Platinum for neural stimulation: voltammetry considerations.

The underlying cause of electrical stimulation-induced tissue trauma is debated. Our focus has been to study effects of generating electrochemical by-products at the electrode-electrolyte interface, using the pulse-clamp technique coupled with voltammetry to analyze charge transfer. The platinum-H(2)SO(4) system has been a standard for analyzing electrochemistry on platinum-stimulating electrodes, even though the chemical differences between H(2)SO(4) and the living body are obvious. Experiments were designed to determine whether phosphate-buffered saline (PBS) could serve as a more accurate emulation of living tissue. It had been rumored that platinum's performance in PBS deviates from that in H(2)SO(4) at larger potentials. Voltammetry in PBS was performed in two potential ranges. In a conventional potential range (-0.6 V to +0.9 V versus Ag/AgCl), characteristic peaks appear very similar to published voltammograms of platinum in H(2)SO(4). However, in an extended range (-1.0 V to +1.7 V versus Ag/AgCl), platinum exhibited additional electrochemical activity: one oxidation peak and two reduction peaks. Therefore, voltammetry was performed in NaCl and a sodium phosphate mixture (i.e. PBS components) to separate their activity. The altered electrochemical performance of platinum in PBS suggests that certain reactions on platinum at potentials outside the water window will not reflect what happens in vivo.

Selective block of external anal sphincter activation during electrical stimulation of the sacral anterior roots in a canine model.

Our aim was to electrically activate small diameter parasympathetic fibres in the sacral anterior roots, without activating the larger somatic fibres to the external anal sphincter (EAS). Electrodes were implanted on selected roots in five adult dogs. Pressures were recorded from the rectum and EAS. Quasitrapezoidal (Qzt) pulses for selective activation of smaller axons and narrow rectangular (Rct) pulses to activate all fibres were applied. Sphincter block was defined as [(P(max) - P(min))/P(max)] x 100%. Roots were also tested with 20 Hz trains. In three animals, evacuation of bowel contents was recorded with artificial fecal material. Stimulation with Qzt pulses showed decrease in sphincter recruitment with increasing pulse amplitudes, indicating propagation arrest in the large fibres. The average sphincter suppression was 94.1% in 16 roots implanted. With Qzt pulse trains, the average evoked sphincter pressure was significantly lower than Rct pulses. Evoked rectal pressures were not significantly different. The mean mass of expelled bowel contents of 51.1 g by Qzt trains was significantly higher than that of 14.8 g expelled by Rct trains. Our results demonstrate that this selective stimuli can activate small diameter fibres innervating the distal bowel and result in significant evacuation of rectal contents.

The learning curve for investigational surgery: lessons learned from laparoscopic diaphragm pacing for chronic ventilator dependence.

BACKGROUND: Electrical stimulation of the phrenic nerve motor point of the diaphragm through laparoscopic implantation of a pacing system is an option for high spinal cord-injured patients with chronic respiratory insufficiency. This study assesses the operative learning curve for the initial series of patients. METHOD: A series of six patients underwent laparoscopic placement of a diaphragm pacing system. The operative procedure was divided into the following four steps for analysis and rapid adjustment after each operation: exposure of the diaphragm, mapping of the phrenic nerve motor point, implantation of the pacing electrodes, and final routing of the wires to the external system. RESULTS: The first case required two operations, and the second case was unsuccessful because of a nonfunctioning phrenic nerve that led to a change in the preoperative screening criteria. The operative time decreased from 469 min for the first operation to 165 min for the sixth operation. The significant time decrease can be attributed to changes in the mapping and routing aspects of the operation. Key changes during this series that helped to reduce the operative time include abandonment of a software-dependent mapping technique, development of a grid algorithm for mapping, software improvement to increase the speed of stimulation and mapping, refinement of the mapping probe to maintain adequate suction on the diaphragm, shortening of the electrode lengths, and experience with the implantation of connections to the external electrodes. Presently, all five of the successfully implanted patients can be maintained on prolonged ventilatory support with the device. CONCLUSION: Analysis of every step of this investigational procedure enabled us to make rapid changes in surgical protocol, leading to decreases in operative times and expectant improvements in patient safety and efficacy. In this series, analysis was the key to developing a low-risk cost-effective outpatient diaphragm pacing system.

Urethral pressure profiles in the female canine implanted with sacral anterior nerve root electrodes.

