A Theoretical Argument for Extended Interpulse Delays in Therapeutic High-Frequency Irreversible Electroporation Treatments

High-frequency irreversible electroporation (H-FIRE) is a tissue ablation modality employing bursts of electrical pulses in a positive phase-interphase delay (d1)-negative phase-interpulse delay (d2) pattern. Despite accumulating evidence suggesting the significance of these delays, their effects on therapeutic outcomes from clinically-relevant H-FIRE waveforms have not been studied extensively.

OBJECTIVE

We sought to determine whether modifications to the delays within H-FIRE bursts could yield a more desirable clinical outcome in terms of ablation volume versus extent of tissue excitation.

METHODS

We used a modified spatially extended nonlinear node (SENN) nerve fiber model to evaluate excitation thresholds for H-FIRE bursts with varying delays. We then calculated non-thermal tissue ablation, thermal damage, and excitation in a clinically relevant numerical model.

RESULTS

Excitation thresholds were maximized by shortening d1, and extension of d2 up to 1,000 μs increased excitation thresholds by at least 60% versus symmetric bursts. In the ablation model, long interpulse delays lowered the effective frequency of burst waveforms, modulating field redistribution and reducing heat production. Finally, we demonstrate mathematically that variable delays allow for increased voltages and larger ablations with similar extents of excitation as symmetric waveforms.

CONCLUSION

Interphase and interpulse delays play a significant role in outcomes resulting from H-FIRE treatment.

SIGNIFICANCE

Waveforms with short interphase delays (d1) and extended interpulse delays (d2) may improve therapeutic efficacy of H-FIRE as it emerges as a clinical tissue ablation modality.

The development and modelling of devices and paradigms for transcranial magnetic stimulation

Magnetic stimulation is a non-invasive neurostimulation technique that can evoke action potentials and modulate neural circuits through induced electric fields. Biophysical models of magnetic stimulation have become a major driver for technological developments and the understanding of the mechanisms of magnetic neurostimulation and neuromodulation. Major technological developments involve stimulation coils with different spatial characteristics and pulse sources to control the pulse waveform. While early technological developments were the result of manual design and invention processes, there is a trend in both stimulation coil and pulse source design to mathematically optimize parameters with the help of computational models. To date, macroscopically highly realistic spatial models of the brain, as well as peripheral targets, and user-friendly software packages enable researchers and practitioners to simulate the treatment-specific and induced electric field distribution in the brains of individual subjects and patients. Neuron models further introduce the microscopic level of neural activation to understand the influence of activation dynamics in response to different pulse shapes. A number of models that were designed for online calibration to extract otherwise covert information and biomarkers from the neural system recently form a third branch of modelling.

Temporal Considerations for Stimulating Spiral Ganglion Neurons with Cochlear Implants

A wealth of knowledge about different types of neural responses to electrical stimulation has been developed over the past 100 years. However, the exact forms of neural response properties can vary across different types of neurons. In this review, we survey four stimulus-response phenomena that in recent years are thought to be relevant for cochlear implant stimulation of spiral ganglion neurons (SGNs): refractoriness, facilitation, accommodation, and spike rate adaptation. Of these four, refractoriness is the most widely known, and many perceptual and physiological studies interpret their data in terms of refractoriness without incorporating facilitation, accommodation, or spike rate adaptation. In reality, several or all of these behaviors are likely involved in shaping neural responses, particularly at higher stimulation rates. A better understanding of the individual and combined effects of these phenomena could assist in developing improved cochlear implant stimulation strategies. We review the published physiological data for electrical stimulation of SGNs that explores these four different phenomena, as well as some of the recent studies that might reveal the biophysical bases of these stimulus-response phenomena.

Optimization of magnetic neurostimulation waveforms for minimum power loss

Magnetic stimulation is a key tool in experimental brain research and several clinical applications. Whereas coil designs and the spatial field properties have been intensively studied in the literature, the temporal dynamics of the field has received little attention. The available pulse shapes are typically determined by the relatively limited capabilities of commercial stimulation devices instead of efficiency or optimality. Furthermore, magnetic stimulation is relatively inefficient with respect to the required energy compared to other neurostimulation techniques. We therefore analyze and optimize the waveform dynamics with a nonlinear model of a mammalian motor axon for the first time, without any pre-definition of waveform candidates. We implemented an unbiased and stable numerical algorithm using variational calculus in combination with a global optimization method. This approach yields very stable results with comprehensible characteristic properties, such as a first phase which reduces ohmic losses in the subsequent pulse phase. We compare the energy loss of these optimal waveforms with the waveforms generated by existing magnetic stimulation devices.

