EFFECT OF ELECTRIC SHOCK ON HEART 467 



to give a somewhat higher threshold current. F'or the pathway from 

 leg to leg, the proportion of current reaching the region of the heart is 

 so small that fibrillation is not liable to result, even at currents of 15 

 amperes or more, although such currents probably would burn the 

 victim unless the contacts were good and the shock of short duration. 



c. Frequency. For shocks of one second or more in duration, the 25- 

 cycle threshold current is about 25 per cent higher than the 60-cycle 

 value, and the d-c. threshold current 5 times the 60-cycle value. For 

 shock durations of a small fraction of a second this relation probably 

 does not hold, all thresholds being expected to approach one another. 



d. Time of Occurrence of Short Shocks in Relation to Cardiac Cycle. 

 The heart is most sensitive to fibrillation for shocks occurring during 

 the partial refractory phase of its cycle, which is about 20 per cent of 

 the whole and which occurs simultaneously with the T wave of the 

 electrocardiogram. With shocks of a duration of about 0.1 second or 

 less, it is practically impossible to produce ventricular fibrillation, un- 

 less such shocks coincide in part at least with this sensitive phase of 

 the cardiac cycle. The middle of the partial refractory phase is more 

 sensitive than its beginning or end. 



e. Duration of Shock. The threshold current varies inversely with 

 shock duration but not uniformly, being most sensitive to change as 

 the duration approaches the duration of one heart beat. Within the 

 sensitive phase of the heart cycle the threshold fibrillating current for 

 shock durations of about 0.1 second or less is 10 or more times the 

 threshold for durations of one second or more. Shocks \i or more of 

 the heart cycle in duration may cause ventricular fibrillation, even 

 though they would not extend into the sensitive phase of the cycle if 

 the heart continued its normal beat after the initiation of the shock. 

 The reason for this is probably the initiation of a premature heart 

 beat which brings about a premature sensitive phase prior to the end 

 of the shock. 



5. Successive shocks have no cumulative efifect on the susceptibility 

 of the heart to fibrillation. 



6. The susceptibility of the heart to fibrillation by short shocks 

 increases with current up to several times the threshold, then di- 

 minishes, becoming very small at currents of the order of 25 amperes 

 through the body in the vicinity of the heart. However, other serious 

 injury may be expected from such currents when brought about by 

 accidental contacts. 



7. Fibrillation produced by an electric shock will in the majority of 

 cases be arrested by a subsequent electric shock of high intensity and 

 short duration through the heart, allowing the resumption of co- 

 ordinate beating with no permanent damage. 



