Electrocardiology Adv. Cardiol., vol. 16, pp. 18-26 (Karger, Basel 1976)
Possibilities of Electrocardiography in the Future HANS SCHAEFER
I. Physiologisches Institut der Universitiit, Heidelberg
The Elementary Generator
This is a brief description of the electric source of the EeG. This source obviously is the single myocardial fibre. On its surface the migrating excitation wave separates a stilI resting part from an already excited portion. This boundary certainly is the elementary generator of all cardiac potentials (fig. 1). The charged surfaces of the fibre with opposite membrane potentials may be electrically replaced by a charged disc, the potential difference of which is equal to the difference of the membrane potentials at rest and in the excited state [SCHAEFER, 1951]. This disc, with the diameter of the myocardial fibre, develops an elementary electric field which, on its side, may be represented by a vector [SCHAEFER and HAAS, 1962] (fig. 2). Whatever will be recordable as an EeG is nothing but the vectorial sum of all elementary generators with their projections on the chosen lead vector.
The intention of the physician must be to come to an interpretation of the EeG, regressing to the elementary events in the heart. The possibilities of this interpretation are limited for various reasons: (a) A resultant vector can never be redistributed into its individual elementary components. (b) Every interpretation of the EeG, therefore, is necessarily based on the
Downloaded by: University of Cambridge 131.111.164.128 - 3/17/2019 5:52:22 PM
The Electric Field of the Heart
19
SCHAEFER
1
p
Fig. 1. Model of an elementary dipole: a myocardial fibre. The right part of the fibre is at rest, the left maximally excited. Both parts of the fibre may be reduced to electrical ineffectiveness by adding charged discs, 1 and 4, having the same potential difference across their surfaces as the membrane of the fibre. The two now completely closed, though oppositely charged, parts of the fibre no longer develop an electric field. The addition of discs 1 and 4 is neutralized, however, by the addition of two more discs, 2 and 3, of opposite polarity but the same potential difference as disc 1 or 4. These two discs, 2 and 3, now produce the same electric field as the action potential. The area between the discs is disregarded here, but it may be analyzed in the same manner, with the same result, by cutting it into infinitesimally small slices. V' and vn are the respective membrane potentials of the resting and the excited membrane. The potential recorded is determined according to figure 2. Fig. 2. The amount of potential recorded at P from the charge at discs 2 and 3 of figure 1, the potential differences of which are summed. The potential is proportional to the solid angle D, under which the discs appear from the electrode point P. Ml is the individual total moment of the two discs.
Downloaded by: University of Cambridge 131.111.164.128 - 3/17/2019 5:52:22 PM
2
SCHAlFER
20
application of models. (c) Due to a certain lack of detailed information, no definite model of the potential generation is yet existing. The experiments of SCHER and YOUNG [1957] and DURRER et al. [1970] have only supplied us with a basic pattern of interpretation. (d) In every attempt of clinical interpretation it remains, therefore, a considerable amount of arbitrariness which leads to a catalogue of possible diagnoses with diminishing probabilities. This uncertainty is accentuated by errors of interpretation due to the irregular shape of the body surface, the asymmetric position of the heart and the inhomogeneity of conductivity in the thorax. Corrections need in all cases an experimental adjustment to individual peculiarities. Since this is scarcely possible, the field irregularities can be corrected only to a certain degree.
The value of an ECG for prognostic or therapeutic measures is limited by this impossibility to translate the informations of its curves unequivocally into descriptions of underlying cardiac events. Both are linked together by a syntax of diagnostic correlations, which is subdue to all problems of validation we know from epidemiologic research. There are, however, only three possible types of functional deviations: (1) The distribution of excitation waves and their mutual cancellation may be abnormal. This is more or less the consequence of local defects, either in the specific system or in the global mass of the myocardium. The general model of interpretation is: the greater the deformation of QRS, either in time or in its vectorial properties, the higher is the probability that the disturbance may be localized in the specific system of the ventricles. (2) A change in the propagation velocity of the average excitation wave generating the ECG. The general model of interpretation is: the lower the propagation velocity, the longer the QRS duration. In the most simple case QRS is unchanged in type and only prolonged in its time shift. Type I and 2 may lead to very similar traces. The discrimination is expressed in a probability function: the more the vector loop has changed its spatial position, the higher is the probability for type 1. (3) The shape of the so-called monophasic action potential of the average excitation wave may be changed, either in general or locally, thus leading to inhomogeneities of the repolarization. The model of interpretation is this: the more pronounced the change in T pattern, the higher the pro-
Downloaded by: University of Cambridge 131.111.164.128 - 3/17/2019 5:52:22 PM
Clinical Interpretation
Future Electrocardiography
21
bability that the disturbance is local and of a primarily metabolic character. The area of T, however, is strongly dependent on that of QRS. Only changes of the ventricular gradients are of functional significance in such cases. Besides such functional data, morphological peculiarities of the heart will strongly influence the electric patterns. Hypertrophy, infarcts, positional abnormalities and similar items will be represented in the form criteria of ECG and VCG. Especially the interpretation of T may more easily be accessible to an analysis, if morphological data (e.g. various fibre diameters) are taken into account as suggested by ABEL and LOEPER [1973].