Analysis and optimization of pulse dynamics for magnetic stimulation

Magnetic stimulation is a standard tool in brain research and has found important clinical applications in neurology, psychiatry, and rehabilitation. Whereas coil designs and the spatial field properties have been intensively studied in the literature, the temporal dynamics of the field has received less attention. Typically, the magnetic field waveform is determined by available device circuit topologies rather than by consideration of what is optimal for neural stimulation. This paper analyzes and optimizes the waveform dynamics using a nonlinear model of a mammalian axon. The optimization objective was to minimize the pulse energy loss. The energy loss drives power consumption and heating, which are the dominating limitations of magnetic stimulation. The optimization approach is based on a hybrid global-local method. Different coordinate systems for describing the continuous waveforms in a limited parameter space are defined for numerical stability. The optimization results suggest that there are waveforms with substantially higher efficiency than that of traditional pulse shapes. One class of optimal pulses is analyzed further. Although the coil voltage profile of these waveforms is almost rectangular, the corresponding current shape presents distinctive characteristics, such as a slow low-amplitude first phase which precedes the main pulse and reduces the losses. Representatives of this class of waveforms corresponding to different maximum voltages are linked by a nonlinear transformation. The main phase, however, scales with time only. As with conventional magnetic stimulation pulses, briefer pulses result in lower energy loss but require higher coil voltage than longer pulses.

Muscle force development after low-frequency magnetic burst stimulation in dogs

INTRODUCTION

Magnetic stimulation allows for painless and non-invasive extrinsic motor nerve stimulation. Despite several advantages, the limited coupling to the target reduces the application of magnetic pulses in rehabilitation. According to experience with electrical stimulation, magnetic bursts could remove this constraint.

METHODS

A novel burst stimulator was used to apply single and burst pulses to the femoral nerve in 10 adult dogs. A figure-of-eight coil was connected, and pulses were applied at 7.5 HZ. Contractions of the quadriceps muscle were measured via an angle force transducer.

RESULTS

Muscle forces were significantly higher upon burst stimulation than after single pulses. Four consecutive burst pulses proved most effective. Stimulation by more bursts resulted in fatigue.

CONCLUSION

Burst stimulation is superior to standard magnetic single pulses, and 4 consecutive burst pulses proved most effective.

Interferential and burst-modulated biphasic pulsed currents yield greater muscular force than Russian current

OBJECTIVE

Previous data regarding neuromuscular electrical stimulation (NMES) have suggested that muscle torque production with interferential current (IFC) is inferior to Russian current; however, waveform parameters specific and critical to NMES were inconsistent, making interpretation of previous findings precarious. The purpose of this investigation was to compare muscle force production of three electrical stimulating waveforms when using equivalent stimulus parameters.

DESIGN

The percent of maximal voluntary isometric knee extensor force (%MVIF) elicited using interferential, Russian, and burst-modulated biphasic pulsed currents were compared in 23 healthy college-aged subjects. A repeated measures single factor design in a university laboratory setting was used.

RESULTS

A significant effect for waveform used was noted. Data showed significantly greater %MVIF of the knee extensors were obtained using IFC or burst-modulated BP current versus conventional Russian current.

CONCLUSIONS

The results of this investigation suggest that IFC and burst-modulated BP current are viable waveform options for purposes of eliciting muscle force. These findings offer significant new evidence with strong clinical implications when selecting waveform parameters for elicitation of muscle force for NMES.

Examining the auditory nerve fiber response to high rate cochlear implant stimulation: chronic sensorineural hearing loss and facilitation

