Search

Electrical stimulation can cause significant damage to clinical electrodes as well as patient injury. In this study, the effects of stimulation on pure metal electrodes were investigated without the complexities introduced by the multiple elements that make up the clinical electrode. As with the clinical electrodes, there was significant decomposition of pure stainless steel anodes with no associated significant changes in the cathodes when stimulation employed long pulse durations. Effects of stimulation were greater when the anode and cathode were closer under constant voltage stimulation but were distance independent under constant current stimulation. High ionic content of the solution also increased the degree of damage to the anode as did the presence of chloride in the solution. Electrode composition also influenced the amount damage to the anode. Platinum and platinum-iridium electrodes showed no damage with any stimulus while stainless steel showed the lowest resistance to corrosion for direct current (DC) stimulation. Tungsten electrodes behaved very differently than stainless steel, decomposing with pulse stimulation and resisting decomposition during DC stimulation because of the formation of surface protective layers. Because platinum was able to maintain high levels of current over time, prolonged stimulation produced dramatic increases in the temperature of the solution; however, even short periods of stimulation were sufficient to produce dramatic changes in pH in the neighborhood of the electrode.

This three part series of articles has discussed mechanisms of electrode related injuries from a theoretical viewpoint in Part 1 (Stecker et al. 2006) and from a clinical experience viewpoint in Part 2 (Patterson et al. 2007). The third and final part of the series discusses practical concepts for the end user to use to both understand the basic principles at work in the creation of electrode related injuries as well as to avoid electrode related injuries. To help accomplish this, an online presentation with animations and voiceover is used to supplement written explanations of these basic principles. Areas of interest include mechanical pressure points, adhesives in contact with skin, electrosurgical unit (ESU) radiofrequency concepts, magnetic resonance imaging (MRI) radiofrequency concepts, electrochemical concepts, and basic electrical safety concepts. The goal of this article series has been to heighten awareness of electrode related lesions in general and to help clinicians become better at injury avoidance. A list of electrode related injury avoidance fundamentals is presented. Finally, practical scenarios are presented to illustrate the applications of the concepts.

In the previous paper in this series, basic mechanisms of electrode related injuries were discussed. In this paper, the discussion begins with some of the clinical aspects of burns. This is followed by a summary of the clinical literature on injuries produced by surface and subdermal electrodes. This clinical literature demonstrates that most electrode burns are related to the presence of high frequency electric fields (RF) created either by an electrosurgical unit or a magnetic resonance imaging (MRI) scanner. A smaller number of lesions are produced by low current, long duration direct current (DC) stimulation and during high current stimulation such as defibrillation. A discussion of the clinical complications from indwelling intracranial electrodes centers on electrodes placed for deep brain stimulation (DBS) that are currently used therapeutically in a wide array of neurologic disorders. The probability of considering a post-implant MRI scan is high and the safety of such scans is the focus of discussion. A very small number of adverse incidents have indicated a downward revision in the specific absorption rate recommendations for MRI examination with those patients who present with indwelling DBS leads and internal pulse generators. Continued vigilance when any type of electrode is used is important.

Electrodes are the essential elements of clinical neurophysiology both in recording of neural activity and in functional electrical stimulation of the nervous system. Therefore it is important to understand the potential complications of using electrodes. In this paper, the factors that influence the chance of electrode related injury are discussed from a theoretical standpoint. The mechanical factors, especially pressure related injury, are discussed first, followed by a discussion of injury that is of chemical origin such as contact dermatitis. Next, the ways in which electrical currents flowing from electrodes can cause injury including: Joule heating, electroporation, electroconformational denaturation, and excitatory neurotoxicity are discussed. The differential effects of constant current and constant voltage stimulation on tissue heating are examined, as are the effects of the conductivity and geometric structure of the stimulated tissue. Finally, the effects of electrochemical reactions are discussed both in the context of surface and implanted electrodes.

Monitoring the vagus nerve and the recurrent laryngeal nerve during surgical procedures may reduce the probability of significant nerve injury. As such, a number of methods to monitor these nerves have been devised including placing electrodes directly into the vocal cords or recording from surface electrodes. In direct comparison, monitoring the identical muscles, bipolar hookwire electrodes displayed approximately one order of magnitude greater amplitude, of both spontaneously occurring and evoked electrical activity than double wire endotracheal tube electrodes. The enhanced sensitivity of the hookwire electrodes, despite the technical difficulties with placement, suggests their use when maximum sensitivity is required.

Objective: The purpose of this study was to investigate the short-term effects of residual dust from the World Trade Center (WTC) on rat sciatic nerve. Methods: Nerve action potentials were recorded in nerves exposed to dust from the WTC as well as control nerves. Results: There was a reduction in the conduction velocity of nerves exposed to a high concentration of the dust from the WTC when compared with controls. Conclusions: Although there are statistically significant reductions in conduction velocity when exposed to the WTC dust in this pilot study, additional studies both clinical and basic will be needed to further understand the significance of these results. [ABSTRACT FROM AUTHOR]

Differences between constant current and constant voltage nerve stimulation are controversial. To elucidate this controversy, exact solutions are found for the electrical potential and current of a conducting electrode of finite size placed near a boundary of altered conductivity. Substantial differences in the effects of a finite and a point stimulator are predicted. This was strongly dependent on the stimulator–boundary distance, the conductivity of the media, and the curvature of the boundary. The difference between constant voltage and constant current stimulation was smaller than the effects of changes in medium conductivity and electrode distance. A poorly conducting boundary layer surrounding the stimulator minimized these differences. [Copyright &y& Elsevier]

The potential recorded by a set of electrodes as an action potential traverses a small axonal segment is proportional to the transmembrane potential produced during stimulation of that axon segment by the same set of recording electrodes, under certain circumstances. First, the membrane must have a constant thickness which is so small that the difference between the surface area of the inner and outer surfaces is minimal. Second, all media must be linear. Third, there must be a monotonically increasing relation between the mean transmembrane potential induced by a stimulus and the maximum transmembrane potential. Fourth, as each axon segment depolarizes, the transmembrane current and change in membrane potential during this lime are same. This principle remains true for magnetic stimulation and recording as long as currents generated at the boundaries between regions of differing conductivity outside the axon contribute minimally to the field at the axon. This allows the identification of the point at which an action potential generates a maximal extracellular potential as the point that is stimulated with the lowest threshold. [ABSTRACT FROM AUTHOR]