The Five Possibilities of Development
Future development should try to improve the methods of translating the language of electrical events on the surface of the body into informations on functional or morphological events in the interior of the heart. There are five possible ways to the goal of such an improvement: (a) the standard curves of the ECG could be made accessible to better translation methods, to improve the interpretation of electric tracings into information about cardiac events; (b) the same standard curves could be used for screening methods in preventive medicine; (c) new methods of vectorial analysis could be developed; (d) mapping of electric potentials on the chest surface could be used, and (e) all these records could be analyzed by computers.
Improvements of standard electrocardiography will be possible only by a more stringent correlation between ECG data and cardiac events based on the observation of morphological or functional deviations. This can be done only with epidemiological methods. The procedure will be the correlation of ECG peculiarities to observed/expected ratios of typical disfunctions or of death. The probable progress will be dubious. Most of the correlative work has already been done. New knowledge can be gained only if new correlations of ECG data with the morphology or physiology of the myocardium could be established. The validity of the ECG in respect to the morphology of infarct, however, is modest, as has been shown by a Swedish epidemiology in Malmo [BJURULF
et al., 1967].
Downloaded by: University of Cambridge 131.111.164.128 - 3/17/2019 5:52:22 PM
The Standard EeG
SCHAEFER
22
The EeG as a Screening Method
Among future developments, the introduction of the standard ECG as screening method will be most probably the most important one, according to what CARRUTHERS and TAGGERT [1974] called 'neocardiology'. The problem may be condensed in the following sentence: it is desired to detect the earliest stages of abnormal events in the heart with a method as simpe and as cheap as possible and with a validity as high as possible in respect to an interpretation model. No doubt that up to now no procedures of such a screening character and of a reliable validity are at hand. The first difficulty is to discriminate between normal and abnormal. Though excellent tables are available with normal limits and standard deviations [CORNFIELD et aI., 1973; OKAMOTO and SIMONSON, 1966; SIMONSON, 1961], the definition of normal can be given with an appropriate accuracy only if the ECG data are available in values referred to an orthogonal lead system. The reason is that each lead may be of borderline character, and only the summation of all borderline values results in the decision of abnormality. Systems which will suffice this condition should present an optimal parallelism of their electric flux lines. The system used should be checked in this respect. This can be done by recording the vector loop of a heart under extreme positions in the chest (e.g. deepest inspiration and expiration). The loops are electronically rotated into their brideside and edgeside views (absolute loop). HILTMANN and SCHAEFER [1975] have described the mathematical procedure with the aid of which a comparison between the correctness of various lead systems can be calculated. The optimal system presents a minimal difference between the absolute form of loops recorded in extreme heart positions. The most urgent task in the future will be to work out a more precise discrimination between normal and abnormal in a prospective epidemiology. The expenses for such an enterprise, however, are high. The problem can be solved only by an international cooperation. The first reasonable attempt in this respect which came to my knowledge is that of BLACKBURN et al. [1973] reported recently.
The vector analysis has by no means already gained all informations incorporated in the vectorial data. KOCH and SCHAEFER [1975] have desc-
Downloaded by: University of Cambridge 131.111.164.128 - 3/17/2019 5:52:22 PM
Description of Vectorial Data
Future Electrocardiography
23
ribed new forms of such an analysis similar to a method first used by SEIDEN and STAHL [1973]. Details of such a procedure cannot be given here. The principle is that three new functions can be calculated: (1) the differential quotient of the QRS vector magnitude in space; (2) the tangent vector to the vector loop, and (3) its projection on the plane orthogonal to the instantaneous vector of QRS. All three functions can be interpreted as indicators of definable cardiac events, like the magnitude of the momentary change of the resulting electric force, its positional change in time as indicator of the propagation velocity of the excitation processes, or the total muscular mass participating in the generation of the electric field. Details should be read in the original paper. It may be hoped that even minor deviations in the excitation process can be detected.