Neural prostheses, such as cochlear and retinal implants, induce perceptual responses by electrically stimulating sensory nerves. These devices restore sensory system function by using patterned electrical stimuli to evoke neural responses. An understanding of their function requires knowledge of the nerves responses to relevant electrical stimuli as well as the likely effects of pathology on nerve function. We describe how sensorineural hearing loss (SNHL) affects the response properties of single auditory nerve fibers (ANFs) to electrical stimuli relevant to cochlear implants. The response of 188 individual ANFs were recorded in response to trains of stimuli presented at 200, 1,000, 2,000, and 5,000 pulse/s in acutely and chronically deafened guinea pigs. The effects of stimulation rate and SNHL on ANF responses during the 0-2 ms period following stimulus onset were examined to minimize the influence of ANF adaptation. As stimulation rate increased to 5,000 pulse/s, threshold decreased, dynamic range increased and first spike latency decreased. Similar effects of stimulation rate were observed following chronic SNHL, although onset threshold and first spike latency were reduced and onset dynamic range increased compared with acutely deafened animals. Facilitation, defined as an increased nerve excitability caused by subthreshold stimulation, was observed in both acute and chronic SNHL groups, although the magnitude of its effect was diminished in the latter. These results indicate that facilitation, demonstrated here using stimuli similar to those used in cochlear implants, influences the ANF response to pulsatile electrical stimulation and may have important implications for cochlear implant signal processing strategies.

Auditory midbrain implant: a review

The auditory midbrain implant (AMI) is a new hearing prosthesis designed for stimulation of the inferior colliculus in deaf patients who cannot sufficiently benefit from cochlear implants. The authors have begun clinical trials in which five patients have been implanted with a single shank AMI array (20 electrodes). The goal of this review is to summarize the development and research that has led to the translation of the AMI from a concept into the first patients. This study presents the rationale and design concept for the AMI as well a summary of the animal safety and feasibility studies that were required for clinical approval. The authors also present the initial surgical, psychophysical, and speech results from the first three implanted patients. Overall, the results have been encouraging in terms of the safety and functionality of the implant. All patients obtain improvements in hearing capabilities on a daily basis. However, performance varies dramatically across patients depending on the implant location within the midbrain with the best performer still not able to achieve open set speech perception without lip-reading cues. Stimulation of the auditory midbrain provides a wide range of level, spectral, and temporal cues, all of which are important for speech understanding, but they do not appear to sufficiently fuse together to enable open set speech perception with the currently used stimulation strategies. Finally, several issues and hypotheses for why current patients obtain limited speech perception along with several feasible solutions for improving AMI implementation are presented.

Effects of phase duration and pulse rate on loudness and pitch percepts in the first auditory midbrain implant patients: Comparison to cochlear implant and auditory brainstem implant results

The auditory midbrain implant (AMI), which is designed for stimulation of the inferior colliculus (IC), is now in clinical trials. The AMI consists of a single shank array (20 contacts) and uses a stimulation strategy originally designed for cochlear implants since it is already approved for human use and we do not yet know how to optimally activate the auditory midbrain. The goal of this study was to investigate the effects of different pulse rates and phase durations on loudness and pitch percepts because these parameters are required to implement the AMI stimulation strategy. Although each patient was implanted into a different region (i.e. lateral lemniscus, central nucleus of IC, dorsal cortex of IC), they generally exhibited similar threshold versus phase duration, threshold versus pulse rate, and pitch versus pulse rate curves. In particular, stimulation with 100 mus/phase, 250 pulse per second (pps) pulse trains achieved an optimal balance among safety, energy, and current threshold requirements while avoiding rate pitch effects. However, we observed large differences across patients in loudness adaptation to continuous pulse stimulation over long time scales. One patient (implanted in dorsal cortex of IC) even experienced complete loudness decay and elevation of thresholds with daily stimulation. Comparing these results with those of cochlear implant and auditory brainstem implant patients, it appears that stimulation of higher order neurons exhibits less and even no loudness summation for higher rate stimuli and greater current leakage for longer phase durations than that of cochlear neurons. The fact that all midbrain regions we stimulated, which includes three distinctly different nuclei, exhibited similar loudness summation effects (i.e. none for pulse rates above 250 pps) suggests a possible shift in some coding properties that is affected more by which stage along the auditory pathway rather than the types of neurons are being stimulated. However, loudness adaptation occurs at multiple stages from the cochlea up to the midbrain.

A low-cost neurostimulator with accurate pulsed-current control

A constant-current stimulator for high-impedance loads using only low-cost standard high-voltage components is presented. A voltage-regulator powers an oscillator built across the primary of a step-up transformer whose secondary supplies, after rectification, the high voltage to a switched current-mirror in the driving stage. Adjusting the regulated voltage controls the pulsed-current intensity. A prototype produces stimulus of amplitude and pulsewidth within 0 < or = I(skin) < or = 20 mA and 50 micros < or = T(pulse) < or = 1 ms, respectively. Pulse-repetition spans from 1 Hz to 10 Hz. Worst case ripple is 3.7% at I(skin) = 1 mA. Overall consumption is 5.6 W at I(skin) = 20 mA.

