ELECTROCARDIOGRAPHY 



351 



thus forming excitation waves of opposite direction 

 with a cancellation of their electric fields. There are 

 several sets of observations which elucidate this 

 general pattern in more detail. Unfortunately, all 

 these details are only concerned with the hearts of 

 animals, mostly dogs. There is only a small amount 

 of information available which deals with the human 

 heart. But the similarity between the anatomy of 

 human and dog hearts does allow generalization of 

 the conclusions drawn from experiments in dogs. 

 These indicate that a considerable synchronization 

 of all parts of the ventricle is effected by the fact 

 that /) the conducting system connects all points of 

 the ventricle on the shortest pathway with a centrally 

 situated point of distribution, at which point the 

 specific system branches; 2) this system conducts 

 with a comparatively high velocity of about 2.5 m 

 per sec, diminishing in an "intermediate" part to 

 about i.o m per sec and reaching then, in the bulk 

 of the myocardial fibers, perhaps much slower rates 

 (171). These facts go\'ern the total duration of QRS. 



Direction of Excitation Wave Recorded ]Vith 

 Bipolar Electrode Combinations at the Surface 



The simplest way to determine the pattern of 

 propagation along the ventricular wall would be by 

 direct measurement of propagation velocities and 

 directions. This, however, can be done only with a 

 minimum of three different electrodes closely placed 

 on a bundle of myocardial fibers. As the orientation 

 of such bundles can never be observed directly in the 

 interior of the ventricular wall, only surface measure- 

 ments promise reliable results. However, the distance 

 between such electrodes must be smaller than i 

 mm, if the behavior of single fibers is to be observed. 

 Any record in which the local R wave or spike poten- 

 tial exceeds even i msec cannot be regarded as a 

 true picture of the elementary processes in the myo- 

 cardium, because too many imsynchronized fibers 

 contribute to the potential. 



The first measurements of this kind with close 

 bipolar electrodes showed small local action poten- 

 tials of a duration as short as i msec (412). A "com- 

 pass-electrode" was used, with which several direc- 

 tions of conduction could be derived. The electrode 

 position yielding the maximal amplitudes could be 

 defined as being parallel to the direction of the excita- 

 tion process. The time difference between the peaks 

 of two potentials recorded with closely paired elec- 

 trodes was an exact measure of the propagation 

 velocity (fig. 33). With a series of such measurements, 



the propagation at the surface of a dog's heart could 

 be investigated, and the result shows that the propaga- 

 tion waves run more or less uniformlv in a direction 

 which seems to start from a common central region, 

 the so-called "source" (Quellpunkt) on the anterior 

 surface of the heart. In figure 34, the directions of the 

 propagation waves are shown in the form of a map; 

 figure 34Z? shows one experiment and its original 

 result. Arrows demonstrate the directions of the waves. 

 The velocities ranged between 0.5 and 1.7 m per 

 sec, with an average of 0.9 m per sec. These observa- 

 tions have been repeated with sufficiently accurate 

 technique only by Meda (344) in frog hearts and 

 with rabbits by Taccardi (479), who achieved similar 

 results. Only Draper & Mya-Tu (171) report much 

 slower conduction velocities in strands of ventricular 

 tissue. Taccardi found that the excitation wave 

 traveled in one direction only over \ery short dis- 

 tances, about 4 mm. This has been confirmed in 

 recent experiments (234). It seems certain that the 

 distance over which there is uniform propagation of 

 myocardial excitation is limited to a few millimeters 

 ("length of free way"). As soon as excessive electrode 

 distances are used, the electrodes touch different 

 muscular bundles and high apparent propagation 

 velocities are recorded (466, 467). 



Latencies at the Ventricular Surface 



The above results have been confirmed by measin-e- 

 ments of local latencies at the epicardial surface. 

 Lewis et al. have investigated these latencies with 

 amazingly precise results, in dogs (323), many other 

 animals (321), and even in man (321). A rather 

 comprehensive series of papers has dealt with the 

 same problem more recently. Unfortunately, most 

 of them recorded the intrinsic deflection in unipolar 

 leads, a method of only restricted value, so that the 

 results are not as accurate as one could wish them 

 to be (64, 81, 388, 397). The intrinsic deflection is 

 by no means synchronous with the arrival of the 

 excitation wave at the electrode (413, 467, 501). 

 Therefore, close bipolar electrodes prov'ide the only 

 method for obtaining conclusive results. All measure- 

 ments made with such electrodes, to date, agree that 

 the latencies of superficial local excitations are 

 minimal in a region on the anterior surface of the 

 dog's heart (64, 237, 412, 420); and in cats (58), 

 rabbits (501), and guinea pigs (169), the same result 

 has been found. This means that the "source" region 

 of figure 34 bears the shortest latency. The differences 

 in latency time, however, are rather small, and very 



