| High frequency biphasic (HFB)(kHz) electrical currents have been shown to be able to locally block nerve conductions and selective activation of nerve fibers. This kind of block could potentially be used to alleviate pain, relieve muscle spasms. And selective nerve activation could be used to raise urinary efficiency and restore muscle movement. Thus, a safe and efficacious method for HFB electric stimulus would have great values for scientific researches and clinical applications. However, the nerve injuries caused by HFB electrical stimulation has not been widespread concerned. This study explored the effects of HFB electrical currents on nerve conductibility through analyzing charges of HFB waveforms and differences between recorded compound action potential (CAP) before and following HFB electrical stimulation. This study analyzed the nerve injuries induced by different parameters of HFB electrical currents statistically, and optimized the stimulus waveforms and stimulus electrode according to this statistical results and possible injury mechanisms, which provided ideas for design of efficacious and safe protocols for HFB electrical stimulation.In this study, we analyzed the trends of nerve injuries caused by different parameters (amplitude, duration and frequency) of HFB electrical current at first. With increasing of HFB electrical current amplitude within0.15-10times of blocking threshold, the injury degree would be increased first, and then decreased, and increased at last. The injury degree approached its local maximum when the amplitude of HFB current met the blocking threshold, and approached its local minimum at2.8times of blocking threshold. When the amplitude of HFB current met10times of blocking threshold, a complete damage would be appeared. When charge per phase of stimulus waveforms was equal, increased in frequency (1.25k,2.5k,5k and10kHz) of the waveforms would cause less injury. Duration of HFB electrical current would affect both nerve injury degree and recovering speed. Longer duration HFB electrical stimulations led to higher injury and slower recovering speed, and the CAP amplitudes at the end of recovering time would decrease. Thus, the nerve injury would be relative less when the parameters of high frequency stimulus met2.8times of blocking threshold, higher frequency and shorter duration.The lower HFB electrical current could modulate nerve fiber excitabilities. It has established that the lower HFB electrical current could be applied in clinical treatment combined with its characteristic of less injury. This study proposed to use high frequency biphasic (HFB) electrical current with small amount of charge per phase to stimulate nerve fiber model based on simulation. The small amount of charge per phase stimulus with small amount of charges is composed of HFB electrical current followed by a pulse with a delay (d). The probability of selective nerve activation around specific diameters between5and16μm were explored by means of adopting appropriate values of the delay (d) for pulse to appear and the frequency for high frequency electrical current. Selective nerve activation of diameters around5,7,8,9,10,11and15μm were successfully achieved by adopting stimulus with the delay (d) of0.83,0.84,0.85,0.86,0.87,0.88and0.90ms respectively. When HFB electrical currents were applied at frequencies of5,6,7,8,9and10kHz, nerve fibers with diameters around7,8,10,11,14and16μm could also be selectively activated respectively.Reasonable parameters of the HFB current would still induced nerve damage. Therefore, we optimized the nerve blocking protocols by changing stimulus waveform (reduced the amplitude of anodic phase of pulses, inserted interpulse interval (IPI) between pulses) and electrode (used polarizable electrode, increased electrode surface area). The nerve injuries had significantly decreased by HFB charge imbalanced current which anodic phase pulse slightly reduced but cathodic one unchanged (Anode/cathode amplitude ratio between0.8-0.95). In addition, the HFB uncontinuous stimulus would reduce charge per phase (maximum:56.8%) or charge per second (maximum:95.2%). Besides, the average injury would reduce by11.5%by using polarizable electrode (Pt) instead of nonpolarizable electrode (Ag/AgCl), and reduce by8.9%at optimal Pt electrode size. And the blocking rate would be stable using Pt electrode.This study has used HFB electrical stimulus to achieve selective nerve activation and conduction blocking based on nerve model simulation and bullfrog’s sciatic nerves stimulation, respectively. In point of nerve selective activations, the stimulus waveform used in this study is characterized by the small amount.of charges per phase. On aspect of nerve conduction blocking, we determined the relative safety of stimulation parameters (2.8times of blocking threshold, shorter duration, higher frequency) by analyzing injuries induced by HFB electrical current. The electro-chemical reaction, changes of ionic concentration and suddenly increase of interface temperature played a dominant role in increase-decrease-increase nerve injury, respectively. This study has designed more efficacious and safe protocols through combined with injury mechanisms described above:reduced the amplitude of anodic phase of electrical stimulus waveforms; inserted IPI between pulses; used polarizable electrode Pt and increased electrode surface area. Either HFB electrical current used in selective nerve activation or HFB uncontinuous stimulus used in nerve conduction blocking was characterized by the small amount of charges per phase, and thus will be especially useful for improving safety prolonging electrical stimulation periods. Besides, the nerve injuries have significantly decreased by reducing the amplitude of anodic phase of electrical stimulus waveforms and inserting IPI between pulses and increasing electrode surface area, and will provide a safe and efficacious protocol for applying in clinical practice. |