The purpose of this work was to study the pressure distribution along the urethra in female canines with and without electrical stimulation of the sacral anterior roots innervating the bladder and urethra. Urethral pressure profiles were recorded in two orientations, dorsal and ventral, with microtransducer catheters. Two pulse types were applied at 1 Hz, 500 micros quasitrapezoidal pulses to selectively activate the small axons and 100 micros rectangular pulses. Four parameters were measured from each profile; maximum urethral pressure (MUP), bladder pressure (Pv), functional urethral length (FUL), and the position of the maximum peak from the bladder neck (PMP). Two derived measures, the estimated maximum urethral closing pressure (UCP) and the position of the maximum as a percentage of the FUL (PM%) were calculated. There were highly significant differences (P<0.01) in the value and position of the estimated UCP in the two orientations of the transducers. The highest pressures were recorded in the ventral orientation near the terminal portions of the urethra. Principal sphincter activity during electrical stimulation of the ventral sacral roots was also confined to this part. Selective small fiber activation did not result in any significant increase in this peak pressure from passive values.

Neural and connective tissue response to long-term implantation of multiple contact nerve cuff electrodes.

The objective of this study was to characterize the tissue response to multiple contact spiral nerve cuff electrodes implanted on the sciatic nerve of seven cats for 28-34 weeks. The cuffs were surrounded by fibrous tissue encapsulation consisting of foreign body cells, collagen, and fibroblasts. Focal areas of abnormal neural morphology including perineurial thickening, endoneurial fibrosis, thinly myelinated axons, and focal reduction in the density of myelinated axons were noted in five of seven nerves. In three implants, the percutaneous lead cable was destroyed by the animal pulling on the external leads. Morphological changes were observed in two of three nerves from implants sustaining no known animal induced trauma (group A), and in three of four nerves from implants damaged by the animal pulling at the leads (group B). All nerves appeared normal 2 cm proximal to the cuff. At the cuff level, small regions of one fascicle in each of two nerves (both group B) exhibited abnormalities, while the proximal and distal sections of both nerves were normal. Distal to the cuff, small regions of seven fascicles distributed among three nerves (two group A, one group B) exhibited abnormalities. These nerves were normal at the cuff level but exhibited abnormalities in individual nerve branches distal to the cuff. The incidence and characteristics of the morphological abnormalities at the cuff level are consistent with those observed in previous studies of nerve cuff electrodes, and support the hypothesis that spiral cuff electrodes can be implanted with an internal diameter less than that of the nerve and expand to accommodate the nerve without compression The pattern of morphological abnormalities indicated that mechanical trauma had occurred at some time in the past, and the distribution suggested animal intervention and the lead cable as possible causes.

Laparoscopic implant instrument for the placement of intramuscular electrodes in the diaphragm.

We have developed an endoscopic instrument that will allow a surgeon to safely, dependably and accurately place intramuscular (IM) electrodes in the diaphragm. This instrument has been used to implant 28 IM electrodes in the diaphragms of eleven acute and four chronic dogs. All electrodes achieved full activation of the diaphragm muscle, producing tidal volumes up to 130% V(TCRIT), the critical volume necessary for basal ventilatory support, with unilateral stimulation. The surgeon is able to control the angle of the IM electrode insertion needle, which enables the needle to approach the diaphragm at an angle that is parallel to the surface of the muscle. This insures good control over the depth of needle penetration into the muscle, which greatly reduces the risk of accidentally passing the needle through the diaphragm and entering the thorax. Endoscopic placement of IM electrodes into the diaphragm opens opportunities to provide cost effective negative pressure ventilation to patients who are unable to effect sufficient ventilation by central nervous system (CNS) control of respiration.

An implantable impedance pneumograph monitor for detection of diaphragm contraction and airway obstruction during diaphragm pacing.

Impedance pneumography signals were characterised during diaphragm pacing using stimulating and recording electrodes placed on the abdominal surface of the diaphragm. These measurements were useful for the detection of muscle contraction without confounding effects from stimulus artifacts. Impedance pneumography signals were measured using 23 epimysial electrodes implanted in seven dogs with 1-5 experiments on each electrode. The polarity of the change in impedance associated with diaphragm pacing differed for each recording electrode and its configuration. Thirty-four of 57 cases produced increased impedance, 11 produced decreased impedance and the remaining 12 depended on the level of diaphragm activation. Impedance pneumography signals were useful for detecting complete airway obstruction. The mean difference between the impedance measured during open and obstructed airway conditions was 80% of the open airway impedance signal. The difference between open and obstructed airway impedance measurements was a mean of 2.3 times larger with a recording electrode on the same hemidiaphragm as the stimulating electrode, compared to an electrode placed on the opposite hemidiaphragm (p < 0.05, paired t test, four dogs). In addition, the differences between open and completely obstructed airways were a mean of 2.8 times larger when the second recording electrode was placed on the thorax at the fifth intercostal space, compared to the ninth intercostal space (p < 0.05, two-factor ANOVA, one dog, two replicates). It was concluded that impedance pneumograph circuitry could be incorporated into an existing diaphragm pacer using electrodes placed on the diaphragm to provide valuable measurements of the function of the device.

