Conduction block of whole nerve without onset firing using combined high frequency and direct current

Abstract

This study investigates a novel technique for blocking a nerve using a combination of direct and high frequency alternating currents (HFAC). HFAC can produce a fast acting and reversible conduction block, but cause intense firing at the onset of current delivery. We hypothesized that a direct current (DC) block could be used for a very brief period in combination with HFAC to block the onset firing, and thus establish a nerve conduction block which does not transmit onset response firing to an end organ. Experiments were performed in rats to evaluate (1) nerve response to anodic and cathodic DC of various amplitudes, (2) degree of nerve activation to ramped DC, (3) a method of blocking onset firing generated by high frequency block with DC, and (4) prolonged non-electrical conduction failure caused by DC delivery. The results showed that cathodic currents produced complete block of the sciatic nerve with a mean block threshold amplitude of 1.73 mA. Ramped DC waveforms allowed for conduction block without nerve activation; however, down ramps were more reliable than up ramps. The degree of nerve activity was found to have a non-monotonic relationship with up ramp time. Block of the onset response resulting from 40 kHz current using DC was achieved in each of the six animals in which it was attempted; however, DC was found to produce a prolonged conduction failure that likely resulted from nerve damage.

Fig. 1

Schematic representation of the HFAC + DC setup. The 100-Ω resistor was used to monitor DC current flow. The isolation transformer and inductor were used to squelch HFAC leakage

Fig. 2

Example timing of proximal stimulation, DC, and high frequency currents for the combined DC and HFAC experiments

Fig. 3

Response of the nerve to ramped a anodic and b cathodic DC waveforms of various amplitudes

Fig. 4

a Individual muscle force recordings from the first 500 ms of a trapezoidal DC pulse from a single experiment. Up arrows demarcate the ramp start time for each trace. b Peak force measurements of DC onset response to each of the up ramp times tested for four animals. Each symbol represents data from an individual animal. The mean is shown with filled circles connected by a bold line

Fig. 5

a Individual muscle force recordings from the final 500 ms of a trapezoidal DC pulse from a single experiment. Down arrows demarcate the ramp end time for each trace. b Peak force measurements of DC onset response to each of the down ramp times tested for four animals. Each symbol represents data from an individual animal. The mean is shown with filled circles connected by a bold line

Fig. 6

Sequence of three consecutive trials demonstrating block of the onset response with DC. Each trial shows a recording of the muscle force and the timing of the HFAC waveform (indicated by the grey bar). a Muscle response to a 40 kHz HFAC waveform. b Muscle response to combined delivery of HFAC and ramped DC. The timing of the DC is indicated by the dashed trace. c Muscle response to a 40 kHz HFAC waveform

Fig. 7

Combined HFAC and DC block trial with signs of nerve damage. Outlined arrow indicates a large transient muscle contraction that occurred immediately following the cessation of the DC delivery. The immediate decrease in the muscle force elicited by proximal stimulation is also indicated

Fig. 8

a Results of repeated DC delivery experiments show a decline in proximally elicited muscle force, indicating nerve conduction failure with cumulative DC delivery. Proximally elicited muscle force was normalized to distally elicited muscle force. b Results of repeated DC delivery experiments show an increase in post-DC contraction force with cumulative DC delivery. Muscle force was normalized to that of a distally elicited twitch