278 



THE CIRCULATION OF THE BLOOD 



site of origin of the premature beats can be determined only by careful 



measurement of the distances between the various beats of the ventricle. 



Whenever an irregularity repeats itself and the duration of one cycle 



of the arrhythmia accurately corresponds to another, the irregularity 



1 I I I I 



Fig. 101. Premature beats (extrasystoles) ventricular in origin at PB. Compare the duration of 

 the intervals marked A and B' with those marked C and D. (From E. P. Carter.) 



may be due to: (1) premature auricular or ventricular contractions; 

 (2) the occasional occurrence of dropped beats (a failure of ventricular 

 response) ; or (3) a high degree of heart-block with a wide variation in 

 the ventricular response. The important point to note here is that, no 

 matter how irregular such a tracing may appear, if the irregularity re- 

 peats itself it can not be due to auricular fibrillation. 



Fig. 102. Paroxysmal tachycardia. The paroxysms start at xx fallowing normal beats and 

 lasting for seven beats. The clue to "a," which falls with "v" after the first premature contrac- 

 tions, is found in the initial beat of the new rhythm. (From E. P. Carter.) 



Paroxysmal Tachycardia. When the rate of a regular pulse is sud- 

 denly altered but the change in rate bears a simple ratio to the previous 

 rhythm, the change may be due to (1) premature ventricular contrac- 

 tions which do not reach the radial, or (2) to the sudden development 