The effects of epimysial electrode location on phrenic nerve recruitment and the relation between tidal volume and interpulse interval.

Electrode location is of vital importance to diaphragm pacing devices using electrodes implanted on the diaphragm. Complete phrenic nerve recruitment with a single epimysial electrode implanted on the abdominal surface of the diaphragm required placement within 1 cm of the motor point. Recruitment could be increased further using multiple electrodes, provided the electrodes were implanted on opposite sides of the phrenic nerve motor point. The location of the implanted electrode relative to the phrenic nerve motor point also affected the relation between the stimulus interpulse interval (IPI) and the measured tidal volume. Specifically, we found that electrodes implanted lateral to the phrenic nerve motor point had different tidal volume--IPI relations than electrodes placed anterior or posterior to the motor point. We concluded that properly placed epimysial electrodes are required to obtain adequate phrenic nerve recruitment for full time ventilation and knowledge of the relative location of the electrode with respect to motor point is necessary to predict the tidal volume produced by a specific IPI.

Functional conditions of micturition induced by selective sacral anterior root stimulation: experimental results in a canine animal model.

Electrical stimulation of the sacral anterior roots using conventional rectangular current pulses results in a simultaneous contraction of the urinary bladder and the striated urethral sphincter. Using a tripolar nerve cuff electrode with quasitrapezoidal current pulses and appropriate stimulation parameters, hyperpolarization of the nerve-fiber cell membrane under the anode of the stimulating electrode can reversibly arrest action potential propagation in large myelinated nerve fibers, innervating the striated urethral sphincter, while leaving action potential propagation unaffected in small nonmyelinated nerve fibers innervating the urinary bladder smooth muscle (anodal arrest). Using this technique in 19 female mongrel dogs, we studied the effect of bladder filling, level of anesthesia, and sacral deafferentation on bladder pressure, urethral pressure, and urinary flow. Effective micturition could be induced only after complete dorsal rhizotomy, abolishing reflex contraction of the striated urethral sphincter, when blocking quasitrapezoidal current pulses were used for stimulation. Stimulation with rectangular current pulses directly induced a rise in distal urethral pressure, preventing micturition during stimulation.

Stability of the input-output properties of chronically implanted multiple contact nerve cuff stimulating electrodes.

The objective of this investigation was to measure the input-output (I-O) properties of chronically implanted nerve cuff electrodes. Silicone rubber spiral nerve cuff electrodes, containing 12 individual platinum electrode contacts, were implanted on the sciatic nerve of seven adult cats for 28-34 weeks. Measurements of the torque generated at the ankle joint by electrical stimulation of the sciatic nerve were made every 1-2 weeks for the first 6 weeks post-implant and every 3-5 weeks between 6 weeks and 32 weeks post-implant. In three implants the percutaneous lead cable was irreparably damaged by the animal within 4 weeks after implant and further testing was not possible. One additional lead cable was irreparably damaged by the animal at 17 weeks post-implant. The three remaining implants functioned for 28, 31, and 32 weeks. Input-output curves of ankle joint torque as a function of stimulus current amplitude were repeatable within an experimental session, but there were changes in I-O curves between sessions. The degree of variability in I-O properties differed between implants and between different contacts within the same implant. After 8 weeks, the session to session changes in the stimulus amplitude required to generate 50% of the maximum torque (I50) were smaller (15+/-19%, mean +/- s.d.) than the changes in I50 measured between 1 week and 8 weeks post-implant (34+/-42%). Furthermore, the I-O properties were more stable across changes in limb position in the late post-implant period than in acutely implanted cuff electrodes. These results suggest that tissue encapsulation acted to stabilize chronically implanted cuff electrodes. Electrode movement relative to the nerve, de- and regeneration of nerve fibers, and the inability to precisely reproduce limb position in the measurement apparatus all may have contributed to the variability in I-O properties.

