394 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



parasystolias are that the coupling of several ectopic 

 with nomotopic beats varies, so that the duration of 

 intervals between two consecutive ectopic beats stand 

 in simple mathematical relationships to one another 

 (though this may vary a bit according to changes of 

 ectopic frequency) and that combination or fusion 

 beats are present (60). (See below.) It should, how- 

 ever, be kept in mind that no explanation for what we 

 call an entrance block is available, but that without 

 the assumption of such a block the whole theory of 

 parasystoles is applicable only in case of changing 

 local generator frequencies, i.e., mainly nodal or 

 ventricular tachycardias, which are comparatively 

 rare. 



Dissociation and Interference 



In case of two or more simultaneously active pace- 

 makers, the normal sequence of the excitation process 

 in sinus, atria, A-V node, and ventricle may disappear. 

 Every interruption of that sequence is called a disso- 

 ciation. This dissociation may be accompanied by a 

 complete block of the dissociated parts of the heart, 

 due to a disturbed conduction or to unilateral con- 

 ductivity (e.g., lacking the retrograde conduction 

 from ventricle or A-V node to the auricle). In such a 

 case the dissociated pacemakers cannot interfere and 

 will beat completely independently of each other (dis- 

 sociation with complete block). As soon as conduction 

 takes place, an excitation wave starts from every pace- 

 maker, so that, in certain cases, the excitation waves 

 meet each other on their way and obliterate each 

 other. In other cases, the more frequent pacemaker 

 forces the other one into a refractory period and, in 

 the simplest case, induces a complete depolarization 

 of its rival which then remains inactive until, by a 

 change in its generator mechanism, it develops a 

 shorter local generator interval and thus becomes the 

 leader of the interfering pacemakers. If, however, the 

 slower pacemaker is protected by an entrance block 

 against the excitation wave coming from its rival, 

 very intricate conditions arise and the pattern of 

 rhythm developing then can scarcely be predicted 

 (dissociation with interference and entrance block). 

 There is a third possibility of dissociation found when 

 the conduction between the two pacemakers varies 

 from beat to beat as a result of delayed conduction or 

 even a partial block, occurring temporarily and dis- 

 appearing after longer diastolic intervals. Such 

 changes of conduction are due in most cases to changes 

 of refractoriness, of which the well-known Wencke- 

 bach periods are an example. 



If during disturbances of rhythm a heterotopic 

 pacemaker suddenly becomes active, it "escapes" from 

 the rhythm imposed on the heart by the ordinary 

 pacemaker. Such an escape means either that the 

 primary pacemaker suddenly failed to discharge, or 

 that a hitherto absent entrance block suddenly came 

 into action. In the former case, the arrhythmic ectopic 

 beat would appear later than the normal beat would 

 have been expected. In the latter case, the ectopic 

 beat would appear too early ("premature"). If one 

 considers that possibly a pacemaker region may not 

 only be blocked by an entrance block, but may be 

 unable as well to excite its environment by what is 

 called an exit block, one recognizes that an immense 

 nuinber of varieties in disorders of rhythm and con- 

 duction is possible, which cannot be described here in 

 detail. 



Extrasystoles 



It seems to be generally accepted that besides para- 

 systolic disturbances of rhythms, completely different 

 "true" extrasystolic beats do exist. Extrasystoles of 

 this kind are always precipitated by a preceding heart 

 beat. The question is whether such extrasystolic beats 

 are caused at the site of their origin by a specific 

 mechanism. Parasystoles are comparatively rare 

 events, whereas extrasystoles with a fixed coupling to 

 the preceding beat are often found in clinical cases. 

 On the other hand, only the pararrhythmia can easily 

 be explained by ordinary physiological events at pace- 

 maker regions, if one disregards difficulties in under- 

 standing entrance block. No comparably simple ex- 

 planation is available in case of true extrasystolic 

 arrhythmias, because records of the action potential 

 of single fibers do not support us, in case of true extra- 

 systoles, with a simple mechanism like the generator 

 depolarization during diastole. There are, neverthe- 

 less, several indicators of a common event at the base 

 of both extrasystolic and parasystolic arrhythmias. 

 The first is that pararrhythmias are converted in the 

 same patient into true extrasystolias, with the same 

 form of QRS for the extrasystolic beats, and therefore, 

 most probably, with the same site of arousal of para- 

 systolic and extrasystolic excitations (34, p. 612). 

 Then, only regions of the heart with a clear diastolic 

 depolarization (generator potential) usually develop 

 extrasystoles. Adrenaline steepens the generator po- 

 tential and strongly facilitates extrasystolic beats, as 

 is well known. Anoxia (491) and stretch (173) both 

 foster the development of generator potentials and 

 elicit coupled extrasystoles. The same is true with 