Mapping procedures of the chest surface potential have been much discussed in the last years. This procedure doubtlessly has advantages. However, it can be shown by simple physical considerations that only a network of close bipolar electrodes, as ERNSTHAUSEN and KIENLE [1953] used for the first time, has a fair chance to detect what in classical electrocardiography has been called proximity potentials or partial derivations. No attempt has been made up to now to give a mathematical approximation of the optimal chance, expressed in percent of a global field, to measure potentials of distinct superficial muscle layers of the myocardium. The pure description offield lines, originally introduced by GROEDEL and KOCH [1934] into the clinical electrocardiography, will scarcely be able to detect much more cardiac lesions than classical methods do. The discussion on mapping as a clinical tool [TACCARDI et al., 1974] certainly was not very encouraging. Mapping without a clear physiological background in form of myocardial structures, elementary cardiac events, etc. is most probably unsuccessful (so SCHMITT in TACCARDI'S paper). The often used model of charged shells separating excited from resting muscular areas certainly is an unpermitted simplification. Figure 3 demonstrates this in a typical model. Three excitation waves travelling in three distinct fibre bundles, but with a difference in latency, may form a fictive boundary represented by a plane which is the geometrical locus of all zones separating the resting from the excited part of the individual fibre. Such boundaries are recorded in experiments like those of SCHER and
Downloaded by: University of Cambridge 131.111.164.128 - 3/17/2019 5:52:22 PM
Surface Mapping
SCHAEFER
Vector of shell
24
i I
Vectors of fibres
~~~ e e e
-8
Fig. 3. Difference between the resultant vector constructed by the electric moment of three individual fibres, producing fields according to figure 2. If a boundary between already excited and still resting areas is constructed and regarded as a 'shell' carrying the electric charge, a vector going into a completely different region is resulting.
[1957] or DURRER et al. [1970], registering the arrival of the excitation by unipolar microelectrodes situated in the ventricular wall. The shells of charges constructed according to such latency measurements, however are incompatible with the histological structure of the heart, with its overall direction of fibre bundles parallel to the epicardial surface. The most serious objection to mapping methods is that their validation, i.e. the proof, what such patterns really mean, is still very scanty. An attempt to validate patterns in respect to standard ECGs is obviously frustrating. The whole bulk of experience incorporated in the diagnostic use of ECGs would have to be translated into the patterning of mapping methods. Moreover, the costs of mapping will be comparatively high. A cost-benefit analysis should be made therefore. My own opinion is not very optimistic. A diagnosis of the exact location of myocardial damages in restricted areas will not very often lead to a more exact prognosis or to a more successful therapy. Prognosis or therapy, however, are the only goals for introducing new methods into medicine, justifying a higher amount of expenses. YOUNG
It may be unnecessary to add a word about computers in electrocardiography. The application of programs based on an ample experience, the reduction in manpower used for diagnosis and, last not least, the liberation
Downloaded by: University of Cambridge 131.111.164.128 - 3/17/2019 5:52:22 PM
Computer Diagnosis
Future Electrocardiography
25
of diagnostic procedure from the inadequacies of individual observers, are strong reasons for the use of computers. Not all problems have been solved. But only with the aid of computers, methods like new vectorial analysis or surface mapping could be successful. Moreover, I scarcely could imagine an institution concerned with financing medicine which, in the near future, will pay for an ECG diagnosis made by an individual doctor. This holds true even more if the ECG will be used as a screening method in preventive medicine. In prevention, the validation of computer programs against the prediction of cardiac attacks, as PIPBERGER et al. [1958, 1972] have done in recent years, will be one of the most urgent tasks. Let me express one idea in closing these few remarks: one of the critiques often heard in respect to medicine is that this medicine develops along the line of the physician's interests. I think this is true to a regrettable degree. The electrocardiography has become a battlefield, where too many technical interests come into the picture, and a cost-benefit analysis would give most probably astonishing results. Those, who are responsible for future development (and we all belong to this group) should carefully reflect the social responsibilities laid upon our shoulders. Salus aegroti suprema lex.
