ELECTROCARDIOGRAPHY 399 



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FIG. 69. Schematic drawing of the mechanism leading to the phenomenon of the compensatory 

 pause. The distance between sinus and ectopic pacemaker is short in the upper picture (atriail 

 extrasystole), long in the lower (ventricular extrasystole). In black brackets the time is indicated 

 during which either the normal beat travels from sinus to the atria or from the ectopic pacemaker 

 back to the sinus. When the antidromic volley reaches the sinus, the sinus pacemaker is stimulated 

 and responds with an excitation, if it is not refractory. In any case, the status of membrane repolari- 

 zation is brought back to a newly induced complete depolarization. The upper line in each picture 

 indicates the local excitatory state of the sinus, which brings the sinus to excitation in reaching the 

 threshold (black dots). If the sinus is prematurely depolarized by an antidromic volley from the 

 ectopic center {upper picture), the phase of the sinus rhythm is shifted, the compensatory pause too 

 short. If the sinus beats before the antidromic volley arrives (lower picture) both nomodromic and 

 antidromic volley meet and cancel each other {crossed circle). The compensatory pause is correct; 

 no shift in the sinus rhythm. [From Schaefer (58).] 



of the atria, which leads to a complicated form of P. 

 If the ectopic pacemaker beats in a very remote point 

 of the ventricles, or shortly after the preceding normal 

 beat, the nomotopic excitation may be just proceed- 

 ing in the ventricle when the retrograde extrasystolic 

 wave arrives, thus causing a ventricular activation 

 which stems partly from the nomotopic, partly from 

 the ectopic excitation. Such beats are called "fusion 

 beats," and may be recognized by their form, which 

 shows them to be a mixture of nomotopic and purely 

 ectopic beats. In this latter case, the sinus remains 

 beating in its normal rhythm without any disturbance, 

 and the compensatory pause exactly completes the 

 preceding ectopic interval to double the normal (58, 

 p. 387). In comparatively rare cases, an ectopic beat 

 is interpolated between two normal beats of normal 

 interval. But this is possible only in bradycardias, 

 where the refractory period of the ectopic beat ends 



before the normal sinus starts its next excitation. 

 Whether or not, in this or in the former cases, the 

 sinus is shifted in phase by the ectopic beat depends 

 merely on chance, i.e., on the time relations between 

 the sinus interval, the retrograde conduction time 

 from the ectopic pacemaker to the sinus, and the in- 

 terval between the ectopic and its preceding normal 

 beat. As innumerable as the combinations in the sites 

 of ectopic beats, in the frequency ratios of ectopic and 

 nomotopic beats, and in the phase shifts benveen the 

 two pacemakers, are the difi'erent electrocardiographic 

 patterns. 



A peculiar phenomenon has been described as 

 "return extrasystoles" or "'ventricular echoes": a ven- 

 tricular extrasystole starts a retrograde excitation 

 running to the atria, being "reflected" here and ex- 

 citing again the ventricles with a second abnormal 

 beat (60, 394). 



