282 



THE CIRCULATION OF THE BLOOD 



Since it is impossible for the ventricle to respond to all the impulses 

 coming from the auricles, a condition of partial heart-block obtains 

 (2:1 3:1 4:1, etc.). The ventricular complexes will occur regularly 

 except when a 3:2 rhythm exists. 



Usually the ventricular complexes are such as to indicate that the 

 stimulus arose in the auricle (supraventricular). The height of the 



Fig. 91. Auricular flutter. Auricular rate 300. Ventricular rate 80. Note the inversion of the P 



individual deflections Q-R-S-T may vary, depending on the predominance 

 of a right or left ventricular hypertrophy. 



Heart-block. There are three degrees of severity in heart-block: (1) 

 delayed conduction, (2) partial dissociation, and (3) complete dissociation. 



DELAYED CONDUCTION. When the conducting tissues of the heart are 

 so affected as to cause an abnormal prolongation of the P-R interval, 



(F-R 



Fig. 92. Delayed conduction. Note the normal appearance of the electrocardiogram except for 

 the prolongation of the P-R interval, which measures .23 seconds. 



the condition is called delayed conduction. The ventricles respond to 

 each stimulus originating at the sinus node, but the time required for the 

 impulse to pass through the conducting tissues is longer than normal. 

 In a simple case the electrocardiogram may appear perfectly normal, but 

 when the P-R interval is measured accurately, it will be found to be length- 

 ened beyond the extreme limits of the normal (0.20 seconds) (Fig. 92). 



PARTIAL DISSOCIATION. In the typical case of partial dissociation the 

 ventricles respond to the impulse coming from the auricle most of the 

 time, but occasionally fail to do so, when the condition is called " dropped 