A stochastic model of the electrically stimulated auditory nerve: pulse-train response

The single-pulse model of the companion paper [1] is extended to describe responses to pulse trains by introducing a phenomenological refractory mechanism. Comparisons with physiological data from cat auditory nerve fibers are made for pulse rates between 100 and 800 pulses/s. First, it is shown that both the shape and slope of mean discharge rate curves are better predicted by the stochastic model than by the deterministic model. Second, while interpulse effects such as refractory effects do indeed increase the dynamic range at higher pulse rates, both the physiological data and the model indicate that much of the dynamic range for pulse-train stimuli is due to stochastic activity. Third, it is shown that the stochastic model is able to predict the general magnitude and behavior of variance in discharge rate as a function of pulse rate, while the deterministic model predicts no variance at all.

Loudness perception with pulsatile electrical stimulation: the effect of interpulse intervals

The effect of interpulse intervals on the perception of loudness of biphasic current pulse trains was investigated in eight adult cochlear implantees at three different stimulus levels encompassing the psychophysical dynamic range. Equal-loudness contours and thresholds were obtained for stimuli in which two biphasic pulses were presented in a fixed repetition period (4 and 20 ms), and also for single-pulse/period stimuli with rates varying between 20 and 750 Hz. All stimuli were of 500-ms duration, and the phase durations of each pulse were 100 microseconds or less. The results of these experiments were consistent with predictions of a three-stage model of loudness perception, consisting of a peripheral refractory effect function, a sliding central integration time window, and a central equal-loudness decision device. Application of the model to the data allowed the estimation of neural refractory characteristics of the subjects' remaining peripheral neural population. The average neural spike probability for a 50-Hz stimulus was predicted to be about 0.77, with an associated neural refractory time of 7.3 ms. These predictions did not vary systematically with level, implying that the effect of increasing current level on loudness results more from recruitment of neurons than from any increase in average spike probability.

Transcutaneous electrical nerve stimulation in the treatment of chronic pain: predictive factors and evaluation of the method

OBJECTIVE

Transcutaneous electrical nerve stimulation (TENS) is a widely used therapeutic approach in acute and chronic pain syndromes. The aim of this study was to investigate the influence of patient management as well as other factors on the outcome of TENS treatment.

DESIGN

The study was carried out as a retrospective analysis of the course of treatment and the therapeutic results of transcutaneous electric nerve stimulation (TENS) in 482 patients with chronic pain of various origins. The follow-up period was up to 48 months. Two groups with differing patient management were compared.

RESULTS

Competent patient evaluation and education (i.e., a long testing and learning phase as well as regular comprehensive after-care) was found to be important. Our analysis of the reasons for the discontinuation of long-term TENS therapy showed that the most important feature was the discrepancy between effort and therapeutic result. Other causes were intermittent depressive states and progression of the underlying disease followed by an aggravation of pain. In addition, numerous factors were identified that adversely affected the outcome of TENS treatment. These factors were listed in order of importance and were included in a prognostic score.

CONCLUSIONS

The prognostic score permits an efficient selection of patients. Moreover, a comprehensive documentation of pain syndromes and their organic, psychogenic, and social features is presented. On the basis of this documentation, an appropriate therapeutic concept may be established. The prognostic score was validated in a subsequent study including 99 patients with chronic pain treated with TENS.

Comparison of the sensory threshold in healthy human volunteers with the sensory nerve response of the rat in vitro hindlimb skin and saphenous nerve preparation on cutaneous electrical stimulation

We report a comparative study of stimulation thresholds of cutaneous fibres of the rat in vitro skin and saphenous nerve preparation with psychophysical measurements of sensibility to cutaneous electrical stimulation in human volunteers. The same clinical diagnostic stimulator and modified skin electrodes were used in both animal and human experiments. Axons were recruited by increasing the stimulus strength, and correlation was made between the stimulus intensity required for unit activation and their conduction velocities. The findings suggest that an initial "tingling" sensation is due to recruitment of A beta fibres and that later sharp "pricking" occurs with recruitment of A delta fibres.

