Counted cycles method to measure the block inception time of kiloHertz frequency mammalian motor nerve block

Abstract

Background: Kilohertz frequency alternating currents (KHFAC) produce rapid nerve conduction block of mammalian peripheral nerves and have potential clinical applications in reducing nerve hyperactivity. However, there are no experimental measurements of the block inception time (BIT) for the complete block of mammalian motor axons, i.e. the time from the start of delivery of the KHFAC to the axons reaching a fully blocked state.

New method: A "counted cycles" method (CCM) was designed to exploit characteristics of the onset response, which is typical of KHFAC block, to measure the BIT with a millisecond time resolution. Randomized and repeated experiments were conducted in an in-vivo rodent model, using trains of KHFAC over a range of complete cycle counts at three frequencies (10, 20, and 40 kHz).

Results: Complete motor nerve conduction block was obtained in the rat sciatic nerve (N = 4) with an average BIT range of 5 ms-10 ms. The fastest BIT measured was 2.5 ms-5 ms. There was no statistical difference between the block inception times for the three frequencies tested.

Comparison with existing methods: There are no comparable methods to measure the KHFAC BIT.

Conclusion: The KHFAC BIT is faster than previously estimated. KHFAC motor nerve block is established in milliseconds. These results may assist in the design of methods to eliminate the onset response produced by KHFAC nerve block.

Keywords: Block onset response; Electrical nerve conduction block; Kilohertz frequency alternating current; Mammalian peripheral nerve; Nerve block inception time.

Fig.1.

Sketch showing the depiction of the response of the nerve-muscle preparation to different cycle counts/durations of KHFAC at two amplitudes, low and high.

Fig.2.

Illustration of the experimental setup showing the position of the stimulating and blocking electrodes along the sciatic nerve.

Fig.3.

A representative randomized trial at 10 kHz showing the gastrocnemius force response to trains of counted cycles of KHFAC at a sub-block threshold of 2 V (L) and a supra-block threshold of 10 V (H). Solid black bars indicate the timing of each KHFAC train and the amplitude and cycle count are indicated above each muscle response. The 26 trains end with 6 twitches elicited from the proximal test stimulus (PS).

Fig.4.

(Top Left) The onset responses of 10 kHz, 10 V KHFAC trains consisting of 1, 10, 25, 250, and 500 complete cycles (0.1, 1, 2.5, 25 and 50 milliseconds) are plotted on top of each other along with a single twitch from the proximal stimulation (PS) to show the single twitch onset response for cycle counts 1 to 10 and the summated twitch onset response for longer cycle counts (note different time scale as compared to the rest of the panels in the Figure). (Top Right) Overlay of the sub-block threshold amplitude (L = 2 V) and supra-block threshold amplitude (H = 10 V) onset response for 25,000 cycles of 10 kHz KHFAC. This comparison illustrates the difference between a Phase I only onset at higher KHFAC amplitudes and the Phase II onset of the lower amplitude KHFAC showing. (Bottom Left) An overlay of the onset responses for all cycle counts to the high amplitude (10 V) KHFAC trains at 10 kHz. Note the onset reaches a maximum peak force and area after 500 cycles (50 ms) when the motor axons reache complete block. (Bottom Right) An overlay of the onset responses for all cycle counts to the low amplitude (2 V) KHFAC trains at 10 kHz. Note the onset responses continue to increase in magnitude and duration with increasing cycle count as the nerve continues the Phase II firing.

Figure.5.

Peak force (Top) and the force-time integral (Bottom) measurements shown for all 26 trains from a single trial at 20 kHz (log-log scales). The x-axis is in milliseconds. The bifurcation points and BIT ranges are marked with black horizontal arrows. In this example both the peak force and the force-time integral shows the bifurcation 2.5 ms indicating a KHFAC motor conduction block time at 20 kHz in the range of 2.5 ms to 5 ms.

Figure 6.

Mean and SE of Peak forces for the Coded counted cycle (CC) groups from all data from all the experiments. Closed circles: High amplitudes (coded as 0) and Open squares: Low Amplitudes (coded as 1). The x-axis is in milliseconds and the peak forces diverge between 5 ms to 10 ms.

Figure 7.

Mean and SE of Onset Area for the Coded counted cycle (CC) groups from all data from all the experiments. Closed circles: High amplitudes (coded as 0) and Open squares: Low Amplitudes (coded as 1). The x-axis is in milliseconds and the force-time integrals diverge between 5 ms to 10 ms.