ABEL, H. und LOEPER, H.: Myokardfaserquerschnitte und T-Welle. Z. Kardiol. 62: 380-387 (1973). BJURULF, R.; GARLIND, R., and STERNBY, N. H.: On epidemiologic methods for recording ischemic heart disease. Acta med. scand., suppl. 474 (1967). BLACKBURN, H.; KEyS, A.; SIMONSON, B.; RAUTAHARJU, P., and PUNSAR, S.: The BCG in population studies. A classification system. Circulation 21: 1160-1175 (1960). BLACKBURN, H.; KEYS, A.; TAYLOR, H. L., and THORSEN, R. D.: Screening and prognostic value of the resting ECG. Circulation 48: suppl. 4, p. 23 (1973). CARRUTHERS, M. and TAGGERT, P.: Paleocardiology and neocardiology. Am. Heart J. 88: 1-3 (1974). CORNFIELD, J.; DUNN, R. A.; BATCHLOR, C. D., and PIPBERGER, H. V.: Multigroup diagnosis of electrocardiograms. Comput. biomed. Res. 6: 97-120 (1973). DURRER, D.; DAM, R. T. VAN; FREUD, G. E.; JANSE, M. J.; MEIJLER, F. L., and ARzBAECHER, R. C.: Total excitation of the isolated human heart. Circulation 41: 899--912 (1970). ERNSTHAUSEN, W. und KIENLE, F.: Das elektrische Herzbild (Rinn, Miinchen 1953). GROEDEL, F. M. und KOCH, E.: Topographie der Aktionspotentiale des Herzens auf der vorderen Brustwand. Z. Kreislaufforsch.26: 18-20 (1934). lIILTMANN, W. D.: Die Variabilitat der Vektorschleifen der orthogonalen Ableitesysteme von Frank und Schmitt (SVEC III). Basic Res. Cardiol. 70: 87-102 (1975).
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References
SCHAEFER
26
Prof. Dr. med. H. SCHAEFER, I. Physiologisches Institut der Universitat, 1m Neuenheimer Feld 326, D-6900 Heidelberg (FRG)
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HILTMANN, W. D. und SCHAEFER, H.: Theorie der Methoden zur Vergleichung von Ableitefeldern in der Elektrokardiographie. Basic Res. Cardio!. 70: 78-86 (1975). KOCH, W.: Datenreduktion, GIattung und Basislinienkorrektur vor der automatischen Vermessung von EKG-Signalen. Basic Res. Cardio!. (in press). KOCH, W. und SCHAEFER, H.: Neue Funktionen und Variablen fUr die Charakterisierung und den Vergleich von Vektorkardiogrammen. Basic Res. Cardio!. 70: 103-114 (1975). OKAMOTO, N. and SIMONSON, E.: Separation of normal and abnormal vectorcardiograms. Am. J. Cardio!. 18: 682-689 (1966). PIPBERGER, H. V.: The normal orthogonal electrocardiogram and vectorcardiogram. Circulation 15: 1102-1111 (1958). PIPBERGER, H. V.; BERSON, A. S.; KLINGEMAN, J. D., and BATCHLOR, C. D.: Diagnostic classifications of orthogonal electrocardiograms and vectorcardiograms. Proc. 11 th Int. Vectorcardiography Symposium, pp. 157-163 (North-Holland, Amsterdam 1971). PIPBERGER, H. V.; DUNN, R. A., and CORNFIELD, J.: First and second generation computer programs for diagnostic ECG and VCG classification; in RIJLANT: Proc. Satellite Symp. 25th Int. Congr. Physio!. Sci., pp. 431-440 (Presses academiques europeennes, Bruxelles 1972). SCHAEFER, H.: Das E1ektrokardiograrnrn (Springer, Berlin 1951). SCHAEFER, H. and HAAS, H. : Electrocardiography. Handbook of Physiology. Circulation I, pp. 323--445 (American Physio!. Society, Washington 1962). SCHER, A. M. and YOUNG, A. c.: Ventricular depolarization and the genesis of QRS. Ann. N.Y. Acad. Sci. 65: 768-778 (1957). SEIDEN, G. E. and STAHL, C.: A new method for diagnosing myocardial damage in patients with normal ECGs and VCGs. Trans. N.Y. Acad. Sci. 35: 283-313 (1973). SIMONSON, E.: Differentiation between normal and abnormal in electrocardiography (Mosby, St. Louis 1961). TACCARDI, B.: Panel session: clinical studies and prognosis for mapping as a clinical tool. Adv. Cardio!., vo!. 10, pp. 296-301 (Karger, Basel 1974). T ACCARDI, B.; AMBROGGI, L. DE, and VIGANOTTI, c.: Characteristic features of surface potential maps during QRS and S-T intervals. Adv. Cardio!., vo!. 10, pp.248-256 (Karger, Basel 1974).
