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HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



it in terms of physical conditions like membrane 

 permeabilities. 



The Wenckebach periods are an excellent example 

 of what is called the formation of a period. "Period"' 

 here means that a given number of beats form a regu- 

 larly repeated pattern (io8). Periods of this kind may 

 be found in the activity of every part of the heart and 

 as ventricular extrasystolic periods as well. In such 

 cases, one or two, or even more extrasystoles (bi- 

 geminus, trigeminus, quadrigeminus, etc.) are coupled 

 with a normal beat, and the period ends obviously 

 with a refractoriness (or exhaustion) of the ectopic 

 pacemaker. 



A peculiar variety of such periods is called the 

 "alternans," which is of interest in this connection 

 only as the '"electrical alternans" (42, 60, 66). The 

 term alternans is applicable only to such periods in 

 which two different forms of an ECG or action poten- 

 tial alternate, but with a completely regular rhythm. 

 This regularity, however, concerns only the rhythms 

 of P or of the beginning of QRS. The behavior of 

 parts of the heart beating later, during the conduction 

 time from the A-V node to the utmost parts of the 

 ventricle, cannot be stated in such records. It has been 

 observed that cooling parts of a frog's heart leads to a 

 mechanical alternans, obviously because the cooled 

 parts of the heart are beating merely in a 1:2 rhythm 

 as compared with the warmer parts (292). It is ques- 

 tionable whether basically different forms of a true 

 alternans can he found, in which the single cell of the 

 heart l^eats with a different amplitude of its membrane 

 action potential, for reasons other than refractoriness. 

 A true alternans has really been described in single 

 cells with microelectrodes (290), so that for such a cell 

 refractoriness cannot play any role. The mechanism 

 of such an alternans is obscure and difficult to explain. 

 Apparently some fundamental process responds in a 

 I '.2 manner. In the whole heart, such events are never 

 shown to exist. In most cases, a partial hypo- or 

 asystole or an alternating disturbance in conduction 

 will be found to be the cause of an electrical alternans 

 (42). Partial refractoriness of the heart will lead to an 

 alternating ECG pattern, if the restoration of the 

 alternately beating parts is completed during twice 

 the normal interval. 



Besides this alternans, a pattern may be described 

 in which the start of P and QRS is absolutely rhyth- 

 mic, but the QRS form alternates on account of 

 ectopic beats alternating in a 1:2 manner. This is a 

 peculiar case of a coupled extrasystole, and as such is 

 a "false" alternans, as is every bigeminus. The alter- 

 nans patterns may be very complicated due to the 



great number of varieties possible in local refractori- 

 ness or fusion beats (313). 



The ECG of Ectopic Beats 



The ECG of ectopic beats is characterized by ab- 

 normalities conditioned by the abnormal spread of 

 excitation. Hereijy, the sequence of P and QRST 

 may be changed, if the atria are excited by a back- 

 ward traveling wave; or only P may be changed if the 

 pacemaker lies between the atria and the ventricular 

 myocardium near the A-V node; or QRST may be 

 changed by an abnormal spread of individual fiber 

 excitations along the ventricular wall (see fig. 42). 



The diagnosis concerning the exact site of the acting 

 pacemaker may be complicated in a single case, be- 

 cause P may be no longer discernible, being sub- 

 merged in the electric events of the ventricular activa- 

 tion. Moreover, the pattern may change from beat to 

 beat due to a change in refractoriness or to an inter- 

 ference of several pacemaker activities. The detail of 

 such pictures cannot be discussed here and is of clini- 

 cal interest only (516). There are, however, some facts 

 to be mentioned which illustrate general physiological 

 laws. The first is that after a premature beat the fol- 

 lowing interval is usually lengthened, so that the in- 

 terval between a beat preceding and following the ec- 

 topic beat is approximately double that of the normal 

 nomotopic interval (fig. 69). This prolonged postec- 

 topic interval is called the "compensatory pause." The 

 mechanism of this pause is simple. The ectopic pace- 

 maker starts an excitation wave running to both sides, 

 antidromically as well as nomodromically. The anti- 

 dromic volley somewhere meets either the excitation 

 started meanwhile by the sinus, or — in case of a very 

 early beat — the sinus itself before it becomes active. 

 In the latter case (upper part of fig. 69), the sinus is 

 excited by this retrograde wave and starts now with a 

 new pacemaker potential, its start being somehow an- 

 ticipated. The interval between the preceding and the 

 following normal beat is less than twice the normal. 

 In the former case (lower part of fig. 69), the anti- 

 dromic wave meets the nomodromic wave started by 

 the sinus somewhere between sinus and ectopic focus. 

 Obviously, this will happen the farther from the sinus, 

 the farther the ectopic pacemaker itself lies from the 

 sinus. In ventricular extrasystoles, the retrograde 

 excitation has to travel up the whole Purkinje system 

 and His bundle, through the A-\' node and the atria, 

 which obviously takes much time. In this retrograde 

 conduction time, the sinus has already started its next 

 nomotopic excitation. Both waves meet in the middle 



