Selective Small Fiber Activation by Anodal Blocking

Obtained With a Step Pulse

A. Vuckovic¹, N.J.M. Rijkhoff¹, D. Stefania²

¹Center for Sensory Motor Interaction ²Institute of Experimental Clinical

Aalborg University Research, Skejby Hospital

9220 Aalborg, Denmark 8200 Aarhus, Denmark

av@smi.auc.dk nr@miba.auc.dk ds@iekf.au.dk

Abstract

The aim of this study is to investigate the use of a step pulse to obtain selective small fiber activation by anodal blocking. The charge of a step pulse is less compared to the conventional rectangular pulse and would be therefore safer in a chronic application. Acute experiments on pigs show that using a step pulse, charge reduction up to 13 % can be achieved.

1. Introduction

Electrical stimulation of nervous tissue using implanted electrodes can be used to induce muscle contraction in neurologically impaired individuals. With conventional stimulation, large diameter nerve fibers are recruited before smaller ones [2]. However, some applications in urology, gastroenterology and skeletal muscle activation require selective activation of small fibers without activating larger ones [3, 5].

Experiments in humans [4] have shown that it is possible to obtain selective stimulation of small fibers using the method of anodal blocking. The principle of selective small fiber activation by anodal blocking is that both small and large fibers are activated and that the propagation of action potentials (AP) in the large fibers is blocked distal to the activation site. To obtain anodal blocking a tripolar cuff electrode is most commonly used. The central contact is the cathode and the lateral contacts are anodes. The fibers are excited close to the cathode and the AP’s of large fibers are blocked close to the anodes.

A drawback of this technique is that it requires relative long pulses (between 300 and 600 ms) and relative high current (1 mA). These currents may cause neural damage and electrode contact corrosion because the charge per

Fig.1. Step pulse: I₁-amplitude of the first step, t₁-duration of the first step, I₂-amplitude of the second step, t₂-duration of the second step, t-duration of a step pulse

pulse is high [1]. To make the technique safer for chronic use, the charge per pulse should be reduced. Computer simulations have shown that the charge per pulse can be reduced if a square pulse is modified with a step pulse of the same duration (Fig. 1) [6]. The idea is based on the fact that an AP needs time to propagate from the cathode to the anodes. Therefore, at the beginning on the pulse it is not necessary to apply the high current, which is needed to obtain blocking.

The aim of this study was to validate the theoretical results in acute animal experiments.

2. Methods

2.1. Animal Preparation

Acute experiments were performed on 6 nonspinalized female Danish Landrace pigs, weighting about 40 kg. Anesthesia was induced by Isoflurane (1 ml/10 kg). The animals were intubated and breathed with the aid of a ventilator (40% O₂ and 60% N₂O). Saline was administered intravenously.

A dorsal laminectomy was performed from the vertebral level L4 to S3 and the dura was opened to the entire length of the laminectomy to expose the sacral roots. Individual nerve roots were identified by their size and by responses of several muscle groups to stimulation with bipolar hook electrode. Ventral and dorsal roots were separated and the cuff was placed around the ventral part.The dura was filled with saline to prevent roots from drying. The wound was closed to keep the roots at the body temperature of 37 ^oC.

2.2. Stimulation and Recording

The split cylinder cuff electrode with three contacts was placed around the nerve. Separation between contacts was 3 mm, contact width was 0.5 mm and the length of a whole cuff was about 1 mm. The stimulator consisted of two synchronized current sources with a common cathode. Anal sphincter pressure was measured with a catheter with the filled balloon. Rectal pressure was measured with the catheter with an open tip. The water was infused through the rectal catheter 5 ml/h to prevent blocking of the catheter. For displaying and recording rectum pressure and pressure in the anal canal a BioBench (National Instruments) program was used. Stimulation was preformed with a pulse train 25 Hz, 2 s. Wire electrodes were placed in the anal sphincter for recording the EMG. EMG recordings were obtained from single pulses, averaged on 5 pulses. The EMG was amplified 10000 times and displayed on a portable oscilloscope (Fluke) and transmitted to an on-line PC for displaying and recording.