Possibilities of Electrocardiography in the Future HANS SCHAEFER
I. Physiologisches Institut der Universitiit, Heidelberg
The Elementary Generator
This is a brief description of the electric source of the EeG. This source obviously is the single myocardial fibre. On its surface the migrating excitation wave separates a stilI resting part from an already excited portion. This boundary certainly is the elementary generator of all cardiac potentials (fig. 1). The charged surfaces of the fibre with opposite membrane potentials may be electrically replaced by a charged disc, the potential difference of which is equal to the difference of the membrane potentials at rest and in the excited state [SCHAEFER, 1951]. This disc, with the diameter of the myocardial fibre, develops an elementary electric field which, on its side, may be represented by a vector [SCHAEFER and HAAS, 1962] (fig. 2). Whatever will be recordable as an EeG is nothing but the vectorial sum of all elementary generators with their projections on the chosen lead vector.
The intention of the physician must be to come to an interpretation of the EeG, regressing to the elementary events in the heart. The possibilities of this interpretation are limited for various reasons: (a) A resultant vector can never be redistributed into its individual elementary components. (b) Every interpretation of the EeG, therefore, is necessarily based on the
Downloaded by: University of Cambridge 131.111.164.128 - 3/17/2019 5:52:22 PM
The Electric Field of the Heart
19
SCHAEFER
1
p
Fig. 1. Model of an elementary dipole: a myocardial fibre. The right part of the fibre is at rest, the left maximally excited. Both parts of the fibre may be reduced to electrical ineffectiveness by adding charged discs, 1 and 4, having the same potential difference across their surfaces as the membrane of the fibre. The two now completely closed, though oppositely charged, parts of the fibre no longer develop an electric field. The addition of discs 1 and 4 is neutralized, however, by the addition of two more discs, 2 and 3, of opposite polarity but the same potential difference as disc 1 or 4. These two discs, 2 and 3, now produce the same electric field as the action potential. The area between the discs is disregarded here, but it may be analyzed in the same manner, with the same result, by cutting it into infinitesimally small slices. V' and vn are the respective membrane potentials of the resting and the excited membrane. The potential recorded is determined according to figure 2. Fig. 2. The amount of potential recorded at P from the charge at discs 2 and 3 of figure 1, the potential differences of which are summed. The potential is proportional to the solid angle D, under which the discs appear from the electrode point P. Ml is the individual total moment of the two discs.
Downloaded by: University of Cambridge 131.111.164.128 - 3/17/2019 5:52:22 PM
2
SCHAlFER
20
application of models. (c) Due to a certain lack of detailed information, no definite model of the potential generation is yet existing. The experiments of SCHER and YOUNG [1957] and DURRER et al. [1970] have only supplied us with a basic pattern of interpretation. (d) In every attempt of clinical interpretation it remains, therefore, a considerable amount of arbitrariness which leads to a catalogue of possible diagnoses with diminishing probabilities. This uncertainty is accentuated by errors of interpretation due to the irregular shape of the body surface, the asymmetric position of the heart and the inhomogeneity of conductivity in the thorax. Corrections need in all cases an experimental adjustment to individual peculiarities. Since this is scarcely possible, the field irregularities can be corrected only to a certain degree.
The value of an ECG for prognostic or therapeutic measures is limited by this impossibility to translate the informations of its curves unequivocally into descriptions of underlying cardiac events. Both are linked together by a syntax of diagnostic correlations, which is subdue to all problems of validation we know from epidemiologic research. There are, however, only three possible types of functional deviations: (1) The distribution of excitation waves and their mutual cancellation may be abnormal. This is more or less the consequence of local defects, either in the specific system or in the global mass of the myocardium. The general model of interpretation is: the greater the deformation of QRS, either in time or in its vectorial properties, the higher is the probability that the disturbance may be localized in the specific system of the ventricles. (2) A change in the propagation velocity of the average excitation wave generating the ECG. The general model of interpretation is: the lower the propagation velocity, the longer the QRS duration. In the most simple case QRS is unchanged in type and only prolonged in its time shift. Type I and 2 may lead to very similar traces. The discrimination is expressed in a probability function: the more the vector loop has changed its spatial position, the higher is the probability for type 1. (3) The shape of the so-called monophasic action potential of the average excitation wave may be changed, either in general or locally, thus leading to inhomogeneities of the repolarization. The model of interpretation is this: the more pronounced the change in T pattern, the higher the pro-
Downloaded by: University of Cambridge 131.111.164.128 - 3/17/2019 5:52:22 PM
Clinical Interpretation
Future Electrocardiography
21
bability that the disturbance is local and of a primarily metabolic character. The area of T, however, is strongly dependent on that of QRS. Only changes of the ventricular gradients are of functional significance in such cases. Besides such functional data, morphological peculiarities of the heart will strongly influence the electric patterns. Hypertrophy, infarcts, positional abnormalities and similar items will be represented in the form criteria of ECG and VCG. Especially the interpretation of T may more easily be accessible to an analysis, if morphological data (e.g. various fibre diameters) are taken into account as suggested by ABEL and LOEPER [1973].
