ELECTROCARDIOGRAPHY 



393 



sinus or Keith-Flack node. The electrical process lead- 

 ing to the generation of the excitation wave, the 

 "generator potential," has been described in the pre- 

 ceding chapter. In regard to the whole heart, the 

 problem rises as to how the pacemaker region is de- 

 termined. Obviously, the region with the most fre- 

 quent rhythmic activity commands the rhythm of the 

 heart. Since all parts of the specific system may 

 develop spontaneous activity (whereas the ordinary 

 myocardial fibers never reveal a generator potential 

 under physiological conditions), the sinus must have 

 the most rapidly developing generator potential or 

 the lowest threshold for the amount of depolarization 

 required for activation, thus leading to the earliest 

 local excitation following a previous beat. This im- 

 plies, however, that every part of the heart with po- 

 tentially automatic activity has been activated by the 

 previous excitation wave, and that thereby all local 

 membrane potentials have been depolarized and re- 

 polarized to the resting level. This being the case, 

 every fiber of the heart with automatic rhythmic ac- 

 tivity is forced to start at the same time from the same 

 resting state. Thus the more slowly a particular local 

 generator potential is developed, or the higher its 

 threshold, the more time it will require to generate a 

 new impulse. Local pacemakers beat more slowly the 

 more distant they are from the sinus venosus. If the 

 sinus is totally removed, the heart beats with a fre- 

 quency reduced to about 30 per cent of the normal, 

 although the often-quoted close relationship between 

 the frequency and the site of local ectopic pacemakers 

 cannot be cited here with conviction (275). 



There are possible and actual conditions under 

 which a certain region of the heart is not completely 

 depolarized by an arriving excitation, so that this part 

 of the heart occasionally escapes total depolarization, 

 thus developing a generator potential which might 

 start a local excitation at a very early moment after an 

 ordinary beat. If this wave meets nonrefractory sur- 

 rounding tissue, the next heart beat starts at this point 

 and an extrasystolic or parasystolic beat appears. The 

 basic mechanism which apparently protects that 

 region of the heart from being excited is called an 

 "entrance block." 



There are several mechanisms which may possibly 

 shift the pacemaker region temporarily away from the 

 sinus. The first is well known and consists of a new 

 balance between the steepness of the generator po- 

 tential and the local threshold (488, 490). If, under 

 any circumstances, a part of the heart develops a 

 swifter diastolic depolarization, this part of the heart 

 may become the pacemaker, if thresholds remain the 



same elsewhere. The strong dependency of the gener- 

 ator potential on the local concentration of adrenaline 

 and acetylcholine makes it easy to explain why every 

 change in the tonic autonomic innervation easily 

 leads to such a shift. Most probably, right in the sinus 

 region, the counterbalancing influence of sympathetic 

 and vagal inner\ation leads to pacemaker shifts over 

 small distances. Such shifts have been directly ob- 

 served (58, p. 377; 180, 223) especially under the 

 influence of the heart nerves (519). The various parts 

 of the auricles are apparently innervated with a 

 different density by vagal and sympathetic endings, 

 so that every v-agal innervation slows down the gen- 

 erator potential at the sinus pacemaker, thus enabling 

 another part of the auricle to command, with its 

 quicker generator potentials, the start of the next 

 excitation wave. 



Such fluctuations in the generator potentials ap- 

 parently occur all over the heart, depending on the 

 fluctuations in autonomic nervous activity. Other in- 

 fluences, as metabolic ones, will act in a similar 

 manner, so that in longer periods the pacemaker may 

 shift about. In normal hearts, such events are rare; in 

 hearts with local disturbances of metabolic or ionic 

 equilibria, however, such things occur much more 

 easily. The time between the preceding stimulus and 

 the moment when the membrane potential reaches 

 the threshold may be called the local "pacemaker 

 interval." Changes of this local pacemaker interval 

 are then responsible for the shift of the pacemaker 

 position, because that region takes over the pace- 

 maker function which at this moment has the shortest 

 local pacemaker interval. Shifts of this kind may 

 generally be regarded as a "dissociation with inter- 

 ference" ("Ersatz-Rhythmus"), because two or more 

 dissociated, i.e., independent, centers with undis- 

 turbed local frequencies interfere in playing the pace- 

 maker. In normal hearts, such events are rare or 

 absent, as all other parts of the heart have a much 

 slower local rate of generator potentials than the sinus. 



This is valid only in hearts in which no entrance 

 blocks are to be found. Whenever such a block occurs, 

 it is only a matter of chance whether the blocked part 

 of the heart or the normal pacemaker will command 

 the next beat. If we call the normal pacemaker 

 "nomotopic," every shifted pacemaker "heterotopic" 

 or "ectopic," a special form of interference is to be 

 expected, depending on the special circumstances 

 given at the moment of investigation. Every rhythm 

 determined by the interference of two or more pace- 

 makers is generally called "parasystolic," the related 

 arrhythmia "pararrhythmia." The characteristics of 



