PAROXYSMAL IRREGULARITY OF THE HEART 9 



to the ventricle by way of the auricle. Two factors are thus involved 

 directly ; the discharge of the impulse from the rhythmic area must be 

 regular, and its path through the auricle unobstructed ; and the ventricle 

 must always be in such a condition that it can respond to the stimulus, 

 while its irritability is too low to permit of its initiating a contraction 

 spontaneously. Irregularity of the heart arises either from irregularity 

 in the discharge of impulses or from an abnormal condition of the 

 ventricle which leads to spontaneous contraction before the descending 

 impulse reaches it ; in rarer cases the ventricle is unable to respond to 

 the descending impulse and a beat lapses. Careful examination of the 

 pulse often determines whether the irregularity arises from ventricular 

 disability or from the irregular action of the originating area.* For in 

 intermittent pulse due to ventricular lesion the intermission is almost 

 exactly twice the interval between two normal beats, while, if the dis- 

 charging area be at fault, the intermission is considerably shorter. The 

 fact that the intermission due to ventricular abnormality must be a 

 multiple of the normal length of a pulse wave, follows directly from the 

 consideration that the impulses descending through the auricle are 

 undisturbed in rhythm, and the intermission can end only on one of 

 these reaching the ventricle and originating the first normal beat after 

 the irregularity. That the intermission is shorter when it is due to an 

 irregular discharge from the rhythm-giving area was first deduced from 

 animal experiments, and has been found to conform with clinical experi- 

 ence in too many cases to permit of further doubt. 



In our case of irregularity there is no reason to suppose that the 

 ventricle failed to respond to the impulses it received. The length of the 

 apparent intermissions or of the periods of short quick pulses is not a 

 multiple of the single beats. For example, in Fig. i, the first single beat 

 is 0*44 sec. in duration, and if the next intermission had been due to the 

 failure of the ventricle to contract on the next regular stimulus its length 

 would have been 88 instead of 73. Similar reasoning in each case shows 

 that the irregularity is due, not to variations in the ventricular receptivity 

 of stimuli, but to an irregular discharge of impulses or to an irregular 

 transmission through the auricle or auriculo-ventricular fibres. 



* Cushny. Journal of Exper. Med, , iv. , p. 327. 

 Mackenzie, loc. cit.^ p. 297. 

 Wenckebach. Ztschr. f. klin. Med., 1899, xxxvi., p. i8i. 



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