The Five Possibilities of Development
Future development should try to improve the methods of translating the language of electrical events on the surface of the body into informations on functional or morphological events in the interior of the heart. There are five possible ways to the goal of such an improvement: (a) the standard curves of the ECG could be made accessible to better translation methods, to improve the interpretation of electric tracings into information about cardiac events; (b) the same standard curves could be used for screening methods in preventive medicine; (c) new methods of vectorial analysis could be developed; (d) mapping of electric potentials on the chest surface could be used, and (e) all these records could be analyzed by computers.
Improvements of standard electrocardiography will be possible only by a more stringent correlation between ECG data and cardiac events based on the observation of morphological or functional deviations. This can be done only with epidemiological methods. The procedure will be the correlation of ECG peculiarities to observed/expected ratios of typical disfunctions or of death. The probable progress will be dubious. Most of the correlative work has already been done. New knowledge can be gained only if new correlations of ECG data with the morphology or physiology of the myocardium could be established. The validity of the ECG in respect to the morphology of infarct, however, is modest, as has been shown by a Swedish epidemiology in Malmo [BJURULF
et al., 1967].
Downloaded by: University of Cambridge 131.111.164.128 - 3/17/2019 5:52:22 PM
The Standard EeG
SCHAEFER
22
The EeG as a Screening Method
Among future developments, the introduction of the standard ECG as screening method will be most probably the most important one, according to what CARRUTHERS and TAGGERT [1974] called 'neocardiology'. The problem may be condensed in the following sentence: it is desired to detect the earliest stages of abnormal events in the heart with a method as simpe and as cheap as possible and with a validity as high as possible in respect to an interpretation model. No doubt that up to now no procedures of such a screening character and of a reliable validity are at hand. The first difficulty is to discriminate between normal and abnormal. Though excellent tables are available with normal limits and standard deviations [CORNFIELD et aI., 1973; OKAMOTO and SIMONSON, 1966; SIMONSON, 1961], the definition of normal can be given with an appropriate accuracy only if the ECG data are available in values referred to an orthogonal lead system. The reason is that each lead may be of borderline character, and only the summation of all borderline values results in the decision of abnormality. Systems which will suffice this condition should present an optimal parallelism of their electric flux lines. The system used should be checked in this respect. This can be done by recording the vector loop of a heart under extreme positions in the chest (e.g. deepest inspiration and expiration). The loops are electronically rotated into their brideside and edgeside views (absolute loop). HILTMANN and SCHAEFER [1975] have described the mathematical procedure with the aid of which a comparison between the correctness of various lead systems can be calculated. The optimal system presents a minimal difference between the absolute form of loops recorded in extreme heart positions. The most urgent task in the future will be to work out a more precise discrimination between normal and abnormal in a prospective epidemiology. The expenses for such an enterprise, however, are high. The problem can be solved only by an international cooperation. The first reasonable attempt in this respect which came to my knowledge is that of BLACKBURN et al. [1973] reported recently.
The vector analysis has by no means already gained all informations incorporated in the vectorial data. KOCH and SCHAEFER [1975] have desc-
Downloaded by: University of Cambridge 131.111.164.128 - 3/17/2019 5:52:22 PM
Description of Vectorial Data
Future Electrocardiography
23
ribed new forms of such an analysis similar to a method first used by SEIDEN and STAHL [1973]. Details of such a procedure cannot be given here. The principle is that three new functions can be calculated: (1) the differential quotient of the QRS vector magnitude in space; (2) the tangent vector to the vector loop, and (3) its projection on the plane orthogonal to the instantaneous vector of QRS. All three functions can be interpreted as indicators of definable cardiac events, like the magnitude of the momentary change of the resulting electric force, its positional change in time as indicator of the propagation velocity of the excitation processes, or the total muscular mass participating in the generation of the electric field. Details should be read in the original paper. It may be hoped that even minor deviations in the excitation process can be detected.
