Reduction of the onset response in high frequency nerve block with amplitude ramps from non-zero amplitudes.

High frequency alternating current (HFAC) waveforms reversibly block conduction in mammalian peripheral nerves. The initiation of the HFAC produces an onset response in the nerve before complete block occurs. An amplitude ramp, starting from zero amplitude, is ineffective in eliminating this onset response. In fact, it makes the onset worse. We postulated that initiating the ramp from a non-zero amplitude would produce a different effect on the onset. This was tested in an in-vivo rat sciatic nerve model. HFAC was applied at supra block threshold amplitudes and then reduced to a lower amplitude (0%, 25% 50 %, 75% and 90% of the suprathreshold amplitude). The amplitude was then increased again to the original supra block threshold amplitude. This normally produces a second period of onset response if increased as a step. However, an amplitude ramp was successful in eliminating this onset. This was always possible for the ramps up from 50%, 75 % and 90% block threshold amplitude, but never from 0% or 25% of the block threshold amplitude. This maneuver can potentially be used to maintain complete nerve block, transition to partial block and then resume complete block without initiating another onset.