Optimization of single electrode tactile codes

The effects of a frequency modulated electrocutaneous signal's (code's) characteristics on the interpretability of the signal were investigated using an electrocutaneous tracking approach. The characteristics investigated include the functional relationship (exponential and hybrid) between an informational signal and the stimulation frequency, the range of stimulation (2-50 Hz and 2-100 Hz), and the impact of pulse width compensation on a code's efficacy. The interpretability of six different single bipolar electrode codes was examined by 30 subjects using a balanced incomplete block experimental design. Codes with exponentially shaped transfer functions resulted in generally lower electrocutaneous tracking errors than codes utilizing hybrid-shaped transfer functions. Hybrid codes had a transfer function that was linear in the lower frequency range and exponential in the higher frequency range. Codes with a 2-100 Hz frequency range were interpreted better than codes with a 2-50 Hz frequency range. The use of pulse width compensation to maintain a more even level of stimulation intensity had a slightly negative effect on the subjects' abilities to cutaneously track the information signal.

Electrotactile and vibrotactile displays for sensory substitution systems

Sensory substitution systems provide their users with environmental information through a human sensory channel (eye, ear, or skin) different from that normally used, or with the information processed in some useful way. We review the methods used to present visual, auditory, and modified tactile information to the skin. First, we discuss present and potential future applications of sensory substitution, including tactile vision substitution (TVS), tactile auditory substitution, and remote tactile sensing or feedback (teletouch). Next, we review the relevant sensory physiology of the skin, including both the mechanisms of normal touch and the mechanisms and sensations associated with electrical stimulation of the skin using surface electrodes (electrotactile (also called electrocutaneous) stimulation). We briefly summarize the information-processing ability of the tactile sense and its relevance to sensory substitution. Finally, we discuss the limitations of current tactile display technologies and suggest areas requiring further research for sensory substitution systems to become more practical.

A model of threshold for pulsatile electrical stimulation of cochlear implants

Threshold measures have been made as a function of the repetition rate and pulse duration of biphasic electrical pulses applied to the cochlea through a cochlear implant (Shannon, 1985). Nonmonotonicities in those data suggest that at least two separate processes are involved in the translation of an electrical stimulus into a threshold perception. This paper presents a phenomenological model which accounts for the key features of the threshold data. The model consists of two parallel processes which are each power-law functions of the instantaneous current amplitude. The output of each process is then integrated with a short time constant (approximately 1-2 ms). The maximum of these two outputs represents the sensory magnitude of that electrical stimulus. Threshold data from 14 patients implanted with three different devices are compared to model predictions over a wide range of pulse durations and pulse rates. Since the model accurately predicts thresholds over such a wide range of stimuli, it is possible that it can predict the threshold of an arbitrary electrical stimulus. This model could be used to construct a speech processor that would convert any acoustic waveform into an equivalent electrical waveform that would preserve threshold relationships.

Threshold and loudness functions for pulsatile stimulation of cochlear implants

Thresholds and loudness estimates were measured for biphasic pulsatile electrical stimulation of the auditory nerve. Measures were collected as a function of the parameters: pulse duration, and pulse rate. The results indicate that the sensations of threshold and loudness are determined by a complex function of the stimulating current waveform. For stimuli with the same charge, maximum loudness is seen at the shortest pulse durations, and a secondary maximum is seen at pulse durations of 2-3 ms/phase. It is possible that the secondary peak in the loudness function and the slow growth of loudness just above threshold for long pulses are indications of dendrite survival near the electrode. If this interpretation is valid, these measures could lead to perceptual tests of peripheral nerve viability. In addition, a speech processor device could use these measures to equalize the loudness of stimuli with different pulse durations and pulse rates.

Electrical stimulation at the wrist as an aid for the profoundly deaf

The development and evaluation of a sensory electrical substitution aid for the profoundly deaf is described. The aid is a small wrist-worn device which converts sound into an electrical stimulus at the wrist. The initial aim was to give profoundly deaf people useful information from ambient sounds. Evaluation of the Mark 1 aid on 15 hearing and five profoundly deaf people indicated that such a device might be of use to some deaf people. Investigation of the sensitivity of the peripheral nervous system to electrical pulse stimulation showed that information could be transmitted in a more efficient manner so a Mark 2 aid was developed which incorporated circuitry to transmit pitch information, potentially useful in lipreading. The Mark 2 aid was shown to give improved results over the Mark 1 aid in transmitting voice intonation, but gave no help in an isolated word lipreading test. A portable Mark 2 aid was developed to enable clinical trials to be carried out on profoundly deaf people.