Mapping procedures of the chest surface potential have been much discussed in the last years. This procedure doubtlessly has advantages. However, it can be shown by simple physical considerations that only a network of close bipolar electrodes, as ERNSTHAUSEN and KIENLE [1953] used for the first time, has a fair chance to detect what in classical electrocardiography has been called proximity potentials or partial derivations. No attempt has been made up to now to give a mathematical approximation of the optimal chance, expressed in percent of a global field, to measure potentials of distinct superficial muscle layers of the myocardium. The pure description offield lines, originally introduced by GROEDEL and KOCH [1934] into the clinical electrocardiography, will scarcely be able to detect much more cardiac lesions than classical methods do. The discussion on mapping as a clinical tool [TACCARDI et al., 1974] certainly was not very encouraging. Mapping without a clear physiological background in form of myocardial structures, elementary cardiac events, etc. is most probably unsuccessful (so SCHMITT in TACCARDI'S paper). The often used model of charged shells separating excited from resting muscular areas certainly is an unpermitted simplification. Figure 3 demonstrates this in a typical model. Three excitation waves travelling in three distinct fibre bundles, but with a difference in latency, may form a fictive boundary represented by a plane which is the geometrical locus of all zones separating the resting from the excited part of the individual fibre. Such boundaries are recorded in experiments like those of SCHER and
Downloaded by: University of Cambridge 131.111.164.128 - 3/17/2019 5:52:22 PM
Surface Mapping
SCHAEFER
Vector of shell
24
i I
Vectors of fibres
~~~ e e e
-8
Fig. 3. Difference between the resultant vector constructed by the electric moment of three individual fibres, producing fields according to figure 2. If a boundary between already excited and still resting areas is constructed and regarded as a 'shell' carrying the electric charge, a vector going into a completely different region is resulting.
[1957] or DURRER et al. [1970], registering the arrival of the excitation by unipolar microelectrodes situated in the ventricular wall. The shells of charges constructed according to such latency measurements, however are incompatible with the histological structure of the heart, with its overall direction of fibre bundles parallel to the epicardial surface. The most serious objection to mapping methods is that their validation, i.e. the proof, what such patterns really mean, is still very scanty. An attempt to validate patterns in respect to standard ECGs is obviously frustrating. The whole bulk of experience incorporated in the diagnostic use of ECGs would have to be translated into the patterning of mapping methods. Moreover, the costs of mapping will be comparatively high. A cost-benefit analysis should be made therefore. My own opinion is not very optimistic. A diagnosis of the exact location of myocardial damages in restricted areas will not very often lead to a more exact prognosis or to a more successful therapy. Prognosis or therapy, however, are the only goals for introducing new methods into medicine, justifying a higher amount of expenses. YOUNG
It may be unnecessary to add a word about computers in electrocardiography. The application of programs based on an ample experience, the reduction in manpower used for diagnosis and, last not least, the liberation
Downloaded by: University of Cambridge 131.111.164.128 - 3/17/2019 5:52:22 PM
Computer Diagnosis
Future Electrocardiography
25
of diagnostic procedure from the inadequacies of individual observers, are strong reasons for the use of computers. Not all problems have been solved. But only with the aid of computers, methods like new vectorial analysis or surface mapping could be successful. Moreover, I scarcely could imagine an institution concerned with financing medicine which, in the near future, will pay for an ECG diagnosis made by an individual doctor. This holds true even more if the ECG will be used as a screening method in preventive medicine. In prevention, the validation of computer programs against the prediction of cardiac attacks, as PIPBERGER et al. [1958, 1972] have done in recent years, will be one of the most urgent tasks. Let me express one idea in closing these few remarks: one of the critiques often heard in respect to medicine is that this medicine develops along the line of the physician's interests. I think this is true to a regrettable degree. The electrocardiography has become a battlefield, where too many technical interests come into the picture, and a cost-benefit analysis would give most probably astonishing results. Those, who are responsible for future development (and we all belong to this group) should carefully reflect the social responsibilities laid upon our shoulders. Salus aegroti suprema lex.