The blocking current I_bl was first determined for a square pulse of 600 ms. This was the lowest current by which the rectum, innervated by small nerve fibers was activated, and the anal sphincter, innervated by large nerve fibers was not activated. The pulse duration was than decreased until an incomplete block developed, i.e. until anal pressure started to increase. In that way, the minimum pulse duration t for a minimum blocking current I_bl was determined. A step pulse of the duration t was then applied. The duration of the first step was increased from 0 ms in steps of 20 ms. The amplitude of the first step was given three predefined values 0.25, 0.5 and 0.75 times I_bl. For each combination of t₁ and I_1, the amplitude of the second step I₂ on which blocking was achieved was determined. All currents are expressed as cathodal currents, which are a sum of anodal currents.

3. Results

Rectal and anal pressure responses to intradural stimulation of S2 left sacral root, recorded from one pig are shown in Fig. 2. Total pulse duration was t=350 ms and the blocking current was I_bl=1.16 mA. For I₁=0.25 I_bl it was possible to obtain blocking with I₂=I_bl up to t₁=60 ms. During partial block with t₁=80 ms, the amplitude of anal pressure was 50 % of the amplitude during activation with a square pulse of duration 100 ms. Rectal pressure remained unchanged during all stimulations. Fig. 3 shows the EMG responses to stimulation with a step pulse of a total duration 350 ms.

Fig. 2. Rectal pressure (a) and anal pressure (b). Vertical bars denote start of stimulation. A minimum current I₂ was 1.15 m. Stimulation was always performed with pulses t= 350 ms, with a pulse train 25 Hz, 2s. Duration of the first step is given under the anal pressure responses. Amplitude of the first step was 0.25 I_bl. Last pressure response, marked by an arrow was obtained with a square pulse of the duration of 100 ms, where no blocking occurs.

Fig. 3. EMG response of external anal sphincter for stimulation with step pulses, with t=350 ms. Last EMG for square pulse with t= 100 ms. Vertical bars denote start of stimulation. Stimulation currents was I₂=1.15 mA

The duration of the first step was varied and the amplitude

was fixed to I₁=0.25 I_bl. The EMG response to stimulation

Fig. 4. Amplitude of the second step I₂ as a function of a duration of the first step t₁, for three different current values of the first step: I₁=0.25 I_bl; 0.5 I_bl; 0.75 I_bl. I_bl=1.15 mA; t=350ms.

with the square pulse of duration t= 100 ms is also shown.

Fig 4. shows relation between I₂ and t₁ for three different values of I₁. Maximum t₁ for minimum I₂ is obtained for I₁=0.75 I₂. Fig. 5. shows the relation between the charge per pulse Q and t₁. Minimum charge per pulse was obtained for I₁=0.25 I₂ and t₁=60 ms. This charge is 13 % less than a charge induced with the square pulse of the same duration.

Fig. 5. Charge per pulse with the step pulse as a function of a duration of the first step t₁, for three different currents of the first step: I₁=0.25 I_bl; 0.5 I_bl; 0.75 I_bl. I_bl=1.15 mA; t=350ms. The charge induced with the square pulse of the same duration and blocking current is 402 nC.

In the other five pigs, the minimum pulse duration to obtain a complete block varied from 350 to 500 ms. Minimum Q was achieved for t₁ from 60 to 80 ms for different I₁. Mean value of a charge reduction was 8.12±3.19 % compared to the square pulse.

4. Discussion

Selective small fibers activation by anodal blocking may cause a neural damage in a long-term use, due to the high charge per pulse. Experiments on pigs have shown that this charge can be reduced by changing the pulse shape from a square to a step pulse

The applied step pulse may have the same blocking current as the square pulse. The reduction of the charge depends on the duration and the amplitude of the first step.

Acknowledgment

This work was supported by Danish Technical Research

Council.

References

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[4] N.J.M. Rijkhoff, H. Wijkstra, P.E.V. van Kerrebroeck, F.M.J. Debryne, “Selective detrusor activation by electrical sacral nerve root stimulation in spiral cord injury”, J. Urol., vol. 16, pp.337-341, 1998.

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[6] A. Vuckovic, N.J.M. Rijkhoff, “A new pulse shape to obtain selective small fiber activation by anodal blocking”, Proc. 23^rd Ann. Int. Conf. IEEE-EBMS, Istanbul, Turkey, 25-28 Oct. 2001.