CLINICAL APPLICATIONS OF ELECTROCARDIOGRAPIIY 



271 



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 

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

 92). 



PARTIAL DISSOCIATION. In the typical case of partial dissociation the 



Fig. 93. Partial dissociation. Note the failure of ventricular response following the second P, 

 which has been preceded by two extrasystoles (x) of Ventricular origin. 



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 

 beat." The electrocardiogram records a P deflection but no ventricular 

 complex, showing that the auricles have contracted at their usual rate 

 but that the ventricles failed to respond to the stimulus coming from 

 the sinoauricular node (Fig. 93). 



Fig. 94. Complete dissociation. Note that the P wave spaces regularly and bears no definite re- 

 lation to the R wave of the ventricular complex. Auricular rate 72. Ventricular rate 40. 



COMPLETE DISSOCIATION. In a simple case of complete dissociation 

 the auricles beat independently of the ventricles; hence the P deflection 

 of the electrocardiograms bears no relation to the ventricular complex 

 (Q-R-S-T) (Fig. 94). The P deflections space regularly and are easily 

 made out when they fall during diastole of the ventricle. Occasionally 



