An investigation was undertaken to determine the ventricular fibrillation (VF) threshold of anaesthetised dogs subjected to external application of electric shocks between a foreleg and a hindleg. The shocks were 2-15 ms sections of 50 Hz sine waveform starting at peak current and were applied at a known time in the heart cycle. The object of the experiment was to determine if there was an increase in cardiac susceptibility to electrically induced VF at 31 atmospheres absolute (atm abs) in a helium and oxygen environment. The duration and position of that part of the cardiac cycle most vulnerable to induction of VF by electrocution was found (seven animals) using 4 ms shocks and then the minimum fibrillating current for shocks of 2-15 ms (min FC2-15) delivered at the most vulnerable point of the cycle (five animals). Body resistance was calculated from the data so gathered. Fibrillation thresholds were not changed by compression and there were no significant changes in the vulnerable period of the cardiac cycle. Min FC2 was significantly higher than for other durations under both control (3.21 A, SD 1.08) and test conditions (3.26 A, SD 0.39), p = 0.001. There was no difference in body resistance at 31 atm abs (395.5, SD 12.9) from control values at 1 atm abs (396.7, SD 10.9). From these data it was concluded that the heart is no more susceptible to the induction of VF at 31 atm abs in a helium oxygen environment and additional safety factors are unnecessary from this point of view.
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