IRREGULARITIES OF CARDIAC RHYTHM 



209 



The conducting bundle or path between the sino-auricular node where 

 the normal beat arises and the auriculo-ventricular node is usually at 

 fault in partial block. In complete block the independent ventricular 

 beats start from a rhythmic center in the auriculo-ventricular node as a 

 rule. But the. block may be in the His bundle itself in which case the 

 rhythm production is low in the conducting system or even in the ven- 

 tricular muscle. 



Extra Ventricular Systoles. When for any reason some portion of the 

 ventricular complex becomes excessively irritable its rhythmicity may 

 be so much increased that it starts an independent contraction before the 

 normal ventricular stimulus reaches the muscle. This leads to a con- 

 traction with the shortened period between beats and is called an extra- 

 ventricular systole. As a rule there is a longer or compensatory pause 

 following an extra systole after which the regular rhythm again becomes 

 dominant. Most people of middle or old age experience occasional extra- 

 systoles. In disease they may become frequent and troublesome but 

 they are of no particular importance. 



Auricular Flutter and Fibrillation. A type of irregularity that is more 

 common is that of an auricular rate far above the normal, i.e., 150 or even 

 more a minute. These are due to hyperirritability of the auricle often 



FIG. i j8b. Electrocardiogram of auricular fibrillation. 



with an ectopic center of rhythm production. When such hearts are 

 examined with the electrocardiograph or with the polygraph it becomes 

 evident that, the auricle is contracting at a very much higher rate than the 

 ventricle. The ventricle does not respond to every auricular contraction, 

 i.e., in this kind of block the stimulus falls within the refractory period of 

 the ventricles. Each auricular contraction is complete and the series is 

 regular but the rate is above that which can be conducted to the ventricle. 

 Auricular flutter is the term that describes this phenomenon. 



The auricular contractions are not always coordinated and com- 

 plete. They sometimes begin in apparently many foci at once so that 

 no rhythmic center controls the entire auricular muscle. Such contrac- 

 tions are called fibrillation. In auricular fibrillation individual muscle 



