ELECTROCARDIOGRAPHY 



325 



pretation of the ECG is based upon a model which 

 simplifies the physiological and physical conditions as 

 much as necessary to permit a clinicalh- relevant and 

 relatively simple mathematical formulation. We 

 should always bear in mind that the ECG is primarily 

 a clinical, diagnostic method that must be translated 

 into the simple language of muscular function (con- 

 tractile force) or prognosis of function under the influ- 

 ence of trauma, drugs, etc. Because, for the most 

 part, translation into such physiological data cannot 

 be achieved, the ECG is merely a toy for some scien- 

 tists. No basic problems of general interest can be 

 solved on the playground of electrocardiographic re- 

 search, excluding the problems of production of the 

 potential at the fiber membrane. Our endeavor, there- 

 fore, will be to demonstrate how far and to what pur- 

 pose modern electrocardiographic research serves 

 elementary clinical application. [For further back- 

 ground, see monographs (1-74).] 



I. TECHNICAL REMARKS 



The basis of every electrocardiographic diagnosis 

 is the record. The reliability of recording svstems de- 

 pends on the properties at the surface of the body of 

 the electrical events generated by the heart. With the 

 usual leads, the ECG has an average potential varia- 

 tion of I mv, but recording of potentials accurately 

 to o. I mv is necessary (438). Most recording systems 

 possess sensitivities of about i mv per cm. The time 

 course of the ECG is comparatively slow, so that a 

 recording velocity of 25 to 50 mm per sec is sufficient. 

 The accuracy of the curve, however, depends mainly 

 on the frequency range of the event to be recorded 

 and of the recording system. Fourier's analyses of the 

 ECG reV'Cal a maximal frequency range of o to 1 40 

 cps (212), but, in general, the frequencies between 0.3 

 and 60 cps are responsible for all waves and notches 

 found in a usual record, and 0.5 to 20 cps for all of 

 clinical importance (211). 



It is no proljlem to construct amplifiers with such a 

 frequency band, whereas the construction of adequate 

 recording systems is much more difficult. The best 

 direct writer is doubtless the "mingograph," which 

 utilizes a moving beam of ink spurted out of a very 

 tiny glass cannula. Systems of this kind move at 

 speeds up to a critical frequency of 200 cps, whereas 

 the stylus technique, at its best, reaches an upper fre- 

 quency of 100 cps. 



Evaluation of an ECG is a somewhat tedious pro- 

 cedure. A great variety of techniques and evaluation 



devices, therefore, have been invented, even going so 

 far as to put the ECG into a computing machine, 

 which analyzes it in all possible directions in an ex- 

 tremely short time.' Details are beyond the scope of 

 this review. 



2. THE IDEAL ELECTRIC FIELD OF A 

 SINGLE CARDIAC FIBER 



The ECG is originated by sources of electrical po- 

 tential within the muscular wall of the heart; it is, in 

 most cases, picked up by electrodes more or less re- 

 mote from the heart. We therefore record, in the 

 ordinary ECG, peculiarities of an electric field. The 

 theory of such a field is extremely complicated, be- 

 cause of the irregular body surface, the inhomogeneous 

 conductivity of the thorax, the eccentric position of 

 the heart as the source of the field, and the com- 

 paratively large diameter of the heart — large, that is, 

 when compared with every electrode distance possible. 

 Nevertheless, a description of the field of the body sur- 

 face must start with the simple physical rules which 

 determine the field under ideal conditions. Such con- 

 ditions are: a homogeneous medium of infinite bound- 

 aries, with a central dipole as a source representing 

 the heart. A first step in adapting this ideal condition 

 to reality can be the assumption of a regular, spher- 

 ical boundary of the medium, thus representing an 

 approximation of the boundaries of the thorax. 



The very first papers on the dipole theory of the 

 heart (161) started with the assumption that the elec- 

 trical source, i.e., the heart, may be represented by a 

 physical dipole consisting of two equal spheres. The 

 fields which are determined by such a doublet of 

 spherical charges of opposite sign are somewhat simi- 

 lar to the fields of single mu.scular fibers, in spite of 

 their completely different geometrical form. 



It was Wilson et a/. (530) who introduced a more 

 realistic model of the heart as a source of electric po- 

 tential. Figure i gives a more correct model of the 

 electrical charges producing the field in the case of a 

 single active fiber which is closed at both ends (410). 

 If, in such a fiber, the potential difference across the 

 membrane is equal at any point along the fiber, no 

 potential field is generated in the surrounding medium 

 (homogeneous double layer). This is true in the resting 

 as well as in the "completely" activated state. If, how- 

 ever, an excitation wave has entered the fiber, so that 

 one part of the fiber (the right in fig. i ) is still resting, 



' Research going on at the National Bureau of .Standards, 

 Washington, D. C. (3,'^6, 370, 478). 