ABEL, H. und LOEPER, H.: Myokardfaserquerschnitte und T-Welle. Z. Kardiol. 62: 380-387 (1973). BJURULF, R.; GARLIND, R., and STERNBY, N. H.: On epidemiologic methods for recording ischemic heart disease. Acta med. scand., suppl. 474 (1967). BLACKBURN, H.; KEyS, A.; SIMONSON, B.; RAUTAHARJU, P., and PUNSAR, S.: The BCG in population studies. A classification system. Circulation 21: 1160-1175 (1960). BLACKBURN, H.; KEYS, A.; TAYLOR, H. L., and THORSEN, R. D.: Screening and prognostic value of the resting ECG. Circulation 48: suppl. 4, p. 23 (1973). CARRUTHERS, M. and TAGGERT, P.: Paleocardiology and neocardiology. Am. Heart J. 88: 1-3 (1974). CORNFIELD, J.; DUNN, R. A.; BATCHLOR, C. D., and PIPBERGER, H. V.: Multigroup diagnosis of electrocardiograms. Comput. biomed. Res. 6: 97-120 (1973). DURRER, D.; DAM, R. T. VAN; FREUD, G. E.; JANSE, M. J.; MEIJLER, F. L., and ARzBAECHER, R. C.: Total excitation of the isolated human heart. Circulation 41: 899--912 (1970). ERNSTHAUSEN, W. und KIENLE, F.: Das elektrische Herzbild (Rinn, Miinchen 1953). GROEDEL, F. M. und KOCH, E.: Topographie der Aktionspotentiale des Herzens auf der vorderen Brustwand. Z. Kreislaufforsch.26: 18-20 (1934). lIILTMANN, W. D.: Die Variabilitat der Vektorschleifen der orthogonalen Ableitesysteme von Frank und Schmitt (SVEC III). Basic Res. Cardiol. 70: 87-102 (1975).
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References
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Prof. Dr. med. H. SCHAEFER, I. Physiologisches Institut der Universitat, 1m Neuenheimer Feld 326, D-6900 Heidelberg (FRG)
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HILTMANN, W. D. und SCHAEFER, H.: Theorie der Methoden zur Vergleichung von Ableitefeldern in der Elektrokardiographie. Basic Res. Cardio!. 70: 78-86 (1975). KOCH, W.: Datenreduktion, GIattung und Basislinienkorrektur vor der automatischen Vermessung von EKG-Signalen. Basic Res. Cardio!. (in press). KOCH, W. und SCHAEFER, H.: Neue Funktionen und Variablen fUr die Charakterisierung und den Vergleich von Vektorkardiogrammen. Basic Res. Cardio!. 70: 103-114 (1975). OKAMOTO, N. and SIMONSON, E.: Separation of normal and abnormal vectorcardiograms. Am. J. Cardio!. 18: 682-689 (1966). PIPBERGER, H. V.: The normal orthogonal electrocardiogram and vectorcardiogram. Circulation 15: 1102-1111 (1958). PIPBERGER, H. V.; BERSON, A. S.; KLINGEMAN, J. D., and BATCHLOR, C. D.: Diagnostic classifications of orthogonal electrocardiograms and vectorcardiograms. Proc. 11 th Int. Vectorcardiography Symposium, pp. 157-163 (North-Holland, Amsterdam 1971). PIPBERGER, H. V.; DUNN, R. A., and CORNFIELD, J.: First and second generation computer programs for diagnostic ECG and VCG classification; in RIJLANT: Proc. Satellite Symp. 25th Int. Congr. Physio!. Sci., pp. 431-440 (Presses academiques europeennes, Bruxelles 1972). SCHAEFER, H.: Das E1ektrokardiograrnrn (Springer, Berlin 1951). SCHAEFER, H. and HAAS, H. : Electrocardiography. Handbook of Physiology. Circulation I, pp. 323--445 (American Physio!. Society, Washington 1962). SCHER, A. M. and YOUNG, A. c.: Ventricular depolarization and the genesis of QRS. Ann. N.Y. Acad. Sci. 65: 768-778 (1957). SEIDEN, G. E. and STAHL, C.: A new method for diagnosing myocardial damage in patients with normal ECGs and VCGs. Trans. N.Y. Acad. Sci. 35: 283-313 (1973). SIMONSON, E.: Differentiation between normal and abnormal in electrocardiography (Mosby, St. Louis 1961). TACCARDI, B.: Panel session: clinical studies and prognosis for mapping as a clinical tool. Adv. Cardio!., vo!. 10, pp. 296-301 (Karger, Basel 1974). T ACCARDI, B.; AMBROGGI, L. DE, and VIGANOTTI, c.: Characteristic features of surface potential maps during QRS and S-T intervals. Adv. Cardio!., vo!. 10, pp.248-256 (Karger, Basel 1974).
