ELECTROCARDIOGRAPHY 



345 



FIG. -27. Demonstration of the proximity efl'ect of the Wilson chest lead V.v The heart is 

 roughly divided into five compartments, each of which forms the part of a spheric shell with various 

 solid angles and thicknesses. The figure to the right shows how these compartments are projected 

 on the sphere F, seen from the electrode. The amount of potential which (assuming random dis- 

 tribution and direction of the individual fibers in the compartment) each compartment contributes 

 to the record V;, equals the product of solid angle and thickness. The various compartments then 

 contribute the following percentile potentials of the record : 



Relative Solid 



Angle 



2-3 

 3-5 

 3-3 

 2-3 



'.7 



47f 

 50) 



One \'entricle in % of the Whole Heart 



Icfi 



-ieht = 28% 



Even in V;,, the right ventricle is represented with one-fourth of the whole potential diflference of 

 the record! [From Schaefer (58).] 



INFORMATION GAINED BY THE VARIOUS ELECTRODE 



SYSTEMS. Against the background of the preceding 

 sections, the problem of information given by any 

 certain lead system may he discussed. The word 

 '"information" here means that the electrocardio- 

 graphic record contains symbols which can be 

 transformed into language describing physiological 

 events. In the first place, the direction of local poten- 

 tials (of their vectors) is of interest. The more the lead 

 field is curved, however (see fig. 23), the more uncer- 

 tain is every statement concerning the site and 

 direction of the fibers which react in an abnormal 

 manner. The quantitative problem of the muscle 

 masses participating in the potential pattern (fig. 27) 

 is a second handicap to interpretation. If in a record 

 the potential differences belonging to the left and to 

 the right heart are equal, no information can be 

 gained about the localization of events in one of the 

 two parts of the heart, even in case of heavy distor- 

 tions. If the proportion of potentials recorded is 



not so well balanced, and a distortion of the potential 

 pattern is ob.served, the probability of course is 

 greater that the distortion belongs to events in the 

 preferentially derived part of the heart. Nevertheless, 

 abnormal events in large, but remote, parts of the 

 heart have the same influence on the total potential 

 production as abnormal events in small, but near, 

 parts. The decision as to which of these two possibili- 

 ties is realized in a peculiar case can only be made 

 with other indirect or empirical signs. One single ex- 

 ception exists : if the direction of a very large bundle 

 of muscle fibers is known, an electrode position at 

 which the lead field flow lines pass the bundle par- 

 allel to its fibers is "optimal" in recording events in 

 that special bundle. Such conditions may exist only 

 during the repolarization process or in the .ST dis- 

 placement, for reasons to be discu.ssed later. 



So, data which allow a physiological interpretation 

 of local events are only occasionally embodied in 

 local leads, and gained only by the strictly precordial 



