PHYSIOLOGICAL PROPERTIES OF CARDIAC MUSCLE 343 



finds the ventricle again in a receptive state and is therefore able to 

 incite a contraction. No further disturbance takes place until another 

 extrasy stole is interposed. 



The refractory period and compensatory pause serve as a protective 

 mechanism which prevents any interference with the cardiac rhythm. 

 But if such a condition has actually arisen (arhythmia), their tendency 

 will be to reestablish normal relationships as quickly as possible. In 

 addition, the refractory period serves to check off the individual 

 discharges of the "pace-maker" and to regulate the length of the 

 successive systoles. Under ordinary conditions, therefore, the latter 

 must retain a twitch-like character and cannot become tetanic. It is 

 possible, however, to prolong them unduly either by stimulating the 

 heart with a series of strong induction shocks, or by exposing it to 

 heat. 



FIG. 180. ELECTROCAHDIOGRAM SHOWING AN EXTRASYSTOLE AT E, AND COMPENSATORY 

 PAUSE AT C. (Cunningham.} 



Extrasystoles are frequently encountered in the human heart 

 without being able to recognize a distinct lesion of the myocardium or 

 of the conducting paths. No special importance need be attached to 

 them as long as they remain infrequent. Most commonly they find 

 their origin in a hyperirritability of the local or general nervous ele- 

 ments. Two types of extrasy stoles are recognized clinically, namely, 

 those which are followed by a distinct compensatory pause and those 

 which are not. The former are more common and are often designated 

 as premature beats. They result in consequence of impulses which 

 start either in the pace-maker itself or high up in the conducting paths 

 and adjoining auricular tissue. The latter are generally called inter- 

 polated systoles, and seem to be due to stimuli which originate either in 

 the substance of the ventricles or in the more distal segment of the 

 conducting bundle. For this reason, they cannot seriously interfere 

 with the regular waves of excitation conveyed downward from the 

 auricle and, hence, cannot give rise to a distinct compensatory pause or 



