EFFECT OF ELECTRIC SHOCK ON HEART 461 



conditions remaining the same as for the 60-cycle cuncni. The 

 results of these tests are illustrated graphically in Fig. ?>. 



Susceptibility of Heart in Different 

 Phases of Cardiac Cycle 



Physiologists have established that the heart is responsive to 

 moderate electrical stimuli during the period of relaxation (diastole) 

 whereas such stimuli during the period of contraction (systole) do not 

 elicit further response. During early systole {QRS to beginning of 1\ 

 Fig. 1) the heart muscle is totally unresponsive, while during diastole 

 (end of T to beginning of QRS) the ventricular muscles are responsive 

 to stimuli. At the time of the T wave of the electrocardiogram, con- 

 traction starts to disintegrate and parts of the ventricular muscle will 

 respond differently to electrical stimuli. In view of these facts and 

 some erratic responses to shocks of three to four cycles of 60-cycle 

 current, differences were expected in the response of the heart to very 

 short shocks during different phases of its cycle. Special apparatus 

 was developed for applying short shocks at predetermined parts of the 

 cardiac cycle. The results of 913 shocks of 0.03-second duration on 

 132 sheep are plotted in Fig. 4, to show the position of the mid-point 

 of each shock in the cardiac cycle, approximate shock current and 

 whether or not fibrillation occurred. None of the shocks occurring 

 during the period of complete contraction or complete relaxation of 

 the heart caused ventricular fibrillation, this result appearing only for 

 shocks falling during the period of diminishing contraction. 



Of 370 shocks of 0.12 second duration applied to 38 sheep, only one 

 shock definitely outside the partial refractory phase resulted in 

 ventricular fibrillation. This shock began at a point in the electro- 

 cardiogram between P and Q waves at which time the ventricles are 

 completely relaxed and resting. 



Effect of Duration 



In the determination of thresholds for shocks of very short durations, 

 a third or less of the duration of one heart beat, the time of occurrence 

 of the shocks was regulated so as always to involve the partial refrac- 

 tory phase, corresponding to the appearance of the T wave. The 

 thresholds were found in the same way as for 3-second shocks, by ap- 

 plying successive shocks at intervals of 5 minutes, each at increased 

 current until ventricular fibrillation resulted. 



The effect of duration on the threshold is illustrated in Fig. 5. It 

 is believed that the current required to initiate fibrillation would in- 

 crease markedly as the duration is decreased below 0.03 second. It 



