ELECTROCARDIOGRAPHY 



361 



length of the myocardial fibers over wliich an un- 

 branched homogeneous wave is running to be about 

 10 mm or less, a simple calculation can be made, 

 which is given in table 2 (58, p. 83). In this table we 

 have tried to calculate the relation between heart 

 weight and QRS duration, and to give at the same 

 time a simple explanation for the observed mag- 

 nitudes.'* As the table shows, the QRS duration 

 changes very little with heart weight, so that a 

 fairly good correlation exists between QRS duration 

 and the cube root of weight, as has been already 

 claimed by Wilson & Hermann (525). The correlation 

 with the sixth root of weight is even better. The 

 reason is obvious: "hypertrophy" acts in the later 

 stages of normal growth, as well as under pathological 

 conditions, to augment fiber diameter, and not so 

 much to augment fiber length. If this is correct, then 

 every increase in the QRS duration does not indicate 

 hypertrophy so much as an increase in conduction 

 times. Two events lead to such an increase : the 

 decrease of conduction velocity in either specific or 

 regular myocardial fibers, and the block of those 

 specific conduction pathways which provide the 

 shortest pathways to different myocardial areas 

 ("blocks" in the general sense). A decision may be 

 made between these two possibilities by analyzing 

 the direction of the heart vector. Every pure de- 

 crease in the conduction velocity would leave direc- 

 tion and the vector loop of the heart vector 

 unchanged. If, however, the decrease were inhomo- 

 geneous, so that some parts of the heart were more 

 involved in it than others, or if some pathways were 

 blocked, the excitation wave of some fiber bundles 

 would have to travel in the opposite direction from 

 normal (fig. 44) and would arrive with a much 

 higher latency. Changes in the vector direction 

 combined with an increase in the QRS duration are 

 due, in most cases, to blocks of the specific system or 

 an otherwise induced inhomogeneous change of the 

 conduction velocity. 



The duration of QRS enables us to estimate the 

 number of simultaneously active fibers. The total 

 length of all myocardial fibers in the left ventricle 

 alone amounts (in a normal heart weighing 130 g) 

 to about 3.10' cm. The surface area of these fibers 

 equals about 25 m- (326). The time of QRS during 

 which a considerable number of fibers are active may 



* If the myocardial conduction velocities were much less, 

 e.g., about 0.55-0.70 m/sec, the estimated length of the myo- 

 cardial fiber in table 2 would have to be reduced to about 

 two-thirds of the values given. The principal results of table 2 

 remain valid, however. 



^e^ der Erregungs- 

 welle beim Block 



tlyokardiale 

 Brijcke 



FIG. 44. Schematic drawing of the excitation pathway in a 

 left bundle branch block. The direction of the excitation wave 

 is inverted in the peripheral parts of the left branch, which is 

 stimulated by a myocardial bridge, indicated by an arrow, 

 from the right bundle. [From Schaefer (408).] 



be listed as 50 msec. During this time the excitation 

 can run over a myocardial fiber of only 5 cm in 

 length. As 3.10' cm are e.xcited, on the average, 

 6.10" fibers must be active at the same time. This 

 number is too high for the rising and declining phases 

 of R, and too low for the peak; and only the left 

 ventricle is taken into account. In leads with parallel 

 lead fields, it should be possible to calculate the 

 absolute value of the voltage resulting from such data, 

 and thereby the amount of mutual cancellation. 



2) Changes in the vector direction can be induced 

 by different processes: by changes in the prepond- 

 erance, by changes in the direction of the excitation 

 waves, by changes in the temporal excitation pattern 

 of the various regions of the heart, and b\- changes in 

 the anatomical position of the whole heart. The 

 vector direction changes in all these cases, the momen- 

 tary vectors as well as the vector of the QRS area. 

 It is, therefore, of much value to be able to scan the 

 vector directions .swiftly and roughly. This is possible 

 with the technique called "t\pology." Several 



