Reversible nerve conduction block using kilohertz frequency alternating current

Abstract

Objectives: The features and clinical applications of balanced-charge kilohertz frequency alternating currents (KHFAC) are reviewed. Preclinical studies of KHFAC block have demonstrated that it can produce an extremely rapid and reversible block of nerve conduction. Recent systematic analysis and experimentation utilizing KHFAC block have resulted in a significant increase in interest in KHFAC block, both scientifically and clinically.

Materials and methods: We review the history and characteristics of KHFAC block, the methods used to investigate this type of block, the experimental evaluation of block, and the electrical parameters and electrode designs needed to achieve successful block. We then analyze the existing clinical applications of high-frequency currents, comparing the early results with the known features of KHFAC block.

Results: Although many features of KHFAC block have been characterized, there is still much that is unknown regarding the response of neural structures to rapidly fluctuating electrical fields. The clinical reports to date do not provide sufficient information to properly evaluate the mechanisms that result in successful or unsuccessful treatment.

Conclusions: KHFAC nerve block has significant potential as a means of controlling nerve activity for the purpose of treating disease. However, early clinical studies in the use of high-frequency currents for the treatment of pain have not been designed to elucidate mechanisms or allow direct comparisons to preclinical data. We strongly encourage the careful reporting of the parameters utilized in these clinical studies, as well as the development of outcome measures that could illuminate the mechanisms of this modality.

Keywords: Electrical stimulation; high frequency; kilo hertz frequency nerve block; nerve block; pain block; spasticity block.

Figure 1

Above: Typical experimental setup. Below: Block of rat sciatic nerve using 30 kHz sinusoidal KHFAC at an amplitude of 10 volts peak-to-peak (V_pp). Grey solid bar shows duration of proximal stimulation at 1 Hz. Black bar (below data) shows timing of KHFAC delivery. Short dashed bar shows timing of distal stimulation. There is 99% motor block during the KHFAC delivery after the brief onset response. Response to distal stimulation shows that the neuromuscular junction is responsive during the block, proving this to be a true localized nerve conduction block.

Figure 2

Comparison of onset response characteristics in the same nerve (rat sciatic) using different frequency and amplitude. The scales are the same in each plot, and block is delivered from 10 seconds to 30 seconds. The plot on the right is with KHFAC of 10 kHz and 10 Vpp and shows onset activity for the whole period. The plot on the left is with a KHFAC of 30 kHz and 10 Vpp and has a very brief onset response of lower amplitude.

Figure 3

The counted cycles method to determine the time to achieve block. Specific numbers of KHFAC cycles (from 1 to 50,000 cycles at 20 kHz) are randomly applied at two different KHFAC amplitudes and the resulting onset response (area under the force curve = to the force-time integral in Newton·seconds) is compared in the plot. “Low” uses an amplitude below the block threshold. With the low amplitude, increasing cycle counts always result in increasing force-time integrals, since there is no nerve block effect. “High” uses an amplitude above the block threshold. Since the high amplitude produces a complete block after a specific number of cycles, the resulting force-time integral reaches a plateau. The point of bifurcation of the two curves defines the lower bound of the number of cycles needed to achieve complete block (100 cycles in the figure). The next data point on the right is therefore chosen as a conservative estimate of the block time (after conversion of the cycle count to absolute time). In this example, the block time is 12.5 milliseconds. The fastest time across multiple trials was 7.5 ms.

Figure 4

KHFAC block at 10 kHz. Gastrocnemius force is shown during proximal stimulation and while block is on (block begins at 10 seconds). After 300 seconds of KHFAC delivery, the block is turned off. The peak force due to the proximal stimulation is identical immediately after the cessation of block when compared to prior to block, showing the instantaneous reversibility of this method.

Figure 5

DC+KHFAC no-onset blocking system. Diagram shows schematic electrode configuration on the nerve, with a proximal stimulating electrode (PS), a KHFAC electrode, and a distal direct current (DC) electrode. “A” shows the no-onset block. Top trace shows tendon tension during trial. Proximal stimulation (PS) at 2 Hz is delivered throughout the trial (middle trace). DC (middle trace) ramps down (cathodic block) and plateaus at 4.5s, producing complete block (note partial block during ramp). DC block allows KHFAC (lowest trace) to be turned on without producing an onset response (7.5s). DC is turned off and block is maintained by KHFAC. KHFAC is turned off at 17.5s and normal conduction is restored. “B” shows the normal KHFAC onset (when DC block is not used). Scale is the same for both graphs.