Laparoscopic placement of electrodes for diaphragm pacing using stimulation to locate the phrenic nerve motor points.

Laparoscopic mapping of the phrenic nerve motor points using test stimulation was conducted for the implant of epimysial electrodes for diaphragm pacing in dogs. Both visual assessment of muscle activation and measurements of recruitment were useful for identifying an implant location resulting in a mean electrode placement approximately 14 mm from the phrenic nerve motor points in 16 dogs. Postmortem analysis of the stimulus test site locations and corresponding recruitment curves suggested that the phrenic nerve motor points could be predicted during the laparoscopic procedure to within 4.5 mm of the anatomical motor point.

Visual sensations produced by optic nerve stimulation using an implanted self-sizing spiral cuff electrode.

A blind volunteer with retinitis pigmentosa was chronically implanted with a self-sizing spiral cuff electrode around an optic nerve. Electrical stimuli applied to the nerve produced localized visual sensations that were broadly distributed throughout the visual field and could be varied by changing the stimulating conditions. These results demonstrate the potential for constructing a visual prosthesis, based on electrical stimulation of the optic nerve, for blind subjects who have intact retinal ganglion cells.

Extraction forces and tissue changes during explant of CWRU-type intramuscular electrodes from rat gastrocnemius.

Intramuscular electrodes are currently in use for clinically implementing several electrotherapeutic and neuroprosthetic protocols. A decrease in motor recruitment is often reported in these systems due to movement of the electrode tip from the initial implant site. In the study reported here, multistrand intramuscular electrodes of the CWRU design were implanted aseptically in the gastrocnemii of adult rats under anesthesia. These electrodes were explanted immediately after implant in one group and after periods of 1 and 4 hr; 1, 3, and 5 days; 1 week; 10 days; and 2 and 4 weeks in others. Force as a function of displacement was recorded during explantation. Analysis of the results showed that there was a significant increase in the force required to dislodge the electrode tip between 5 and 7 days of implant. Electrodes seemed to be vulnerable to movement in the first 5 days when the barb provided the only fixation. Histology of muscles from which electrodes had been explanted did not show any increase in the area of tissue changes, compared with control muscles in which the electrode remained in situ. These results indicated that electrode removal occurred within the encapsulation tissues, and the surrounding muscle was mainly unaffected by the explant process.

The tissue response to epimysial electrodes for diaphragm pacing in dogs.

Epimysial electrodes stapled to the abdominal surface of the diaphragm produced a chronic inflammatory response that appeared to be mediated by mechanical stresses placed on the encapsulation tissue by periodic diaphragm contraction. The tissue response surrounding 34 epimysial electrodes implanted in 11 dogs was studied three months post implant. The tissue response was characterized by a capsule having a mean thickness of 1.24 mm between the electrode and the muscle, while having only a very thin capsule on the back, or abdominal side of the electrode. The tissue response between the electrode and the muscle was comprised of two tissue layers: a layer of granulation tissue and a layer of collagen. The granulation tissue layer contained evidence of acute inflammatory processes including the presence of polymorphonuclear leukocytes in 68% of the samples. Granulation layer thickness was inversely correlated with back encapsulation indicating a reduction in granulation tissue for mechanically stabilized electrodes. In addition, encapsulation tissue surrounding the granulation layer was comprised of collagen fibers with an oblique orientation and an extraperitoneal locale suggesting mechanical load transfers between the electrode and the surrounding tissue. As a result, the histological response to epimysial electrodes implanted on the diaphragm suggests that mechanical loading, induced by movement associated with the contraction of adjacent muscle, must be a consideration for devices that employ epimysial electrodes.

Inversion of the current-distance relationship by transient depolarization.

The objective of this research was to develop a technique to excite selectively nerve fibers distant from an electrode without exciting nerve fibers close to the electrode. The shape of the stimulus current waveform was designed based on the nonlinear conductance properties of neuronal sodium channel. Models of mammalian peripheral myelinated axons and experimental measurements on cat sciatic nerve were used to determine the effects of subthreshold polarization on neural excitability and recruitment. Subthreshold membrane depolarization generated a transient decrease in neural excitability and thus an increase in the threshold for stimulation by a subsequent stimulus pulse. The decrease in excitability increased as the duration and amplitude of the subthreshold depolarization were increased, and the increase in threshold was greater for fibers close to the electrode. When a depolarizing stimulus pulse was applied immediately after the subthreshold depolarization, nerve fibers far from the electrode could be stimulated without stimulating fibers close to the electrode. Subthreshold depolarizing prepulses inverted the current-distance relationship and allowed selective stimulation of nerve fibers far from the electrode.

