbacks of conventional analysis of the electrocardiogram has always been the inconsistency in interpretation.22 It is not very realistic to use the skills of trained electrocardiographers as a general standard. The increasing replacement of subjective evaluations by quantitative statements may lead to another risk, however. The persuasiveness of numbers is great and should not be underestimated. As long as the limitations of the electrocardiogram as a diagnostic tool are kept in mind, the advantages of quantitative analysis The will always outweigh the disadvantages. electrocardiogram should always remain one building stone only in the over-all evaluation of the patient. SUMMARY
A review of the most common types of electrocardiographic display and analysis, including those developed more recently, was given. VOLUME
14, SEPTEMBER 1964
of ECG
Data
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Advantages and disadvantages of scalar lead recordings, vector loop displays, curves of spatial magnitude, orientation and velocity, polar vectors and Eigenvectors were evaluated. Analysis of data from these displays was compared with analysis by digital computer, which is based almost exclusively on numerical terms. Since graphic display of numerical operations proved impractical, the question was raised, whether or not display of electrocardiographic data represents an unnecessary restraint for efficient analysis. A new method of differential electrocardiography is described. It is based on computed entities. Leads ranges of various diagnostic which discriminate best between diagnostic groups can be obtained by resolution of orthogonal leads. Some of the results of large scale computations can thus be incorporated in routine electrocardiography. Quantitative comparison of various analytical procedures showed a marked superiority of spatial parameters over scalar leads recorded in sequence. Rates of diagnostic recognition were best when a series of instantaneous vectors was available for QRS, S-T and T. The ventricular gradient, polar vectors, Eigenvectors and curves of spatial magnitude, orientation and velocity were found less efficient for diagnostic classification. REFERENCES 1. EINTHOVEN,W., FAHR, G. and DEWAART, A. ober die Richtung und die manifeste G&se der Potentialschwankungen in men&lichen Herzen und iiber den Einfluss der Herzlage auf die Form des Elektrokardiogramms. Arch. ges. Physiol., 150 : 275, 1913. 2. WILSON, F. N., MCLEOD, A. G., BARKER, P. S. and JOHNSTON,F. D. The determination and the significance of the areas of ventricular deflections of the electrocardiogram. Am. Heart J., 10: 46, 1934. 3. GRANT, R. P. Clinical Electrocardiography. The Spatial Vector Approach. New York, 1957. McGraw-Hill. 4. BALL, M. F. and PIPBERGER, H. V. The normal spatial QRS-T angle of the orthogonal vectorcardiogram. Am. Heart J., 56: 611, 1958. 5. PIPBERGER, H. V. Current status and persistent problems of electrode placement and lead systems for vectorcardiography and electrocardiography. Prog. Cardiovas. Dir., 2: 248, 1959. 6. BURGER, H. C. and VAANE, J. P. A criterion characterizing the orientation of a vectorcardiogram in space. Am. Heart J., 56: 29, 1958. Quantitative Vector Electrocardi7. BRI~BERG, L oaraohv. Baltimore. Md. 1960. Waverlv Press. 8. SC&IT~, ‘0. H. Cathode-ray presentation of three dimensional data. J. Appl. Physics, 18: 819, 1947.
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Pipberger H. V., STALLMANN, I’. LV., YAN(J, K. and DRAPER, H. W. Digital computer analysis of the normal and abnormal electrocardiogram. Prog. Cardiouas. Dis.. 5: 378, 1963. RIJLANT, P. L’i?lectrogendse globale du coeur chez l’homme. Electrocardiographic vectorielle et vectorcardiographie. Acta cardiol., 13: 349, 1958. MCFEE, R., WILKINSON, R. S. and ABILDSKOV,J. A. On the normalization of the electrical orientation of the heart and the representation of electrical axis by means of an axis map. Am. Heart J., 62: 391, 1961. PIPBERGER,H. V. and CARTER,T. N. Analysis of the normal and abnormal vectorcardiogram in its own reference frame. Circulation,25 : 827, 1962. ABILDSKOV,J. A., MCFEE, R. and SCHECTER,G. L. Further observations with the normalized electrocardiogram and axis map. Am. Heart J., 65: 220, 1963. PIPBERGER, H. V. and STALLMANN, F. W. Computation of differential diagnosis in electrocardiography. Ann. New York Acad. SC., in press. MCFEE, R. A trigonometric computer with electrocardiographic application. Rev. Scientifi Z&r., 21: 420, 1950. SAYERS, B. McA., SILBERBERG,F. G. and DURIE,
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and Piptxrger I). F. The electrocardiographic spatial maqnitude curve in man. .4m. Heart J., 49: 323, 1955. 17. ARILDSKOV, .J. A., INGERSON,W. B. and HISEY, B. L. A linear time scale for spatial vectorcardiographic data. Circulation, 14: 556, 1956. 18. HELLERSTEIN, H. K. and HAMLIN, R. QRS component of the spatial vectorcardiogram and of the spatial magnitude and velocity electrocardiograms of the normal dog. Am. J. Cordial., 6: 1049, 1960. 19. YANO, K. and PIPBERGER, H. V. Spatial magnitude, orientation and velocity of the normal and abnormal QRS complex. Circulation, 29: 107, 1964. 20. LANGNER, P. H., JR., GESELOWITZ, D. V. and MANSURE, F. T. High frequency components in the electrocardiograms of normal subjects and of patients with coronary heart disease. Am. Heart J., 62: 746, 1961. 21. FRANKE, E. K., BRAUNSTEIN, I. F. and ZILLNER, D. C. Study of high frequency components in electrocardiogram by power spectrum analysis. Circulation Res., 10: 870, 1962. 22. SEGALL, H. N. The electrocardiogram and its interpretation: A study of reports by 20 physicians on a set of 100 electrocardiograms. Canad. M. A. J., 82: 2, 1960.
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