ELECTROCARDIOGRAPHY 



36: 



indicate more than tlie change in QRS indicates, and 

 of course is no sign of abnormal myocardial processes. 

 In extrasystoles (216), in the WPW syndrome (105) 

 and bundle branch blocks (365), G may remain 

 normal, indicating that changes of T are only con- 

 ditioned by the changes of QRS. Where repolariza- 

 tion is distorted by refractoriness (in premature 

 beats and some extrasystoles) G will change (484). 



Only changes of T without concomitant changes 

 of QRS, therefore, bring about primary alterations 

 of the ventricular gradient ultimately due to mech- 

 anisms which influence the inhomogeneities of 

 repolarization. As has been pointed out before, a 

 dilatation of the heart could reduce the inhomo- 

 geneities by equalizing all mechanical and thermal 

 differences between the various parts of the heart. 

 The movements of the heart (elevations and rotations) 

 are certainlv reduced as well, when the heart is 

 enlarged. This will lead to a flattening of T and a 

 reduction of the ventricular gradient. The most 

 important changes of T, however, are caused by new 

 inhomogeneities in the monophasic action potentials 

 which change the cHrection of G.,, and increase its 

 numerical value in most cases. Such changes are 

 induced mostly by "iocar' events, which alone lead 

 to an inhomogeneous behavior of the muscle cells. 

 Therefore local metabolic differences, caused Ijy local 

 ischemic damage, local inflammation, etc., are the 

 important reasons for strong alterations of the mag- 

 nitude and the direction of T . 



A peculiar cause of a diminished ventricular 

 gradient is the loss of the plateau (ST interval), which 

 itself has to be ascribed to disturbances of the ionic 

 exchange, especially to an accumulation of potassium 

 in the extracellular space. In such cases, the records 

 show constant decline in voltage during the whole 

 repolarization, every difference between action 

 potentials and plateaus of different fibers being 

 reduced. Such a loss of plateau always leads to a 

 change in the contour of T. The T wave becomes a 

 small, long lasting, and (in the case of a linear decline 

 of the action potential) constant, negative deflection 

 starting at the end of S. In such cases, it is very 

 difficult to find a correct line of reference. The 

 determination of the T area therefore becomes 

 erroneous. 



Summarizing, it becomes clear that T and the 

 ventricular gradient respond most strongly to local 

 changes or damage, since only these induce an 

 augmentation or change in the inhomogeneities of 

 the repolarization. If, therefore, a previously local 



area of damage spreads and becomes generalized, 

 the EGG may look more normal than before, in 

 spite of an impaired heart (41). 



10. DESCRIPTION OF THE PqRS PART 



Any attempt to describe the normal EGG meets 

 with two difficulties: defining "normality" and 

 selecting methods of description which present all 

 information contained in the EGG in the shortest, 

 but nevertheless most comprehensive, form. 



The form of the EGG varies considerably from 

 person to person and even in the same person during 

 the day. The reason is that functional differences in 

 the spread of excitation, the local excitation process, 

 and the inhomogeneity of repolarization (because of 

 differences in the autonomic innervation) exist. The 

 strongly diverging excitation waves cancel their 

 respecti\'e fields to such a degree that even small 

 individual differences or small changes in the spread 

 of excitation lead to large differences in the potential 

 pattern. In addition, differences in the anatomical 

 properties of the thoracic surface, conductivity 

 distribution, heart position, etc., lead to a great 

 variety in the form of the lead fields, the direction 

 and magnitude of the lead vectors, and the magnitude 

 and direction of the heart vector itself. Individual 

 variability, therefore, is very large, and a single 

 normal form does not exist. We do not even know 

 its borderlines (449, 454), because "normality" 

 obviously can be defined only from the point of view 

 of the heart's function or anatomical structure, which 

 cannot both be judged in all cases. Moreover, the 

 separation of disturbances in the peripheral circula- 

 tion from those of the heart is frequently somewhat 

 intricate. As a matter of fact, it is not unusual to 

 find an EGG pattern which we are accustomed to 

 regard as abnormal in apparently healthy and even 

 young persons (452, 502). 



A reliable presentation of electrocardiographic 

 data can avoid a tiring enumeration of facts only by 

 presenting the electrical events of the heart in their 

 simplest form: as the temporal change of the heart 

 vector, recorded as the ''vector loop." 



The vectors and the vector loop are nevertheless 

 insufficient to interpret in an easily understandai)le 

 manner all information available in the EGG. All 

 details concerning the time course are more con- 

 cealed than disclosed in vector loops, even when 

 time marks are given. As the spread of excitation is 

 the most important physiological event in the heart. 