Staple penetration and staple histological response for attaching an epimysial electrode onto the abdominal surface of the diaphragm using a laparoscopic approach.

BACKGROUND: Laparoscopic stapling was found to be a viable option for attaching epimysial electrodes onto the abdominal surface of the diaphragm. Stapling was preferable to suturing due to its simplicity and speed. METHODS: Of the two staplers tested in this study, the Ethicon Endopath was preferred over the Autosuture Endo Hernia because the staples did not penetrate the diaphragm when an electrode tab thickness greater than 0.75 mm was used. RESULTS: The thickness of the electrode tab was an important factor in determining staple penetration but large variation in penetration depth indicated that other factors may also play a role. An electrode tab thickness of 1.0-1.25 mm was suggested to minimize the risk of diaphragm perforation. CONCLUSIONS: The histological reaction to staples implanted up to 14 months was unremarkable, reflecting the safety of laparoscopic staples for permanently anchoring electrodes on the diaphragm.

Quantification of recruitment properties of multiple contact cuff electrodes.

Nerve-based stimulating electrodes provide the technology for advancing the function of motor system neural prostheses. The goal of this work was to measure and quantify the recruitment properties of a 12 contact spiral nerve cuff electrode. The cuff was implanted on the cat sciatic nerve trunk, which consists of at least four distinct motor fascicles, and the torque generated at the ankle joint by selective stimulation of the nerve was recorded in nine acute experiments. Comparisons of torques generated with the cuff to torques generated by selective stimulation of individual nerve branches indicated that the cuff allowed selective activation of individual nerve fascicles. Selectivity was dependent on the relative location of the electrode contacts and the nerve fascicles, as well as the size and relative spacing of neighboring fascicles. Selective stimulation of individual nerve fascicles allowed independent and graded control of dorsiflexion and plantarflexion torques in all nine experiments. Field steering currents improved selectivity as reflected by significant increases in the maximum torques that could be generated before spillover to other fascicles, significant increases in the difference between the current amplitude at spillover and the current amplitude at threshold, and significant increases in the slope of the current distance relationship.

The effects of work intensity on adolescent mental health, achievement, and behavioral adjustment: new evidence from a prospective study.

This article examines the effects of work intensity on adolescent mental health, academic achievement, and behavioral adjustment. Questionnaire data were collected yearly from an initial panel of 1,000 randomly selected ninth graders (14-15 years old). Consistent with other studies, students who worked at higher intensity engaged in more alcohol use. The methodological strengths of this research (a representative panel studied prospectively over a 4-year period with minimal attrition and an analysis incorporating key control and lagged variables) provide strong evidence that adolescent work fosters alcohol use. The contention that work of high intensity has deleterious effects on mental health, academic achievement, and 2 other indicators of behavioral adjustment did not withstand our stringent tests. However, high school seniors who worked at moderate intensity (1-20 hours per week) had higher grades than both nonworkers and students who worked more hours per week.

Non-invasive measurement of the input-output properties of peripheral nerve stimulating electrodes.

A non-invasive method was developed to determine the input-output (I/O) properties of peripheral nerve stimulating electrodes. An apparatus was fabricated to measure the 3-dimensional (3-D) isometric torque generated at the cat ankle joint by electrical activation of the sciatic nerve. The performance of the apparatus was quantified, and the utility of the method was demonstrated by measuring the recruitment properties of multiple contact nerve cuff electrodes. Torque-twitch waveforms, recruitment curves of peak torque as a function of stimulus current amplitude, and 2-D joint torque vectors were used to analyze the recruitment properties of the cuff. The peak of the twitch torque was an accurate measure of excitation even for muscles having fibers with varying speeds of contraction. The evoked twitch waveforms and torque vectors generated by selective stimulation of individual nerve branches with a hook electrode were compared to those produced by stimulation of the nerve trunk with the cuff electrode. These data allowed determination of the regions of the nerve trunk that were activated by different electrode geometries and stimulus parameters. The positional stability of electrode recruitment properties could be quantified by measuring I/O characteristics at different limb positions. The methods described are useful for characterization of neural stimulating electrodes and for studies of motor system physiology.