The use of direct current to cause selective block of large fibres in peripheral nerves.

The effect of direct current on the propagated compound action potential in a cutaneous branch of the radial nerve of the dog was studied and a convenient method for applying electrical current to a nerve is described. Changes in the flow of blocking current, even when these were made slowly during a period of 5-10 sec, caused stimulation of the nerve. The method was satisfactory for producing differential nerve block within the myelinated fibres. It was confirmed that direct current could be used to eliminate selectively the group II/III potentials. When these potentials had been eliminated for a period of 5 min the mean height of the group IV potentials was reduced to 65% of the control value, and their conduction velocity was decreased by 10%. Relatively small variations in current flow caused a marked change in conduction. In the early stages of block, frequent relatively large reductions in current flow were necessary to produce a constant response and it was impossible to cause a stable effect on conduction until the blocking current had been applied for at least 1 min. Because of nerve damage the method is unsuitable for clinical use.