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BELL SYSTEM TECHNICAL JOURNAL 



except that the current was reduced to between 4 and 5 amperes. 

 Five developed ventricular fibrillation on the first shock, and 3 on the 

 second shock. A single animal survived 5 shocks, and another animal 

 2, without fibrillating. A comparison and combination of these 

 results and those previously obtained definitely establish that the 

 susceptibility of the heart to ventricular fibrillation becomes very 

 much less when the current is increased to about 25 amperes, 10 times 

 the average threshold value for shocks of short duration. This 

 variation of susceptibility to fibrillation is shown graphically in Fig. 6. 



100 



8 10 12 14 16 



CURRENT AMPERES 



20 22 24 26 



Fig. 6 — Effect of current on susceptibility of sheep hearts to ventricular fibrilla- 

 tion. Shocks of 0.03 second, 60 cycles, applied in partial refractory period of cardiac 

 cycle. Number of shocks and current spread indicated for each point on curve. 



To determine whether such a decrease in susceptibility to fibrillation 

 would occur also for shocks of about 3 seconds duration if the current 

 were increased about 15 times the average threshold, 5 sheep were 

 subjected to 4 ampere shocks of 3 seconds duration. In all cases 

 ventricular fibrillation resulted on the initial shock, indicating either 

 that at this duration no such decrease in susceptibility takes place 

 with increase of current, or that 4 amperes is not a sufficiently high 

 current to bring it about. The apparatus was not capable of giving 

 much higher currents for this duration. 



