Control of Fibrillation in the Mammalian Heart. 311 



The coarsening of the rapid oscillation in the process of recovery is quite 

 different from a coarse slow movement that is not on the way to recovery at 

 all and where the muscle is lax and feeble. It is also different from the 

 apparent coarsening with slowing of the oscillations in the graphic record due, 

 as direct inspection of the heart shows, to irregular summation of fine feeble 

 twitchings which are present with a high degree of dissociatioii and which 

 may gradually become weakened to extinction. It is important to correlate 

 the information derived from (a) inspection, (5) palpation, and (c) graphic 

 records. 



Bates of Stimulation Necessary to Establish the Meclianism of Circulating 



Excitations. 



With excitable ventricles in good condition high rates of excitation by 

 induction shocks of moderate strength are necessary to overpass the phase of 

 pseudo-fibrillation and induce true fibrillation, e.g., single induction shocks at 

 rates of 450-500 per minute are commonly effective, but the duration of the 

 application of the series of shocks has an influence in this respect ; with longer 

 application lower rates may suffice. When faradic currents are employed the 

 current has to be of such a strength and duration as to raise the rate of 

 responsive contractions to about the above rates. Beyond such rates the 

 state of pseudo-fibrillation is not as a rule maintained, but gives place to true 

 fibrillation as soon as the mechanism of circulating excitation has been 

 established, this point being often recognisable on the tracing by a change 

 from the rapid and more or less irregular curves of small excursion that are 

 present during rapid tachycardia or pseudo-fibrillation to the much smaller 

 and entirely irregular oscillations of true fibrillation (fig. 11). 



The conductivity of the muscle plays an essential part in regard to the 

 rate of stimulation needed to cause fibrillation ; the necessary rate is not a 

 constant or absolute one, but varies much in relation to the state of the 

 conductivity at the time. The lower the conducting power, the lower is the 

 rate of stimulation required to establish the circulating mechanism, since 

 under these conditions the normal relations between conduction time and 

 refractory period are more readily upset, a relatively low grade of acceleration 

 sufficing to cause slowed excitation waves to reach different parts of the 

 fascicular systems after the refractory period is over in these situations. 

 Agents that depress conduction, e.g., potassium salts, bile, cooling, etc., can be 

 used in such a way and to such a degree as not to induce fibrillation by 

 themselves, but to render the muscle prone to fibrillate with unusually low 

 rates of excitation. Thus the minimal rate of stimulation which induces 

 true fibrillation affords an indication of the state of conductivity. In 



