78 



the conditions of circulating excitation do not arise. Thus 

 artificial cooling of the heart, stopping short of a certain 

 extreme point (about 23° C. in the perfused cat's heart), 

 does not induce fibrillation, there being a concurrent 

 lengthening of refractory period and conduction time as the 

 cooling goes on — within the limit stated. 



Conversely there may be, as results of a rise of tempera- 

 ture, etc., a marked shortening of the refractory period 

 without fibrillation occurring, the rate of conduction of the 

 excitatory impulse being also accelerated. The essential 

 dependence of the mechanism of fibrillation on the factors 

 named makes it clear why there should be no constant or 

 necessaiy relation between the incidence of fibrillation, and 

 even great alterations in contractile force, tone, etc. Dila- 

 tation of the ventricles is credited by Levy with a protective 

 influence ; this view is negatived by various observed facts— 

 among others the proneness to fibrillation seen in ventricles 

 weakened, relaxed, and distended as a result of potassium 

 poisoning; and conversely, the marked resistance or insus- 

 ceptibility to fibrillation in ventricles that are of small 

 volume, acting strongly and rapidly and well emptied at 

 each beat, in sequence to a large dose of adrenaline, etc. 

 So long as the essential relationship, described above, is not 

 upset a heart may be profoundly influenced in many ways 

 without fibrillating : it may beat very rapidly or very slowly, 

 regularly or very irregularly, powerfully or feebly ; it may 

 be well emptied or imperfectly emptied at each beat — with 

 consequent distension in the latter case ; it may be very sensi- 

 tive or very dull to direct stimulation, and its muscle may 

 be lax or firm, etc. 



Pseudo-Fihrillation. 



Apart from the disastrous event of true fibrillation there 

 is also to be observed under certain experimental conditions 

 (for example, rapid artificial excitation by a series of elec- 

 trical shocks, mechanical stimulation, etc.) a temporary 

 condition in the ventricles presenting many points of 

 resemblance to true fibrillation — a degree of inco-ordination 

 or asynchronous contraction of the musculature, recognizable 

 on inspection and on palpation of the ventricular substance, 

 attended by great reduction in the range of contraction 

 movement and very little expulsion of blood at each beat, a 

 great fall in arterial pressure, and a failure of recognizable 

 pulsation in the peripheral arteries, etc. It is impossible 

 to distinguish this condition from true fibrillation by 

 examination of the arterial pulse. 



Usually this condition is soon recovered from when the 

 artificial stimulation is discontinued, though it may persist 

 for variable periods. It apparently differs essentially from 

 true fibrillation in its dependence on a rapid series of 

 excitatory impulses emanating from a stimulated area or 

 " spontaneously " from one or more irritable foci, these 

 impulses forcing a succession of ventricular beats at a rate 



