270 



THE CIRCULATION OF THE BLOOD 



origin of the auricular impulse (when arising from some other source 

 than the S-A node the impulse is said to be ectopic). Usually a regular 

 succession of P deflections can be traced throughout the record (Fig. 

 91). 



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. 



l ? ig. 91. Auricular flutter. Auricular rate 300. Ventricular rate 80. Note the inversion of the P 



deflections. 



Usually the ventricular complexes are such as to indicate that the 

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

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

 of a right or left ventricular hypertrophy. 



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

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



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

 delayed conduction, (2) partial dissociation, and (3) complete dissocia- 

 tion. 



Any one of these conditions may be present in the same patient at 

 successive intervals. 



DELAYED CONDUCTION. When the conducting tissues of the heart are 

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

 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. 



